0001692115-21-000008 | 10-K | XBRL Viewer | Southwest GAS Holdings INC

Revenue from Contract with Customer [Abstract]

Utility Plant and Leases

Note 2 - Utility Plant and Leases

Net Utility Plant

Net utility plant as of December 31, 2020 and 2019 was as follows:


	December 31,
(Thousands of dollars)	2020		2019
Gas plant:
Storage	$	101,203	$	100,908
Transmission	400,657		391,864
Distribution	7,078,656		6,581,043
General	515,879		467,274
Software and software-related intangibles	270,883		256,299
Other	16,722		15,833
	8,384,000		7,813,221
Less: accumulated depreciation and amortization	(2,419,348)		(2,313,050)
Construction work in progress	211,429		185,026
Net utility plant	$	6,176,081	$	5,685,197

Utility plant depreciation is computed on the straight-line remaining life method at composite rates considered sufficient to amortize costs over estimated service lives, including components which are intended to compensate for removal costs (net of salvage value), and retirements, based on the processes of regulatory proceedings and related regulatory commission approvals and/or mandates. In 2020, 2019, and 2018, annual utility depreciation and amortization expense averaged 2.7% of the original cost of depreciable and amortizable property. Transmission and Distribution plant are associated with the core natural gas delivery infrastructure, and combined, constitute the majority of gas plant. Annual utility depreciation expense averaged approximately 2.3% of the original cost of depreciable transmission and distribution plant during the period 2018 through 2020.

Depreciation and amortization expense on gas plant, including intangibles, was as follows:


(Thousands of dollars)	2020		2019		2018
Depreciation and amortization expense	$	215,636	$	197,358	$	185,719

Included in the figures above is amortization of utility intangibles of $13.7 million, $13.2 million, and $13.6 million for the years ended December 31, 2020, 2019, and 2018, respectively. The amounts above exclude regulatory asset and liability amortization.

Leases

Southwest and Centuri determine if an arrangement is a lease at inception. ROU assets represent the right to use an underlying asset for the lease term; lease liabilities represent obligations to make lease payments arising from the lease. Operating lease ROU assets and lease liabilities are recognized at the commencement date based on the present value of lease payments over the lease term. When Southwest’s and Centuri’s leases do not provide an implicit interest rate, an incremental borrowing rate based on information available at commencement is used in determining the present value of lease payments; an implicit rate, if readily determinable, is used. Lease terms utilized in the computations may include options to extend or terminate the lease when it is reasonably certain that the option will be exercised. When lease agreements include non-lease components, they are included with the lease component and accounted for as a single component, for all asset classes.

Southwest’s leases are comprised primarily of operating leases of buildings, land, and equipment. Southwest has no finance leases and no significant short-term leases. Southwest’s leases have a remaining term of up to six years, some of which include optional renewal periods. Southwest is currently not a lessor in any significant lease arrangements.

Centuri has operating and finance leases for corporate and field offices, construction equipment, and transportation vehicles. Centuri is currently not a lessor in any significant lease arrangements. Centuri’s leases have remaining lease terms of up to 18 years. Some of these include options to extend the leases, generally for optional terms of up to 5 years, and some include options to terminate the leases within 1 year. Centuri’s equipment leases may include variable payment terms in addition to the fixed lease payments if machinery is used in excess of the standard work periods. These variable payments are not probable of occurring under the current operating environment and have not been included in consideration of lease payments. Due to the seasonality of Centuri’s business, expense for short-term leases will fluctuate throughout the year with higher expense incurred during the warmer months. As of December 31, 2020, Centuri executed lease agreements that had not yet commenced. These lease agreements primarily relate to real estate leases that have terms ranging from January 2021 through May 2025. Total future lease payments over the lease terms are approximately $1 million.

The components of lease expense were as follows:


(Thousands of dollars)	2020		2019
Southwest:
Operating lease cost	$	1,251	$	1,531

Centuri:
Operating lease cost	$	14,294	$	12,235

Finance lease cost:
Amortization of ROU assets	$	140	$	137
Interest on lease liabilities	37		34
Total finance lease cost	177		171
Short-term lease cost	19,806		16,217
Total lease cost - Southwest Gas Holdings, Inc.	$	35,528	$	30,154

Supplemental cash flow information related to leases for the years ended December 31, 2020 and 2019 was as follows:


	2020
(Thousands of dollars)	Southwest		Centuri		Total
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	766	$	12,889	$	13,655
Operating cash flows from finance leases	—		36		36
Financing cash flows from finance leases	—		199		199

ROU assets obtained in exchange for lease obligations:
Operating leases	$	1,547	$	19,372	$	20,919
Finance leases	—		361		361


	2019
(Thousands of dollars)	Southwest		Centuri		Total
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	1,278	$	11,166	$	12,444
Operating cash flows from finance leases	—		33		33
Financing cash flows from finance leases	—		212		212

ROU assets obtained in exchange for lease obligations:
Operating leases	$	862	$	23,825	$	24,687
Finance leases	—		13,839		13,839

Supplemental information related to leases, including location in the Consolidated Balance Sheets, is as follows:


(Thousands of dollars)	December 31,
	2020			2019
Southwest:
Operating leases:
Net utility plant	$	2,195		$	1,443

Other current liabilities	$	656		$	723
Other deferred credits and other long-term liabilities	1,586			730
Total operating lease liabilities	$	2,242		$	1,453

Weighted average remaining lease term (in years)	4.24			2.88
Weighted average discount rate	4.49		%	3.18		%

Centuri:
Operating leases:
Other property and investments	$	81,010		$	78,954

Other current liabilities	$	10,032		$	8,851
Other deferred credits and other long-term liabilities	75,247			73,323
Total operating lease liabilities	$	85,279		$	82,174

Finance leases:
Other property and investments	$	752		$	14,264

Other current liabilities	$	202		$	13,769
Other deferred credits and other long-term liabilities	490			355
Total finance lease liabilities	$	692		$	14,124

Weighted average remaining lease term (in years)
Operating leases	10.08			10.25
Finance leases	2.12			2.13

Weighted average discount rate
Operating leases	4.05		%	4.03		%
Finance leases	5.55		%	6.10		%

With regard to the finance lease balance as of December 31, 2020, there exist lease provisions for purchase options that meet the “reasonably certain” threshold related to exercise of such options. These amounts were not included in the calculations of the weighted average remaining lease term and discount rate for finance leases above.

The following are schedules of maturities of lease liabilities as of December 31, 2020:


(Thousands of dollars)	Operating Leases
Southwest:
2021	$	735
2022	543
2023	426
2024	404
2025	330
Thereafter	36
Total lease payments	2,474
Less imputed interest	232
Total	$	2,242


(Thousands of dollars)	Operating Leases		Finance Leases
Centuri:
2021	$	13,144	$	235
2022	12,153		241
2023	10,864		275
2024	9,904		1
2025	8,051		—
Thereafter	50,605		—
Total lease payments	104,721		752
Less imputed interest	19,442		60
Total	$	85,279	$	692

Revenue

12 Months Ended

Dec. 31, 2020

Revenue

Note 3 - Revenue

The following information about the Company’s revenues is presented by segment. Southwest encompasses the natural gas operations segment and Centuri encompasses the utility infrastructure services segment.

Natural Gas Operations Segment:

Southwest recognizes revenue when it satisfies its performance by transferring gas to the customer. Revenues also include the net impacts of margin tracker/decoupling accruals based on criteria in U.S. GAAP for rate-regulated entities associated with alternative revenue programs. Revenues from customer arrangements and from alternative revenue programs are described below.

Southwest acts as an agent for state and local taxing authorities in the collection and remittance of a variety of taxes, including sales and use taxes and surcharges. These taxes are not included in Gas operating revenues. Management uses the net classification method to report taxes collected from customers to be remitted to governmental authorities.

Southwest generally offers two types of services to its customers: tariff sales and transportation–only service. Tariff sales encompass sales to many types of customers (primarily residential) under various rate schedules, subject to cost-of-service ratemaking, which is based on the rate-regulation of state commissions and the Federal Energy Regulatory Commission (the “FERC”). Southwest provides both the commodity and the related distribution service to nearly all of its approximate 2 million customers, and only several hundred customers (who are eligible to secure their own gas) subscribe to transportation-only service. Also, only a few hundred customers have contracts with stated periods. Natural gas is delivered and consumed by the customer simultaneously. The provision of service is represented by the turn of the meter dial and is the primary representation of the satisfaction of performance obligations of Southwest. The amount billable via regulated rates (both volumetric and fixed monthly rates as part of rate design) corresponds to the value to the customer, and management believes that the amount billable (amount Southwest has the right to invoice) is appropriate to utilize for purposes of recognizing revenue. Estimated amounts remaining unbilled since the last meter read date are restricted from being billed due only to the passage of time and therefore are also recognized for service provided through the balance sheet date. While natural gas service is typically recurring, there is generally not a contract term for utility service. Therefore, the contract term is not generally viewed to extend beyond the service provided to date, and customers can generally terminate service at will.

Transportation-only service is also governed by tariff rate provisions. Transportation-only service is generally only available to very large customers under requirements of Southwest’s various tariffs. With this service, customers secure their own gas supply and Southwest provides transportation services to move the customer-supplied gas to the intended location. Southwest concluded that transportation/transmission service is suitable to an “over time” recognition model. Rate structures under Southwest’s regulation for transportation customers include a combination of volumetric charges and monthly “fixed” charges (including charges commonly referred to as capacity charges, demand charges, or reservation charges) as part of the rate design of regulated jurisdictions. These types of fixed charges represent a separate performance obligation associated with standing ready over the period of the month to deliver quantities of gas, regardless of whether the customer takes delivery of any quantity of gas. The performance obligations under these circumstances are satisfied over the course of the month under an output measure of progress based on time, which correlates to the period for which the charges are eligible to be invoiced.

Under its regulation, Southwest enters into negotiated rate contracts for those customers located in proximity to another pipeline, which pose a threat of bypassing its distribution system. Southwest may also enter into similar contracts for customers otherwise able to satisfy their energy needs by means of alternative fuel to natural gas. Less than two dozen customers are party to contracts with rate components subject to negotiation. Many rate provisions and terms of service for these less common types of contracts are also subject to regulatory oversight and tariff provisions. The performance obligations for these customers are satisfied similarly to those for other customers by means of transporting/delivering natural gas to the customer. Many or most of the rate components, and structures, for these types of customers are the same as those for similar customers without negotiated rate components; and the negotiated rates are within the parameters of the tariff guidelines. Furthermore, while some of these contracts include contract periods extending over time, including multiple years, as amounts billable under the contract are based on rates in effect for the customer for service provided to date, no significant financing component is deemed to exist.

As indicated above, revenues also include the net impacts of margin tracker/decoupling accruals. All of Southwest’s service territories have decoupled rate structures (also referred to as alternative revenue programs) that are designed to eliminate the direct link between volumetric sales and revenue, thereby mitigating the impacts of unusual weather variability and conservation on margin. The primary alternative revenue programs involve permissible adjustments for differences between stated tariff benchmarks and amounts billed through revenue from contracts with customers via existing rates. Such adjustments are recognized monthly in revenue and in the associated regulatory asset/liability accounts in advance of rate adjustments intended to collect or return amounts recognized. Revenues recognized for the adjustment to the benchmarks noted are required to be presented separately from revenues from contracts with customers, and as such, are provided below and identified as related to alternative revenue programs (which excludes recoveries from customers).

Gas operating revenues on the Consolidated Statements of Income of both the Company and Southwest include revenue from contracts with customers, which is shown below disaggregated by customer type, and various categories of revenue:


	December 31,
(Thousands of dollars)	2020		2019		2018
Residential	$	958,520	$	972,788	$	887,220
Small commercial	221,541		249,117		255,083
Large commercial	44,633		48,935		53,192
Industrial/other	26,242		22,074		23,489
Transportation	88,215		92,380		86,990
Revenue from contracts with customers	1,339,151		1,385,294		1,305,974
Alternative revenue program deferrals	12,140		(25,112)		45,979
Other revenues (a)	(706)		8,757		5,775
Total Gas operating revenues	$	1,350,585	$	1,368,939	$	1,357,728

(a) Comprised of late fees, certain regulatory mechanism impacts, such as cost-of-service components in current customer rates that are expected to be returned to customers in the future, and various other revenue impacts, including $(5) million, $(4.9) million, and $(13.5) million for 2020, 2019, and 2018, respectively, related to tax reform savings reserves/adjustments. In 2020, late fees and certain other fees were reduced due to a moratorium on late fees and disconnection for nonpayment during the COVID-19 pandemic.

Utility Infrastructure Services Segment:

The majority of Centuri contracts are performed under unit-price contracts. Generally, these contracts state prices per unit of installation. Typical installations are accomplished in a few weeks or less. Revenues are recorded as installations are completed. Revenues are recorded for long-term fixed-price contracts in a pattern that reflects the transfer of control of promised goods and services to the customer over time. The amount of revenue recognized on fixed-price contracts is based on costs expended to date relative to anticipated final contract costs (a method of recognition based on inputs). Some unit-price contracts contain caps that if encroached, trigger revenue and loss recognition similar to a fixed-price contract model.

Centuri is required to collect taxes imposed by various governmental agencies on the work performed for its customers. These taxes are not included in Utility infrastructure services revenues. Management uses the net classification method to report taxes collected from customers to be remitted to governmental authorities.

Centuri derives revenue from the installation, replacement, repair, and maintenance of energy distribution systems. Centuri has operations in the U.S. and Canada. The majority of Centuri’s revenues are related to contracts for natural gas pipeline replacement and installation work for natural gas utilities. In addition, Centuri performs certain industrial construction activities for various customers and industries. Centuri has two types of agreements with its customers: master services agreements (“MSAs”) and bid contracts. Most of Centuri’s customers supply many of their own materials in order for Centuri to complete its work under the contracts.

An MSA identifies most of the terms describing each party’s rights and obligations that will govern future work authorizations. An MSA is often effective for multiple years. A work authorization is issued by the customer to describe the location, timing, and any additional information necessary to complete the work for the customer. The combination of the MSA and the work authorization determines when a contract exists and revenue recognition may begin. Each work authorization is generally a single performance obligation as Centuri is performing a significant integration service.

A bid contract is typically a one-time agreement for a specific project that has all necessary terms defining each party’s rights and obligations. Each bid contract is evaluated for revenue recognition individually. Control of assets created under bid contracts generally passes to the customer over time. Bid contracts often have a single performance obligation as Centuri is providing a significant integration service.

Centuri’s MSA and bid contracts are characterized as either fixed-price contracts or unit-price contracts for revenue recognition purposes. The cost-to-cost input method is used to measure progress towards the satisfaction of a performance obligation for fixed-price contracts. Input methods result in the recognition of revenue based on the entity’s expended effort toward satisfaction of the performance obligation relative to the total expected effort to satisfy it in full. For unit-price contracts, an output method is used to measure progress towards satisfaction of a performance obligation (based on the completion of each unit that is required under the contract).

Actual revenues and project costs can vary, sometimes substantially, from previous estimates due to changes in a variety of factors, including unforeseen circumstances. These factors, along with other risks inherent in performing fixed-price contracts may cause actual revenues and gross profit for a project to differ from previous estimates, and could result in reduced profitability or losses on projects. Changes in these factors may result in revisions to costs and earnings, the impacts for which are recognized in the period in which the changes are identified. Once identified, these types of conditions continue to be evaluated for each project throughout the project term, and ongoing revisions in management’s estimates of contract value, cost, and profit are recognized as necessary in the period determined.

Centuri categorizes work performed under MSAs and bid contracts into three primary service types: gas construction, electrical construction, and other construction. Gas construction includes work involving previously existing gas pipelines and the installation of new pipelines or service lines. Electrical construction includes work involving installation and maintenance of transmission and distribution lines, including storm restoration services. Other construction includes all other work and can include industrial and water utility services.

Contracts can have compensation/consideration that is variable. For MSAs, variable consideration is evaluated at the customer level as the terms creating variability in pricing are included within the MSA and are not specific to a work authorization. For multi-year MSAs, variable consideration items are typically determined for each year of the contract and not for the full contract term. For bid contracts, variable consideration is evaluated at the individual contract level. The expected value method or most likely amount method is used based on the nature of the variable consideration. Types of variable consideration include liquidated damages, delay penalties, performance incentives, safety bonuses, payment discounts, and volume rebates. Centuri will typically estimate variable consideration and adjust financial information, as necessary.

Change orders involve the modification in scope, price, or both to the current contract, requiring approval by both parties. The existing terms of the contract continue to be accounted for under the current contract until such time as a change order is approved. Once approved, the change order is either treated as a separate contract or as part of the existing contract, as appropriate, under the circumstances. When the scope is agreed upon in the change order but not the price, Centuri estimates the change to the transaction price.

The following tables display Centuri’s revenue from contracts with customers disaggregated by service type and contract type:


	December 31,
(Thousands of dollars)	2020		2019		2018
Service Types:
Gas infrastructure services	$	1,261,160	$	1,238,974	$	1,123,682
Electric power infrastructure services	411,826		247,717		32,629
Other	275,302		264,287		365,974
Total Utility infrastructure services revenues	$	1,948,288	$	1,750,978	$	1,522,285


	December 31,
(Thousands of dollars)	2020		2019		2018
Contract Types:
Master services agreement	$	1,490,009	$	1,383,377	$	1,102,412
Bid contract	458,279		367,601		419,873
Total Utility infrastructure services revenues	$	1,948,288	$	1,750,978	$	1,522,285

Unit price contracts	$	1,356,640	$	1,380,256	$	1,258,419
Fixed price contracts	157,701		112,924		117,298
Time and materials contracts	433,947		257,798		146,568
Total Utility infrastructure services revenues	$	1,948,288	$	1,750,978	$	1,522,285

The following table provides information about contracts receivable and revenue earned on contracts in progress in excess of billings (contract assets), both of which are included within Accounts receivable, net of allowances, and provides information about amounts billed in excess of revenue earned on contracts (contract liabilities), which are included in Other current liabilities as of December 31, 2020 and 2019 on the Company’s Consolidated Balance Sheets:


	December 31,
(Thousands of dollars)	2020		2019
Contracts receivable, net	$	278,316	$	223,904
Revenue earned on contracts in progress in excess of billings	96,996		99,399
Amounts billed in excess of revenue earned on contracts	4,507		4,525

The revenue earned on contracts in progress in excess of billings (contract asset) primarily relates to Centuri’s rights to consideration for work completed but not billed and/or approved at the reporting date. These contract assets are transferred to contracts receivable when the rights become unconditional. These contract assets are recoverable from Centuri’s customers based upon various measures of performance, including achievement of certain milestones, completion of specified units or completion of a contract. In addition, many of Centuri’s time and materials arrangements are billed in arrears pursuant to contract terms that are standard within the industry, resulting in contract assets and/or unbilled receivables being recorded, as revenue is recognized in advance of billings. Due to the lag in invoicing associated with contractual provisions (or other economic or market conditions that may impact a customer’s business), Centuri’s ability to bill and subsequently collect amounts due may be impacted. These changes may result in the need to record an estimated valuation allowance to adjust contract asset balances to their net realizable value.

The amounts billed in excess of revenue earned (contract liability) primarily relate to the advance consideration received from customers for which work has not yet been completed. The change in this contract liability balance from December 31, 2019 to December 31, 2020 was due to revenue recognized of $4.5 million that was included in this balance as of January 1, 2020, after which time it became earned and the balance was reduced, and to increases due to cash received, net of revenue recognized during the period related to contracts that commenced during the period.

For contracts that have an original duration of one year or less, Centuri does not consider/compute an interest component based on the time value of money. Further, because of the short duration of these contracts, the Company has not disclosed the transaction price for the remaining performance obligations as of the end of each reporting period or when the Company expects to recognize the revenue.

As of December 31, 2020, Centuri has 16 contracts with an original duration of more than one year. The aggregate amount of the transaction price allocated to the unsatisfied performance obligations of these contracts as of December 31, 2020 was $76.4 million. Centuri expects to recognize the remaining performance obligations over approximately the next two years;

however, the timing of that recognition is largely within the control of the customer, including when the necessary equipment and materials required to complete the work will be provided by the customer.

Utility infrastructure services contracts receivable consists of the following:


	December 31,
(Thousands of dollars)	2020		2019
Billed on completed contracts and contracts in progress	$	273,778	$	216,268
Other receivables	6,692		8,456
Contracts receivable, gross	280,470		224,724
Allowance for doubtful accounts	(2,154)		(820)
Contracts receivable, net	$	278,316	$	223,904

The entire disclosure of revenue from contract with customer to transfer good or service and to transfer nonfinancial asset. Includes, but is not limited to, disaggregation of revenue, credit loss recognized from contract with customer, judgment and change in judgment related to contract with customer, and asset recognized from cost incurred to obtain or fulfill contract with customer. Excludes insurance and lease contracts.

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Receivables and Related Allowances

Receivables and Related Allowances

12 Months Ended

Dec. 31, 2020

Receivables [Abstract]

Note 4 - Receivables and Related Allowances

Business activity with respect to natural gas utility operations is conducted with customers located within the three-state region of Arizona, Nevada, and California. Southwest’s accounts receivable are short-term in nature with no billing due dates customarily extending beyond one month, with customers’ credit worthiness assessed upon account creation by evaluation of other utility service and related payment history. Although Southwest seeks generally to minimize its credit risk related to utility operations by requiring security deposits from new customers, imposing late fees, and actively pursuing collection on overdue accounts, some accounts are ultimately not collected. Customer accounts are subject to collection procedures that vary by jurisdiction (late fee assessment, noticing requirements for disconnection of service, and procedures for actual disconnection and/or reestablishment of service). After disconnection of service, accounts are customarily written off approximately two months after inactivation. Dependent upon the jurisdiction, reestablishment of service requires both payment of previously unpaid balances and additional deposit requirements. Provisions for uncollectible accounts are recorded monthly based on experience, consideration of current and expected future conditions, customer and rate composition, and write-off processes. They are included in the ratemaking process as a cost of service. The Nevada jurisdictions have a regulatory mechanism associated with the gas-cost-related portion of uncollectible accounts. Such amounts are deferred and collected through a surcharge in the ratemaking process. Due to the ongoing COVID-19 pandemic, Southwest initiated a moratorium in March 2020 on disconnection of natural gas service for non-payment and also ceased charging late fees until further notice. While the moratorium continues to be in place, Southwest is actively working with customers experiencing financial hardship by means of flexible payment options and by coordinating with certain governmental and nonprofit entities for customer payment assistance. Management continues to monitor expected credit losses in light of the evolving financial impact of COVID-19. The allowance as of December 31, 2020 reflects the expected impact from the pandemic on balances as of that date, including consideration of customers’ ability to pay currently and once the moratorium is lifted.

Utility infrastructure services contracts receivable are recorded at face amounts less an allowance for doubtful accounts. Centuri’s customers are generally investment-grade gas and electric utility companies for which Centuri has historically recognized an insignificant amount of write-offs. Centuri’s trade accounts receivable balances carry standard payment terms of up to 60 days. Centuri maintains an allowance that is an estimate based on historical collection experience, current and estimated future economic and market conditions, and a review of the current status of each customer's trade accounts receivable balance. Account balances are monitored at least monthly, and are charged off against the allowance when management determines it is probable the balance will not be recovered. Centuri has not been significantly impacted, nor does it anticipate it will experience significant difficulty in collecting amounts due, given the nature of its customers, as a result of the current environment surrounding COVID-19.

The table below contains information about the gas utility customer accounts receivable balance (net of allowance) at December 31, 2020 and 2019, and the percentage of customers in each of the three states, which was consistent with the prior year.


	December 31,
(Thousands of dollars)	2020		2019
Gas utility customer accounts receivable balance	$	145,108	$	148,173

The following table represents customers by state at December 31, 2020:


Percent of customers by state:
Arizona	53	%
Nevada	37	%
California	10	%

Activity in the allowance account for uncollectibles is summarized as follows:


(Thousands of dollars)	Allowance for Uncollectibles
Balance, December 31, 2017	$	2,111
Additions charged to expense	2,959
Accounts written off, less recoveries	(2,902)
Balance, December 31, 2018	2,168
Additions charged to expense	3,507
Accounts written off, less recoveries	(3,580)
Balance, December 31, 2019	2,095
Additions charged to expense	4,693
Accounts written off, less recoveries	(2,454)
Balance, December 31, 2020	$	4,334

At December 31, 2020, the utility infrastructure services segment (Centuri) had $375.3 million in combined customer accounts and contracts receivable. Both the allowance for uncollectibles and write-offs related to Centuri customers have been insignificant and are not reflected in the table above.

Regulatory Assets and Liabilities

12 Months Ended

Dec. 31, 2020

Regulatory Assets and Liabilities

Note 5 - Regulatory Assets and Liabilities

Southwest is subject to the regulation of the Arizona Corporation Commission (“ACC”), the Public Utilities Commission of Nevada (“PUCN”), the California Public Utilities Commission (“CPUC”), and the FERC. Accounting policies of Southwest conform to U.S. GAAP applicable to rate-regulated entities and reflect the effects of the ratemaking process. Accounting treatment for rate-regulated entities allows for deferral as regulatory assets, costs that otherwise would be expensed, if it is probable that future recovery from customers will occur. If rate recovery is no longer probable, due to competition or the actions of regulators, Southwest is required to write-off the related regulatory asset. Regulatory liabilities are recorded if it is probable that revenues will be reduced for amounts that will be refunded to customers through the ratemaking process. Southwest management records regulatory assets and liabilities based on decisions of the commissions noted above, including the issuance of regulatory orders and precedents established by these commissions. Southwest has generally been successful in seeking recovery of regulatory assets, and regularly files rate cases in all jurisdictions, in part to establish the basis for recovering regulatory assets reflected in accounting records.

The following table represents existing regulatory assets and liabilities:


	December 31,
(Thousands of dollars)	2020		2019
Regulatory assets:
Accrued pension and other postretirement benefit costs (1)	$	427,550	$	420,114
Unrealized net loss on non-trading derivatives (Swaps) (2)	—		10,951
Deferred purchased gas costs (3)	2,053		44,412
Accrued purchased gas costs (4)	29,000		8,000
Unamortized premium on reacquired debt (5)	17,124		18,249
Accrued absence time (8)	15,565		14,519
Margin, interest- and property tax-tracking (9)	4,759		33,380
Other (10)	45,596		33,134
	541,647		582,759
Regulatory liabilities:
Deferred purchased gas costs (3)	(54,636)		(60,755)
Accumulated removal costs	(404,000)		(395,000)

Unamortized gain on reacquired debt (6)	(7,644)		(8,181)
Regulatory excess deferred/other taxes and gross-up (7)	(461,023)		(455,625)
Margin, interest- and property tax-tracking (9)	(17,132)		(22,650)
Other (10)	(3,503)		(4,438)
Net regulatory liabilities	$	(406,291)	$	(363,890)

(1)Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovery period is greater than five years. (See Note 11 - Pension and Other Postretirement Benefits).

(2)Included in Prepaid and other assets on the Consolidated Balance Sheets. The actual amounts, when realized at settlement, become a component of purchased gas costs under Southwest’s PGA mechanisms. (For specific details, see Derivatives in Note 1 - Background, Organization, and Summary of Significant Accounting Policies).

(3)Balance recovered or refunded on an ongoing basis with interest.

(4)Included in Prepaid and other current assets on the Consolidated Balance Sheets. Balance recovered or refunded on an ongoing basis.

(5)Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovered over life of debt instruments.

(6)Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. Amortized over life of debt instruments.

(7)Includes remeasurement/reduction of the net accumulated deferred income tax liability from U.S. tax reform. The reduction (excess accumulated deferred taxes, or “EADIT”) became a regulatory liability with tax gross-up. EADIT reduces rate base, and is expected to be returned to utility customers in accordance with IRS and regulatory requirements. Included generally, in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets, except for $15 million in 2020 which is in Other current liabilities. Amount also includes difference in current taxes required to be returned to customers and a separate $2.6 million gross-up related to contributions in aid of construction.

(8)Regulatory recovery occurs on a one-year lag basis through the labor loading process. Included in Prepaid and other current assets on the Consolidated Balance Sheets.

(9)Margin tracking/decoupling mechanisms are alternative revenue programs; revenue associated with under-collections (for the difference between authorized margin levels and amounts billed to customers through rates currently) is recognized as revenue so long as

recovery is expected to take place within 24 months. Total category asset balances are included in Prepaid and other current assets and Deferred charges and other assets on the Consolidated Balance Sheets. Total category liability balances are included in Other current liabilities and Other deferred credits and other long-term liabilities.

(10)The following tables detail the components of Other regulatory assets and liabilities. Other regulatory assets are included in either Prepaid and other current assets or Deferred charges and other assets on the Consolidated Balance Sheets (as indicated). Recovery periods vary. Other regulatory liabilities are included in either Other current liabilities or Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets (as indicated).


	December 31,
(Thousands of dollars)	2020		2019
Other Regulatory Assets:
State mandated public purpose programs (including low income and conservation programs) (a) (e)	$	11,527	$	9,172
Infrastructure replacement programs and similar (b) (e)	7,731		8,236
Environmental compliance programs (c) (e)	5,235		5,768
Pension tracking mechanism (b)	3,075		—
Other (d)	18,028		9,958
	$	45,596	$	33,134

a)Included in Prepaid and other current assets on the Consolidated Balance Sheets.

b)Included in Deferred charges and other assets on the Consolidated Balance Sheets.

c)In 2020, approximately $4.2 million of these balances included in Prepaid and other current assets and $998,000 included in Deferred charges and other assets on the Consolidated Balance Sheets. In 2019, approximately $5.0 million included in Prepaid and other current assets and $782,000 included in Deferred charges and other assets on the Consolidated Balance Sheets.

d)In 2020, $3.6 million included in Prepaid and other current assets and $14.4 million included in Deferred charges and other assets on the Consolidated Balance Sheets. In 2019, $1.6 million included in Prepaid and other current assets and $8.3 million included in Deferred charges and other assets on the Consolidated Balance Sheets.

e)Balance recovered or refunded on an ongoing basis, generally with interest.


	December 31,
(Thousands of dollars)	2020		2019
Other Regulatory Liabilities:
State mandated public purpose programs (including low income and conservation programs) (a) (c)	$	(834)	$	(308)
Environmental compliance programs (c) (d)	(405)		(527)
Pension tracking mechanism (b)	—		(2,476)
Other (b) (c)	(2,264)		(1,127)
	$	(3,503)	$	(4,438)

a)Included in Other current liabilities, except for $339,000 which is included in Other deferred credits and long-term liabilities on the Consolidated Balance Sheets.

b)Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.

c)Balance typically recovered or refunded on an ongoing basis, generally with interest.

d)In 2020 and 2019, included in Other current liabilities on the Consolidated Balance Sheet.

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI")

12 Months Ended

Dec. 31, 2020

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI")

Note 6 - Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI")

The following information provides insight into amounts impacting the Company’s Other comprehensive income (loss), both before and after-tax impacts, within the Consolidated Statements of Comprehensive Income, which also impact Accumulated other comprehensive income (“AOCI”) in the Consolidated Balance Sheets and the Consolidated Statements of Equity.

Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss)


	Year Ended December 31,
	2020						2019						2018
(Thousands of dollars)	Before- Tax Amount		Tax (Expense) or Benefit (1)		Net-of- Tax Amount		Before- Tax Amount		Tax (Expense) or Benefit (1)		Net-of- Tax Amount		Before- Tax Amount		Tax (Expense) or Benefit (1)		Net-of- Tax Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(57,539)	$	13,809	$	(43,730)	$	(71,087)	$	17,061	$	(54,026)	$	(20,426)	$	4,902	$	(15,524)
Amortization of prior service cost	1,155		(277)		878		1,271		(305)		966		1,335		(320)		1,015
Amortization of net actuarial (gain)/loss	37,830		(9,079)		28,751		23,376		(5,610)		17,766		33,617		(8,068)		25,549
Prior service cost	—		—		—		(1,878)		452		(1,426)		—		—		—
Regulatory adjustment	7,435		(1,785)		5,650		36,944		(8,867)		28,077		(8,233)		1,976		(6,257)
Pension plans other comprehensive income (loss)	(11,119)		2,668		(8,451)		(11,374)		2,731		(8,643)		6,293		(1,510)		4,783
FSIRS (designated hedging activities):
Amounts reclassified into net income	3,247		(780)		2,467		3,344		(803)		2,541		3,345		(804)		2,541
FSIRS other comprehensive income (loss)	3,247		(780)		2,467		3,344		(803)		2,541		3,345		(804)		2,541
Total other comprehensive income (loss) –Southwest Gas Corporation	(7,872)		1,888		(5,984)		(8,030)		1,928		(6,102)		9,638		(2,314)		7,324
Foreign currency translation adjustments:
Translation adjustments	1,713		—		1,713		2,038		—		2,038		(3,010)		—		(3,010)
Foreign currency other comprehensive income (loss)	1,713		—		1,713		2,038		—		2,038		(3,010)		—		(3,010)
Total other comprehensive income (loss) – Southwest Gas Holdings, Inc.	$	(6,159)	$	1,888	$	(4,271)	$	(5,992)	$	1,928	$	(4,064)	$	6,628	$	(2,314)	$	4,314

(1) Tax amounts are calculated using a 24% rate. With regard to foreign currency translation adjustments, the Company has elected to indefinitely reinvest the earnings of Centuri’s Canadian subsidiaries in Canada, thus preventing deferred taxes on such earnings. As a result of this assertion, and no repatriation of earnings anticipated, the Company is not recognizing a tax effect or presenting a tax expense or benefit for currency translation adjustments in Other comprehensive income (loss).

Approximately $1.7 million of realized losses (net of tax) related to the remaining balance of FSIRS, included in AOCI at December 31, 2020, will be reclassified into interest expense within the next twelve months as the related interest payments on long-term debt occur.

The following table represents a rollforward of AOCI, presented on the Company’s Consolidated Balance Sheets and its Consolidated Statements of Equity:


	Defined Benefit Plans						FSIRS						Foreign Currency Items
(Thousands of dollars)	Before-Tax		Tax (Expense) Benefit (4)		After-Tax		Before- Tax		Tax (Expense) Benefit (4)		After-Tax		Before- Tax		Tax (Expense) Benefit		After-Tax		AOCI
Beginning Balance AOCI December 31, 2019	$	(66,601)	$	15,985	$	(50,616)	$	(5,966)	$	1,431	$	(4,535)	$	(1,581)	$	—	$	(1,581)	$	(56,732)
Net actuarial gain/(loss)	(57,539)		13,809		(43,730)		—		—		—		—		—		—		(43,730)
Translation adjustments	—		—		—		—		—		—		1,713		—		1,713		1,713
Other comprehensive income before reclassifications	(57,539)		13,809		(43,730)		—		—		—		1,713		—		1,713		(42,017)
FSIRS amounts reclassified from AOCI (1)	—		—		—		3,247		(780)		2,467		—		—		—		2,467
Amortization of prior service cost (2)	1,155		(277)		878		—		—		—		—		—		—		878
Amortization of net actuarial loss (2)	37,830		(9,079)		28,751		—		—		—		—		—		—		28,751

Regulatory adjustment (3)	7,435		(1,785)		5,650		—		—		—				—		—		5,650
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.	(11,119)		2,668		(8,451)		3,247		(780)		2,467		1,713		—		1,713		(4,271)
Ending Balance AOCI December 31, 2020	$	(77,720)	$	18,653	$	(59,067)	$	(2,719)	$	651	$	(2,068)	$	132	$	—	$	132	$	(61,003)

(1)The FSIRS reclassification amounts are included in Net interest deductions on the Company’s Consolidated Statements of Income.

(2)These AOCI components are included in the computation of net periodic benefit cost (see Note 11 - Pension and Other Postretirement Benefits for additional details).

(3)The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in Deferred charges and other assets on the Company’s Consolidated Balance Sheets).

(4) Tax amounts are calculated using a 24% rate.

The following table represents a rollforward of AOCI, presented on Southwest’s Consolidated Balance Sheets:


	Defined Benefit Plans						FSIRS
(Thousands of dollars)	Before-Tax		Tax (Expense) Benefit (9)		After- Tax		Before- Tax		Tax (Expense) Benefit (9)		After- Tax		AOCI
Beginning Balance AOCI December 31, 2019	$	(66,601)	$	15,985	$	(50,616)	$	(5,966)	$	1,431	$	(4,535)	$	(55,151)
Net actuarial gain/(loss)	(57,539)		13,809		(43,730)		—		—		—		(43,730)
Other comprehensive loss before reclassifications	(57,539)		13,809		(43,730)		—		—		—		(43,730)
FSIRS amounts reclassified from AOCI (6)	—		—		—		3,247		(780)		2,467		2,467
Amortization of prior service cost (7)	1,155		(277)		878		—		—		—		878
Amortization of net actuarial loss (7)	37,830		(9,079)		28,751		—		—		—		28,751

Regulatory adjustment (8)	7,435		(1,785)		5,650		—		—		—		5,650
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation	(11,119)		2,668		(8,451)		3,247		(780)		2,467		(5,984)
Ending Balance AOCI December 31, 2020	$	(77,720)	$	18,653	$	(59,067)	$	(2,719)	$	651	$	(2,068)	$	(61,135)

(6) The FSIRS reclassification amounts are included in Net interest deductions on Southwest’s Consolidated Statements of Income.

(7) These AOCI components are included in the computation of net periodic benefit cost (see Note 11 - Pension and Other Postretirement Benefits for additional details).

(8) The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included Deferred charges and other assets on Southwest’s Consolidated Balance Sheets).

(9) Tax amounts are calculated using a 24% rate.

The following table represents amounts (before income tax impacts) included in AOCI (in the tables above), that have not yet been recognized in net periodic benefit cost:


	Year Ended December 31,
(Thousands of dollars)	2020		2019
Net actuarial loss	$	(502,783)	$	(483,074)
Prior service cost	(2,487)		(3,641)
Less: amount recognized in regulatory assets	427,550		420,114
Recognized in AOCI	$	(77,720)	$	(66,601)

See Note 11 - Pension and Other Postretirement Benefits for more information on the defined benefit pension plans and Note 1 - Background, Organization, and Summary of Significant Accounting Policies for more information on the FSIRS.

Common Stock

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Common Stock

Note 7 - Common Stock

Only shares of the Company’s common stock are publicly traded on the New York Stock Exchange, under the ticker symbol “SWX.” Share-based compensation related to Southwest and Centuri is based on awards to be issued in shares of Southwest Gas Holdings, Inc.

In May 2019, the Company filed with the SEC an automatic shelf registration statement on Form S-3 (File No. 333-231297), which became effective upon filing, for the offer and sale of up to $300 million of common stock from time to time in at-the-market offerings under the prospectus included therein and in accordance with the Sales Agency Agreement, dated May 2019, between the Company and BNY Mellon Capital Markets, LLC (the “Equity Shelf Program”). The following table provides the activity in the Equity Shelf Program for the three-month and life-to-date periods ended December 31, 2020:


	Three Months Ended		Life-To-Date Ended
	December 31, 2020
Gross proceeds	$	46,425,623	$	253,551,490
Less: agent commissions	(464,256)		(2,535,515)
Net proceeds	$	45,961,367	$	251,015,975
Number of shares sold	691,081		3,396,457
Weighted average price per share	$	67.18	$	74.65

As of December 31, 2020, the Company had up to $46,448,510 of common stock available for future issuance under the program. Net proceeds from the sale of shares of common stock under the Equity Shelf Programs are intended for general corporate purposes, including the acquisition of property for the construction, completion, extension, or improvement of pipeline systems and facilities located in and around the communities served by Southwest. Net proceeds during the twelve months ended December 31, 2020 were contributed to, and reflected in the records of, Southwest (as a capital contribution from Southwest Gas Holdings, Inc.).

Aside from the equity shelf registration, in December 2020, the Company and Southwest jointly filed with the SEC an automatic shelf registration statement (File No. 333-251074), or a “Universal Shelf,” which became effective upon filing and includes a prospectus detailing the Company’s ability to offer and sell, from time to time in amounts at prices and on terms that will be determined at the time of such offering, any combination of common stock, preferred stock, debt securities (which may or may not be guaranteed by one or more of its directly or indirectly wholly owned subsidiaries if indicated in the relevant prospectus supplement), guarantees of debt securities issued by Southwest, depository shares, warrants to purchase common stock, preferred stock or depository shares issued by the Company or debt securities issued by the Company or Southwest, units and rights. Additionally as part of the Universal Shelf, Southwest may offer and sell, from time to time in amounts at prices and on terms that will be determined at the time of such offering, any combination of debt securities (which may or may not be guaranteed by one or more of its directly or indirectly wholly owned subsidiaries if indicated in the relevant prospectus supplement) and guarantees of debt securities issued by the Company or by one or more of its directly or indirectly wholly owned subsidiaries if indicated in the relevant prospectus supplement.

During 2020, the Company issued approximately 96,000 shares of common stock through the Restricted Stock/Unit Plan, Omnibus Incentive Plan, and Management Incentive Plan.

Additionally during 2020, the Company issued 172,000 shares of common stock through the Dividend Reinvestment and Stock Purchase Plan, raising proceeds of approximately $11 million.

As of December 31, 2020, there were 4.5 million shares of common stock registered and available for issuance under the provisions of the various stock issuance plans, which does not include the amount of common stock available that is separately disclosed with respect to the Equity Shelf Program above.

Debt

12 Months Ended

Dec. 31, 2020

Debt Disclosure [Abstract]

Debt

Note 8 - Debt

Long-Term Debt

Long-term debt is recognized in the Company’s and Southwest’s Consolidated Balance Sheets generally at the carrying value of the obligations outstanding. However, details surrounding the fair value and individual carrying values of instruments are discussed below and provided in the table that follows.

The fair values of Southwest’s revolving credit facility (including commercial paper) and the variable-rate Industrial Development Revenue Bonds (“IDRBs”) approximate their carrying values. The fair values of the revolving credit facility and IDRBs are categorized as Level 1 based on the FASB’s fair value hierarchy, due to Southwest’s ability to access similar debt arrangements at measurement dates with comparable terms, including variable/market rates. Additionally, the borrowings by Southwest under the revolving credit facility are generally repaid quickly, and the IDRBs have interest rates that reset frequently.

The fair values of Southwest’s debentures (which include senior and medium-term notes) were determined utilizing a market-based valuation approach, where fair values are determined based on evaluated pricing data, such as broker quotes and yields for similar securities adjusted for observable differences. Significant inputs used in the valuation generally include benchmark yield curves, credit ratings, and issuer spreads. The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable. The fair values of debentures are categorized as Level 2 in the hierarchy.

The Centuri secured revolving credit and term loan facility and Centuri’s other debt obligations (not actively traded) are categorized as Level 3. Because Centuri’s debt is not publicly traded, fair values for the secured revolving credit and term loan facility and its other debt obligations were based on a conventional discounted cash flow methodology and utilizing current market pricing yield curves, across Centuri’s debt maturity spectrum, of other industrial bonds with an assumed credit rating comparable to the Company’s.

Carrying amounts of long-term debt and related estimated fair values as of December 31, 2020 and 2019 are disclosed in the following table. The fair value hierarchy is described in Note 1 - Background, Organization, and Summary of Significant Accounting Policies.


	December 31,
	2020				2019
	Carrying Amount		Fair Value		Carrying Amount		Fair Value
(Thousands of dollars)
Southwest Gas Corporation:
Debentures:
Notes, 4.45%, due 2020	$	—	$	—	$	125,000	$	126,673
Notes, 6.1%, due 2041	125,000		174,858		125,000		162,666
Notes, 3.875%, due 2022	250,000		258,825		250,000		258,550
Notes, 4.875%, due 2043	250,000		317,190		250,000		291,928
Notes, 3.8%, due 2046	300,000		347,046		300,000		308,307
Notes, 3.7%, due 2028	300,000		344,553		300,000		320,685
Notes, 4.15%, due 2049	300,000		370,278		300,000		330,138
Notes, 2.2%, due 2030	450,000		474,552		—		—
8% Series, due 2026	75,000		99,723		75,000		96,905
Medium-term notes, 7.78% series, due 2022	25,000		26,663		25,000		27,500
Medium-term notes, 7.92% series, due 2027	25,000		33,802		25,000		32,543
Medium-term notes, 6.76% series, due 2027	7,500		9,613		7,500		9,156
Unamortized discount and debt issuance costs	(17,822)				(14,450)
	2,089,678				1,768,050
Revolving credit facility and commercial paper	150,000		150,000		150,000		150,000
Industrial development revenue bonds:
Tax-exempt Series A, due 2028	50,000		50,000		50,000		50,000
2003 Series A, due 2038	50,000		50,000		50,000		50,000
2008 Series A, due 2038	50,000		50,000		50,000		50,000
2009 Series A, due 2039	50,000		50,000		50,000		50,000
Unamortized discount and debt issuance costs	(1,472)				(1,717)
	198,528				198,283
Less: current maturities	—				(125,000)
Long-term debt, less current maturities – Southwest Gas Corporation	$	2,438,206			$	1,991,333
Centuri:
Centuri term loan facility	$	226,648	230,824		$	244,812	252,182
Unamortized debt issuance costs	(820)				(1,101)
	225,828				243,711
Centuri secured revolving credit facility	26,626		26,645		60,021		60,057
Centuri other debt obligations	81,973		84,246		43,929		44,787
Less: current maturities	(40,433)				(38,512)
Long-term debt, less current maturities – Centuri	$	293,994			$	309,149
Consolidated Southwest Gas Holdings, Inc.:
Southwest Gas Corporation long-term debt	$	2,438,206			$	2,116,333
Centuri long-term debt	334,427				347,661
Less: current maturities	(40,433)				(163,512)
Long-term debt, less current maturities – Southwest Gas Holdings, Inc.	$	2,732,200			$	2,300,482

Southwest has a $400 million credit facility, for which it has designated $150 million of associated capacity as long-term debt and the remaining $250 million for working capital purposes. Interest rates for the credit facility are calculated at either LIBOR or an “alternate base rate,” plus in each case an applicable margin that is determined based on Southwest’s senior unsecured debt rating. At December 31, 2020, $150 million was outstanding on the long-term portion (including $50 million under the commercial paper program discussed below). The effective interest rate on the long-term portion of the credit facility was 0.92% at December 31, 2020. Borrowings under the credit facility ranged from none at various times throughout 2020 to a high of $344 million during the first quarter of 2020.

On April 10, 2020, Southwest amended its credit facility agreement; total borrowing capacity under the amended agreement remains at $400 million. The amended agreement extended the maturity date from March 2022 to April 2025. Under the amended agreement, the applicable margin ranges from 0.750% to 1.500% for loans bearing interest with reference to LIBOR and from 0.000% to 0.500% for loans bearing interest with reference to an alternate base rate. At December 31, 2020, the applicable margin is 1% for loans bearing interest with reference to LIBOR and 0% for loans bearing interest with reference to the alternative base rate. Upon the occurrence of certain events providing for a transition away from LIBOR, or if LIBOR is no longer a widely recognized benchmark rate, Southwest may further amend the credit facility with a replacement rate as set forth in the amended agreement. Southwest is also required to pay a commitment fee on the unfunded portion of the commitments based on its senior unsecured long-term debt rating. The commitment fee on the unfunded portion of the commitments ranges from 0.075% to 0.200% per annum, and was not significant for the year ended December 31, 2020. The amended agreement contains certain representations and warranties and affirmative and negative covenants similar to those contained in the previous agreement. In addition, the amended agreement contains a financial covenant requiring Southwest to maintain a ratio of funded debt to total capitalization not to exceed 0.70 to 1.00 as of the end of any quarter of any fiscal year.

Southwest has a $50 million commercial paper program. Issuances under the commercial paper program are supported by Southwest’s current revolving credit facility and, therefore, do not represent additional borrowing capacity. Borrowings under the commercial paper program are designated as long-term debt. Interest rates for the program are calculated at the then current commercial paper rate. At December 31, 2020, as noted above, $50 million of borrowings were outstanding under the commercial paper program.

In June 2020, Southwest issued $450 million aggregate principal amount of 2.20% Senior Notes at a discount of 0.126%. The notes will mature in June 2030. A portion of the net proceeds was used to reduce borrowings under Southwest’s credit facility and to redeem the 4.45% $125 million notes due in December 2020, which were redeemed in September 2020 after Southwest provided advance notice to the holders of its intention to redeem the notes in full at a redemption price of 100% plus accrued and unpaid interest.

Centuri has a $590 million senior secured revolving credit and term loan facility, scheduled to expire in November 2023. The capacity of the line of credit portion of the facility is $325 million; related amounts borrowed and repaid are available to be re-borrowed. The term loan portion of the facility has a limit of approximately $265 million; amounts borrowed and repaid under this portion of the facility are not able to be re-borrowed. It is secured by substantially all of Centuri’s assets except those explicitly excluded under the terms of the agreement (including owned real estate and certain certificated vehicles). Centuri’s assets securing the facility at December 31, 2020 totaled $1.4 billion. At December 31, 2020, $253 million in borrowings were outstanding under Centuri’s combined facility. During 2020, Centuri also received proceeds of $70 million in equipment loans.

Interest rates for Centuri’s $590 million secured revolving credit and term loan facility are calculated at LIBOR, the Canadian Dealer Offered Rate (“CDOR”), or an alternate base rate or Canadian base rate, plus in each case an applicable margin that is determined based on Centuri’s consolidated leverage ratio. The applicable margin ranges from 0.875% to 2.25% for loans bearing interest with reference to LIBOR or CDOR and from 0.00% to 1.25% for loans bearing interest with reference to the alternate base rate or Canadian base rate. Centuri is also required to pay a commitment fee on the unfunded portion of the commitments based on their consolidated leverage ratio. The commitment fee ranges from 0.125% to 0.35% per annum. Borrowings under the secured revolving credit portion of the facility ranged from a low of $27 million during the fourth quarter of 2020 to a high of $77 million during the third quarter of 2020.

All amounts outstanding are considered long-term borrowings. The effective interest rate on the secured revolving credit and term loan facility was 1.3% at December 31, 2020.

It is currently anticipated that LIBOR may be discontinued as a benchmark or reference rate after 2021. As of December 31, 2020, $50 million of outstanding borrowings for the holding company under its credit facility (see Short-term Debt below), $157 million of Southwest’s outstanding borrowings under its credit facility (other than from its commercial paper program), and $145 million of Centuri’s outstanding borrowings under its combined facility have interest rates with reference to LIBOR and maturity dates that extend beyond 2021. The outstanding amounts reflect approximately 6% of Southwest’s total debt and 12% of total debt (including current maturities) for the Company overall. Southwest and Southwest Gas Holdings, Inc., in accordance with the April 2020 amendments to their respective facilities, may make further amendments with replacement rates

if LIBOR is discontinued. However, replacement rates are not currently determinable. In order to mitigate the impact of the discontinuance on the Company’s and Southwest’s financial condition and results of operations, management will continue to monitor developments and work with lenders to determine the appropriate replacement/alternative reference rate for variable rate debt. At this time the Company and Southwest can provide no assurances as to the impact a LIBOR discontinuance will have on their financial condition or results of operations. Any alternative rate may be less predictable or less attractive than LIBOR.

The effective interest rates on Southwest’s variable-rate IDRBs are included in the table below:


	December 31,
	2020		2019
2003 Series A	0.80	%	2.51	%
2008 Series A	0.83	%	2.46	%
2009 Series A	0.76	%	2.37	%
Tax-exempt Series A	0.87	%	2.32	%

In Nevada, interest fluctuations due to changing interest rates on Southwest’s 2003 Series A, 2008 Series A, and 2009 Series A variable-rate IDRBs are tracked and recovered from customers through a variable interest expense recovery mechanism.

None of Southwest’s debt instruments have credit triggers or other clauses that result in default if bond ratings are lowered by rating agencies. Interest and fees on certain debt instruments are subject to adjustment depending on Southwest’s bond ratings. Certain debt instruments are subject to a leverage ratio cap and the 6.1% Notes due 2041 are also subject to a minimum net worth requirement. At December 31, 2020, Southwest was in compliance with all of its covenants. Under the most restrictive of the financial covenants, approximately $2.7 billion in additional debt could be issued while still meeting the leverage ratio requirement. Relating to the minimum net worth requirement, as of December 31, 2020, there is at least $1.7 billion of cushion in equity. No specific dividend restrictions exist under the collective covenants. None of the debt instruments contain material adverse change clauses.

Certain Centuri debt instruments have leverage ratio caps and fixed charge ratio coverage requirements. At December 31, 2020, Centuri was in compliance with all of its covenants. Under the most restrictive of the covenants, Centuri could issue over $318 million in additional debt and meet the leverage ratio requirement. Centuri has at least $115 million of cushion relating to the minimum fixed charge ratio coverage requirement. Centuri’s covenants limit its ability to provide cash dividends to Southwest Gas Holdings, Inc., its parent. The dividend restriction is equal to a maximum of 60% of its rolling twelve-month consolidated net income.

Estimated maturities of long-term debt for the next five years are:


(Thousands of dollars)	Southwest		Centuri		Total
2021	$	—	$	40,433	$	40,433
2022	275,000		45,433		320,433
2023	—		219,638		219,638
2024	—		11,284		11,284
2025	150,000		9,076		159,076

Short-Term Debt

Southwest Gas Holdings, Inc. has a $100 million credit facility that is primarily used for short-term financing needs. Interest rates for this facility are calculated at either LIBOR or the “alternate base rate,” plus in each case an applicable margin that is determined based on the Company’s senior unsecured debt rating. Borrowings under the credit facility ranged from a low of $17 million during the first quarter of 2020 to a high of $67 million during the first quarter of 2020. There was $50 million and $17 million outstanding under this facility with a weighted average interest rate of 1.225% and 2.749% at December 31, 2020 and 2019, respectively.

Similar to Southwest amending its credit facility agreement, on April 10, 2020, Southwest Gas Holdings, Inc. also amended its existing credit facility, extending the maturity date to April 2025. The revolving borrowing capacity under the amended agreement remained at $100 million, the same as before the amendment. Interest rate benchmarks (LIBOR or an alternative) as well as related ranges, including with regard to the applicable margin, largely mirror those included in Southwest’s amended facility agreement noted above, determined in this case based on Southwest Gas Holdings, Inc.’s senior unsecured long-term debt rating. At December 31, 2020, the applicable margin is 1.125% for loans bearing interest with reference to LIBOR and 0.125% for loans bearing interest with reference to the alternative base rate. Similar to the Southwest facility amendment, upon the occurrence of certain events providing for a transition away from LIBOR, or if LIBOR is no longer a widely recognized

benchmark rate, Southwest Gas Holdings, Inc. may amend its credit facility agreement with a replacement rate, as set forth in the amended agreement. The commitment fee rates, terms, and covenants, noted above for Southwest are also applicable to Southwest Gas Holdings, Inc. in its amended credit facility, including the noted ratio of funded debt to total capitalization as of the end of any quarter of any fiscal year. The commitment fee under this credit facility was not significant for the year ended December 31, 2020.

At December 31, 2020, Southwest Holdings, Inc. was in compliance with all of its credit facility covenants. Interest and fees on the credit facility are subject to adjustment depending on its bond ratings. The credit facility is subject to a leverage ratio cap. No specific dividend restrictions exist under the collective covenants. The credit facility does not contain a material adverse change clause.

As indicated above, under Southwest’s $400 million credit facility, of which $250 million has been designated by management for working capital purposes. Southwest had $57 million and $194 million of short-term borrowings outstanding with weighted average interest rates of 1.10% and 2.61%, at December 31, 2020 and 2019, respectively.

Share-Based Compensation

12 Months Ended

Dec. 31, 2020

Share-Based Compensation

Note 9 - Share-Based Compensation

At December 31, 2020, the following share-based compensation plans existed at the Company: an omnibus incentive plan and a restricted stock/unit plan. All share grants in 2020, including time-lapse restricted stock units and performance shares, occurred under the omnibus incentive plan. The table below shows total share-based plan compensation expense which was recognized in the Consolidated Statements of Income:


	Year Ended December 31,
(Thousands of dollars)	2020		2019		2018
Share-based compensation plan expense, net of related tax benefits	$	4,816	$	5,154	$	4,644
Share-based compensation plan related tax benefits	1,521		1,627		1,467

Omnibus Incentive Plan

The omnibus incentive plan is used to promote the long-term growth and profitability of the Company by providing directors, employees, and certain other individuals with incentives to increase stockholder value and otherwise contribute to the success of the Company. In addition, the plan enables the Company to attract, retain, and reward the best available persons for positions of responsibility. The omnibus incentive plan provides for the grant of stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares, and other equity-based and cash awards. Employees, directors, and consultants who provide services to the Company or any subsidiary may be eligible under this plan. For grants under the omnibus incentive plan, directors continue to immediately vest in the shares upon grant but are provided the option to defer receipt of equity compensation until they leave the Board of Directors.

Performance-based incentive opportunities under the omnibus plan were granted to all officers of Southwest in the form of performance shares and are based, depending on the officer, on consolidated earnings per share, utility net income, and utility return on equity, with an adjustment based on relative total shareholder return, in each case, measured over a three-year performance period. Southwest recorded $2.8 million, $2.3 million, and $2.1 million of estimated compensation expense associated with these shares during 2020, 2019, and 2018, respectively.

Restricted Stock/Unit Plan

Restricted stock/units under the restricted stock/unit plan were granted to attract, motivate, retain, and reward key employees of the Company with an incentive to attain high levels of individual performance and improved financial performance. As noted above, grants of restricted stock during 2020 occurred under the omnibus incentive plan. The restricted stock/units vest 40% at the end of year one and 30% at the end of years two and three and are issued annually as common stock in accordance with the percentage vested. The restricted stock/unit plan was also established to attract, motivate, and retain experienced and knowledgeable independent directors. Vesting for grants of restricted stock/units to directors occurred immediately upon grant. The issuance of common stock for directors occurs when their service on the Board ends. No new grants are made under the legacy restricted stock/unit plan as all future incentive compensation, including restricted stock, is granted under programs of the omnibus incentive plan, which subject to advance election, provides that issuance to directors may occur upon grant. With regard to management, grants of time-lapse restricted stock under the omnibus plan vest based on the same percentages indicated above under the legacy program.

Management Incentive Plan

Under the management incentive plan, awards were historically granted to encourage key employees of the Company to remain as employees and to achieve short-term and long-term performance goals. Plan participants were eligible to receive a cash bonus (i.e., short-term incentive) and a portion in shares (i.e., long-term incentive). The share grants vested three years after grant and were then issued as common stock. No new share grants are made under the management incentive plan as all future incentive share compensation is granted under the omnibus incentive plan. The remaining shares vesting under the management incentive plan were issued during the first quarter of 2020.

The following table summarizes the activity of the management incentive plan shares and restricted stock/units as of December 31, 2020:


(thousands of shares)	Management Incentive Plan Shares	Weighted- average grant date fair value		Restricted Stock/ Units (1)	Weighted- average grant date fair value
Nonvested/unissued at December 31, 2019	29	$	79.16	365	$	60.94
Granted	—	—		129	76.85
Dividends	—			8
Forfeited or expired	—	—		—	—
Vested and issued (2)	(29)	79.16		(101)	71.06
Nonvested/unissued at December 31, 2020	—	$	—	401	$	62.23

(1)The number of performance shares includes 77,400 granted and 35,500 vested and issued, which was derived by assuming that target performance will be achieved during the relevant performance period.

(2)Includes shares for retiree payouts and those converted for taxes.

The weighted average grant date fair value of all restricted stock/units granted in 2019 and 2018 was $81.75 and $69.16, respectively.

As of December 31, 2020, total compensation cost related to all nonvested restricted stock/units not yet recognized is $4.5 million, which is expected to be recognized over a weighted average period of 1.7 years.

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Note 10 - Commitments and Contingencies The Company and Southwest are defendants in miscellaneous legal proceedings. The Company and Southwest are also parties to various regulatory proceedings. The ultimate dispositions of these proceedings are not presently determinable; however, it is the opinion of management that no litigation or regulatory proceeding to which the Company and Southwest are currently subject will have a material adverse impact on their financial position, results of operations, or cash flows. Southwest maintains liability insurance for various risks associated with the operation of its natural gas pipelines and facilities. In connection with these liability insurance policies, Southwest is responsible for an initial deductible or self-insured retention amount per incident, after which the insurance carriers would be responsible for amounts up to the policy limits. For the policy year August 2020 to July 2021, these liability insurance policies require Southwest to be responsible for the first $1 million (self-insured retention) of each incident plus the first $4 million in aggregate claims above its self-insured retention in the policy year. Centuri maintains liability insurance for various risks associated with its operations. In connection with these liability insurance policies, Centuri is responsible for an initial deductible or self-insured retention amount per occurrence, after which the insurance carriers would be responsible for amounts up to the policy limits. For the policy year May 2020 to April 2021, Centuri is responsible for the first $750,000 (self-insured retention) per occurrence under these liability insurance policies. Through an assessment process, the Company and Southwest may determine that certain costs are likely to be incurred in the future related to specific legal matters. In these circumstances and in accordance with accounting policies, the Company and Southwest will make an accrual, as necessary.

Pension and Other Postretirement Benefits

12 Months Ended

Dec. 31, 2020

Retirement Benefits [Abstract]

Pension and Other Postretirement Benefits

Note 11 - Pension and Other Postretirement Benefits

Southwest Gas Corporation

Employees’ Investment Plan

An Employees’ Investment Plan (“EIP”) is offered to eligible employees of Southwest through deduction of a percentage of base compensation, subject to IRS limitations. The EIP provides for purchases of various mutual fund investments and Company common stock. One-half of amounts deferred by employees are matched, up to a maximum matching contribution of 3.5% of an employee’s annual compensation. There are no employer matching contributions for officer deferrals into the EIP.

Contributions to the plan by Southwest were $5.9 million, $5.7 million, and $5.5 million for 2020, 2019, and 2018, respectively.

Deferred Compensation Plan

A deferred compensation plan is offered to all officers of Southwest and a separate deferred compensation plan is offered to members of the Company’s Board of Directors. The plans provide the opportunity to defer up to 100% of annual cash compensation. One-half of amounts deferred by officers are matched, up to a maximum matching contribution of 3.5% of an officer’s annual base salary. Upon retirement, payments of compensation deferred, plus interest, are made in equal monthly installments over 10, 15, or 20 years, as elected by the participant. Directors have an additional option to receive such payments over a five-year period. Deferred compensation earns interest at a rate determined each January. The interest rate equals 150% of Moody’s Seasoned Corporate Bond Rate Index.

Pension and Postretirement Plans

A noncontributory qualified retirement plan with defined benefits covering substantially all Southwest employees is available, in addition to a separate unfunded supplemental executive retirement plan (“SERP”), which is limited to Southwest’s officers. Postretirement benefits other than pensions (“PBOP”) are provided to qualified retirees for health care, dental, and life insurance benefits.

The overfunded or underfunded positions of defined benefit postretirement plans, including pension plans, are recognized in the Consolidated Balance Sheets. Any actuarial gains and losses, prior service costs, and transition assets or obligations are recognized in Accumulated other comprehensive income under Stockholders’ equity, net of tax, until they are amortized as a component of net periodic benefit cost.

A regulatory asset has been established for the portion of the total amounts otherwise chargeable to Accumulated other comprehensive income that are expected to be recovered through rates in future periods. Changes in actuarial gains and losses and prior service costs pertaining to the regulatory asset will be recognized as an adjustment to the regulatory asset account as these amounts are amortized and recognized as components of net periodic pension costs each year.

The qualified retirement plan invests the majority of its plan assets in common collective trusts, which include a well-diversified portfolio of domestic and international equity securities and fixed income securities, and which are managed by a professional investment manager appointed by Southwest. The investment manager has full discretionary authority to direct the investment of plan assets held in trust within the specific guidelines prescribed by Southwest through the plan’s investment policy statement. In 2016, Southwest adopted a liability driven investment (“LDI”) strategy for part of the portfolio, a form of investing designed to better match the movement in pension plan assets with the impact of interest rate changes and inflation assumption changes on the pension plan liability. The implementation of the LDI strategy will be phased in over time by using a glide path. The glide path is designed to increase the allocation of the plan’s assets to fixed income securities, as the funded status of the plan increases, in order to more closely match the duration of the plan assets to that of the plan liability. Pension plan assets are held in a Master Trust. The pension plan funding policy is in compliance with the federal government’s funding requirements.

Pension costs for these plans are affected by the amount and timing of cash contributions to the plans, the return on plan assets, discount rates, and by employee demographics, including age, compensation, and length of service. Changes made to the provisions of the plans may also impact current and future pension costs. Actuarial formulas are used in the determination of pension costs and are affected by actual plan experience and assumptions about future experience. Key actuarial assumptions include the expected return on plan assets, the discount rate used in determining the projected benefit obligation and pension costs, and the assumed rate of increase in employee compensation. Relatively small changes in these assumptions, particularly the discount rate, may significantly affect pension costs and plan obligations for the qualified retirement plan. In determining the discount rate, management matches the plan’s projected cash flows to a spot-rate yield curve based on highly rated corporate bonds. Changes to the discount rate from year-to-year, if any, are generally made in increments of 25 basis points.

Due to an historically low interest rate environment, there was a 75 basis points decrease in the discount rate between years, as reflected below. This decrease in the discount rate was the most significant contributor to the actuarial loss for the qualified retirement plan, SERP, and PBOP benefit obligations as of December 31, 2020. The methodology utilized to determine the discount rate was consistent with prior years. The weighted-average rate of compensation increase was lowered from the prior year by 25 basis points. The asset return assumption (which impacts the following year’s expense) was also lowered by 25 basis points. The rates are presented in the table below:


	December 31,
	2020		2019
Discount rate	2.75	%	3.50	%
Weighted-average rate of compensation increase	3.00	%	3.25	%
Asset return assumption	6.50	%	6.75	%

Future years’ expense level movements (up or down) will continue to be greatly influenced by long-term interest rates, asset returns, and funding levels.

The following table sets forth the retirement plan, SERP, and PBOP funded statuses and amounts recognized on the Consolidated Balance Sheets and Consolidated Statements of Income.


	Year Ended December 31,
	2020									2019
(Thousands of dollars)	Qualified Retirement Plan			SERP			PBOP			Qualified Retirement Plan			SERP			PBOP
Change in benefit obligations:
Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$	1,329,577		$	47,397		$	76,111		$	1,116,014		$	40,603		$	69,956
Service cost	34,299			389			1,581			25,864			266			1,276
Interest cost	45,555			1,604			2,582			49,006			1,760			3,046
Plan amendments	—			—			—			—			—			1,878
Actuarial loss (gain)	145,440			7,240			6,547			192,416			7,974			3,156
Benefits paid	(55,632)			(2,999)			(4,616)			(53,723)			(3,206)			(3,201)
Benefit obligation at end of year (PBO/PBO/APBO)	1,499,239			53,631			82,205			1,329,577			47,397			76,111
Change in plan assets:
Market value of plan assets at beginning of year	974,993			—			52,838			790,614			—			47,341
Actual return on plan assets	165,072			—			5,320			186,102			—			9,757
Employer contributions	102,000			2,999			—			52,000			3,206			—
Benefits paid	(55,632)			(2,999)			(5,872)			(53,723)			(3,206)			(4,260)
Market value of plan assets at end of year	1,186,433			—			52,286			974,993			—			52,838
Funded status at year end	$	(312,806)		$	(53,631)		$	(29,919)		$	(354,584)		$	(47,397)		$	(23,273)
Weighted-average assumptions (benefit obligation):
Discount rate	2.75		%	2.75		%	2.75		%	3.50		%	3.50		%	3.50		%
Weighted-average rate of compensation increase	3.00		%	3.00		%	N/A			3.25		%	3.25		%	N/A

Estimated funding for the plans above during calendar year 2021 is approximately $105 million, of which $102 million pertains to the retirement plan, and which includes a supplemental discretionary contribution of $50 million in January 2021, similar to the supplemental discretionary contribution made in January 2020. Management monitors plan assets and liabilities and may, at its discretion, increase plan funding levels above the minimum in order to achieve a desired funded status and avoid or minimize potential benefit restrictions. As a result of the impact of the historically low discount rates at December 31, 2020 and December 31, 2019, Southwest made the discretionary supplemental contributions, which are intended to mitigate the impacts on the funded status and the increase in pension costs in both years, through the ability to provide returns on the increased level of plan investments.

The accumulated benefit obligation for the retirement plan and the SERP is presented below:


	December 31,
(Thousands of dollars)	2020		2019
Retirement plan	$	1,367,179	$	1,219,989
SERP	50,471		46,067

Benefits expected to be paid for pension, SERP, and PBOP over the next 10 years are as follows:


(Millions of dollars)	2021		2022		2023		2024		2025		2026-2030
Pension	$	58.0	$	60.0	$	61.0	$	62.0	$	63.0	$	339.0
SERP	3.2		3.2		3.2		3.2		3.1		15.1
PBOP	4.9		5.0		4.9		4.9		4.9		24.0

No assurance can be made that actual funding and benefits paid will match these estimates.

For PBOP measurement purposes, the per capita cost of the covered health care benefits medical rate trend assumption is 6.0%, declining to 4.5%. Specific contributions are made for health care benefits of employees who retire after 1988, but Southwest pays all covered health care costs for employees who retired prior to 1989. The medical trend rate assumption noted above applies to the benefit obligations of pre-1989 retirees only.

The service cost component of net periodic benefit costs included in the table below is part of an overhead loading process associated with the cost of labor. The overhead process ultimately results in allocation of that portion of overall net periodic benefit costs to the same accounts to which productive labor is charged. As a result, service costs become components of various accounts, primarily Operations and maintenance expense, Net utility plant, and Deferred charges and other assets for both the Company and Southwest. The non-service cost components of net periodic benefit cost are reflected in Other income (deductions) on the Consolidated Statements of Income of each entity, based on accounting guidance for the presentation of such costs.

Components of net periodic benefit cost:


	Qualified Retirement Plan									SERP									PBOP
(Thousands of dollars)	2020			2019			2018			2020			2019			2018			2020			2019			2018
Service cost	$	34,299		$	25,864		$	28,555		$	389		$	266		$	245		$	1,581		$	1,276		$	1,473
Interest cost	45,555			49,006			44,174			1,604			1,760			1,658			2,582			3,046			2,748
Expected return on plan assets	(65,296)			(60,244)			(58,755)			—			—			—			(3,408)			(3,156)			(3,718)
Amortization of prior service cost	—			—			—			—			—			—			1,155			1,271			1,335
Amortization of net actuarial loss	36,025			22,356			32,115			1,805			1,020			1,502			—			—			—
Net periodic benefit cost	$	50,583		$	36,982		$	46,089		$	3,798		$	3,046		$	3,405		$	1,910		$	2,437		$	1,838
Weighted-average assumptions (net benefit cost)
Discount rate	3.50		%	4.50		%	3.75		%	3.50		%	4.50		%	3.75		%	3.50		%	4.50		%	3.75		%
Expected return on plan assets	6.75		%	7.00		%	7.00		%	N/A			N/A			N/A			6.75		%	7.00		%	7.00		%
Weighted-average rate of compensation increase	3.25		%	3.25		%	3.25		%	3.25		%	3.25		%	3.25		%	N/A			N/A			N/A

Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income


	Year Ended December 31,
	2020								2019								2018
(Thousands of dollars)	Total		Qualified Retirement Plan		SERP		PBOP		Total		Qualified Retirement Plan		SERP		PBOP		Total		Qualified Retirement Plan		SERP		PBOP
Net actuarial loss (gain) (a)	$	57,539	$	45,665	$	7,240	$	4,634	$	71,087	$	66,557	$	7,975	$	(3,445)	$	20,426	$	23,607	$	(3,940)	$	759
Amortization of prior service cost (b)	(1,155)		—		—		(1,155)		(1,271)		—		—		(1,271)		(1,335)		—		—		(1,335)
Amortization of net actuarial loss (b)	(37,830)		(36,025)		(1,805)		—		(23,376)		(22,356)		(1,020)		—		(33,617)		(32,115)		(1,502)		—
Prior service cost	—		—		—		—		1,878		—		—		1,878		—		—		—		—
Regulatory adjustment	(7,435)		(3,956)		—		(3,479)		(36,944)		(39,782)		—		2,838		8,233		7,657		—		576
Recognized in other comprehensive (income) loss	11,119		5,684		5,435		—		11,374		4,419		6,955		—		(6,293)		(851)		(5,442)		—
Net periodic benefit costs recognized in net income	56,291		50,583		3,798		1,910		42,465		36,982		3,046		2,437		51,332		46,089		3,405		1,838
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$	67,410	$	56,267	$	9,233	$	1,910	$	53,839	$	41,401	$	10,001	$	2,437	$	45,039	$	45,238	$	(2,037)	$	1,838

The table above discloses the net gain or loss and prior service cost recognized in Other comprehensive income, separated into (a) amounts initially recognized in Other comprehensive income, and (b) amounts subsequently recognized as adjustments to Other comprehensive income as those amounts are amortized as components of net periodic benefit cost. See also Note 6 - Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI").

The following table sets forth, by level within the three-level fair value hierarchy, the fair values of the assets of the qualified pension plan and the PBOP as of December 31, 2020 and 2019. The SERP has no assets.


	December 31,
	2020						2019
(Thousands of dollars)	Qualified Retirement Plan		PBOP		Total		Qualified Retirement Plan		PBOP		Total
Assets at fair value:
Level 1 – Quoted prices in active markets for identical financial assets
Mutual funds	$	—	$	30,358	$	30,358	$	—	$	29,188	$	29,188
Total Level 1 Assets (1)	—		30,358		30,358		—		29,188		29,188
Level 2 – Significant other observable inputs
Private commingled equity funds (2)
Global	324,084		5,878		329,962		266,908		6,338		273,246
International	141,290		2,563		143,853		117,086		2,780		119,866
U.S. equity securities	223,374		4,051		227,425		184,642		4,386		189,028
Emerging markets	76,679		1,391		78,070		62,943		1,494		64,437
Private commingled fixed income funds (3)	412,230		7,476		419,706		335,138		7,959		343,097
Pooled funds and mutual funds	5,990		565		6,555		5,359		689		6,048
Government fixed income and mortgage backed securities	201		4		205		181		4		185
Total Level 2 assets (4)	1,183,848		21,928		1,205,776		972,257		23,650		995,907
Total Plan assets at fair value	1,183,848		52,286		1,236,134		972,257		52,838		1,025,095
Insurance company general account contracts (5)	2,585		—		2,585		2,736		—		2,736
Total Plan assets	$	1,186,433	$	52,286	$	1,238,719	$	974,993	$	52,838	$	1,027,831

(1)The Mutual funds category above is a balanced fund that invests in a diversified portfolio of common stocks, preferred stocks, and fixed-income securities. Under normal circumstances the balanced fund will hold no more than 75%, and no less than 25%, of its total assets in equity securities. The fund seeks regular income, conservation of principal, and an opportunity for long-term growth of principal and income.

(2)The private commingled equity funds include common collective trusts that invest in a diversified portfolio of securities regularly traded on securities exchanges. These funds are shown in the above table at net asset value (“NAV”), which is the value of securities in the fund less the amount of any liabilities outstanding. Strategies employed by the funds include investment in:

▪Global equities, including domestic equities

▪International developed countries equities

▪Domestic equities

▪Emerging markets equities

Shares in the private commingled equity funds may be redeemed given one business day notice. While they are private equity funds and reported at NAV, due to the short redemption notice period, the lack of redemption fees, the fact that the underlying investments are exchange-traded, and that substantial liabilities do not exist subject to the NAV calculation, these investments are viewed as indirectly observable (Level 2) in the fair value hierarchy and are therefore not excluded from the body of the fair value table as a reconciling item.

The global fund provides diversified exposure to global equity markets. The fund seeks to provide long-term capital growth by investing primarily in securities listed on the major developed equity markets of the U.S., Europe, and Asia, as well as within those listed on emerging country equity markets on a tactical basis.

The international fund invests in international financial markets, primarily those of developed economies in Europe and the Pacific Basin. The fund invests primarily in equity securities issued by foreign corporations, but may invest in other securities perceived as offering attractive investment return opportunities.

The domestic equities securities funds include a large and medium capitalization fund and a small capitalization fund. The large and medium capitalization fund is designed to track the performance of the large and medium capitalization companies contained in the index, which represents approximately 90% of the market capitalization of the U.S. stock market. The small capitalization fund is designed to provide maximum long-term appreciation through investments that are well diversified by industry.

The emerging markets fund was developed to invest in emerging market equities worldwide. The purposes of the fund’s operations, “emerging market countries,” include every country in the world except the developed markets of the U.S., Canada, Japan, Australia, New Zealand, Hong Kong, and Singapore, and most countries located in Western Europe. Fund investments are made directly in each country or, where direct investment is inefficient or prohibited, through appropriate financial instruments or participation in commingled funds.

(3)The private commingled fixed income funds consist primarily of fixed income debt securities issued by the U.S. Treasury, government agencies, and fixed income debt securities issued by corporations. The fixed income fund investments may include the use of

high yield, international fixed income securities and other instruments, including derivatives, to ensure prudent diversification over a broad spectrum of investments. The changes in the value of the fixed income funds are intended to offset the changes in the pension plan liabilities due to changes in the discount rate.

These funds are shown in the above table at NAV. Investments in the private commingled fixed equity funds may be redeemed given one business day notice. While they are private fixed income funds and reported at NAV, due to the short redemption notice period, the lack of redemption fees, the fact that the underlying investments are exchange-traded, and that substantial liabilities do not exist subject to the NAV calculation, these investments are viewed as indirectly observable (Level 2), and are also not excluded from the body of the fair value table as a reconciling item.

(4)With the exception of items (2) and (3), which are discussed above, the Level 2 assets consist mainly of pooled funds and mutual funds. These funds are collective short-term funds that invest in Treasury bills and money market funds and are used as a temporary cash repository.

(5)The insurance company general account contracts are annuity insurance contracts used to pay the pensions of employees who retired prior to 1989. The balance of the account disclosed in the above table is the contract value, which is the result of deposits, withdrawals, and interest credits.

Centuri

Defined Contribution Plans

Centuri offers defined contribution plans under Section 401(k) of the Internal Revenue Code to its eligible employees, regardless of whether they are covered under collective-bargaining agreements. Eligibility requirements vary, as does timing of participation, matching, vesting, and profit-sharing features of the plans. Contributions by Centuri to these plans for the years ended December 31, 2020, 2019, and 2018 were $9 million, $8 million, and $7 million, respectively.

Deferred Compensation Plan

Centuri sponsors a nonqualified deferred compensation plan that is offered to a select group of management and highly-compensated employees. The plan allows participants to defer up to 80% of base salary and provides a match of 100% of contributions up to 5% of a participant’s salary. The plan also allows Centuri, at its election, to credit participant accounts with discretionary contributions. Participants are 100% vested in salary deferrals, contributions, and all earnings. Participant accounts include a return based on the performance of the underlying investment options selected. Payments from the plan are designated at each annual enrollment period based on specified triggering events and are payable by lump sum or on an annual installment basis.

Multiemployer Pension Plans

Centuri makes defined contributions to several multiemployer defined benefit pension plans under the terms of collective bargaining agreements (“CBAs”) with various unions representing certain employees. Contribution rates are generally specified in the CBAs and are made to the plans on a “pay-as-you-go” basis. Such contributions correspond to the number of union employees and the particular plans in which they participate, and vary depending upon the location, number of ongoing projects, and the need for union resources in connection with those projects.

The risks of participating in multiemployer plans are different from single-employer plans, including: (i) assets contributed to the multiemployer plan by one employer may be used to provide benefits to employees of other participating employers; (ii) if a participating employer stops contributing to the multiemployer plan, the unfunded obligations of the plan may become the obligation of the remaining participating employers; and (iii) if a participating employer chooses to stop participating in these multiemployer plans, the employer may be required to pay those plans an amount based on the underfunded status of the plan.

The Pension Protection Act of 2006 requires special funding and operational rules for multiemployer plans in the U.S., including classification of the plans (based on multiple factors, including the funded status of the plan), the most severe of which is “critical.” Depending upon the classification, plans may be required to adopt measures to improve their funded status through a funding improvement or rehabilitation plan, which may require additional contributions from employers (in the form of a surcharge on benefit contributions) and/or modification of retiree benefits. The amount of additional funds, if any, that Centuri may be obligated to contribute to these plans in the future cannot be estimated due to the uncertainty regarding future levels of work that may require the utilization of union employees covered by these plans, as well as uncertainty as to the future contribution levels and possible surcharges on contributions that may apply to these plans at that time.

Centuri contributed $44.3 million, $41.3 million, and $38.2 million collectively to the plans for the years ended December 31, 2020, 2019, and 2018, respectively. Substantially all of the contributions made by Centuri during these years were to U.S. plans that were not classified as critical, and for which no special surcharges were assessed. Only three plans were classified as critical and required special surcharges; however, the contributions overall related to these plans in all periods were insignificant.

The entire disclosure for retirement benefits.

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Income Taxes

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Income Taxes

Note 12 - Income Taxes

Southwest Gas Holdings, Inc.:

The following is a summary of income before taxes and noncontrolling interest for domestic and foreign operations:


	Year ended December 31,
(Thousands of dollars)	2020		2019		2018
U.S.	$	282,489	$	261,525	$	235,120
Foreign	22,249		11,145		8,216
Total income before income taxes	$	304,738	$	272,670	$	243,336

Income tax expense (benefit) consists of the following:


	Year Ended December 31,
(Thousands of dollars)	2020		2019		2018
Current:
Federal	$	6,287	$	622	$	(13,476)
State	8,617		(1,510)		(3,219)
Foreign	4,666		5,013		2,563
	19,570		4,125		(14,132)
Deferred:
Federal	44,547		45,593		67,784
State	414		8,212		8,901
Foreign	1,222		(1,907)		(869)
	46,183		51,898		75,816
Total income tax expense	$	65,753	$	56,023	$	61,684

Deferred income tax expense (benefit) consists of the following significant components:


	Year Ended December 31,
(Thousands of dollars)	2020		2019		2018
Deferred federal and state:
Property-related items	$	50,504	$	60,449	$	94,899
Purchased gas cost adjustments	(5,726)		3,834		(3,507)
Employee benefits	459		7,680		(7,334)
Regulatory adjustments	(9,885)		(11,962)		2,412
Deferred payroll taxes	(9,055)		—		—
Alternative minimum tax	4,409		441		849
All other deferred	15,529		(8,298)		(10,890)
Total deferred federal and state	46,235		52,144		76,429
Deferred ITC, net	(52)		(246)		(613)
Total deferred income tax expense	$	46,183	$	51,898	$	75,816

References above and below to Deferred payroll taxes relate to the employer portion of Social Security tax, for which deferment of remittance was permissible under the Coronavirus Aid, Relief, and Economic Security (“CARES”) Act.

A reconciliation of the U.S. federal statutory rate to the consolidated effective tax rate (and the sources of these differences and the effect of each) are summarized as follows:


	Year Ended December 31,
	2020		2019		2018
U.S. federal statutory income tax rate	21.0	%	21.0	%	21.0	%
Net state taxes	3.0		2.1		2.9
Tax credits	(0.5)		(0.3)		(0.3)
Company-owned life insurance	(0.8)		(1.5)		0.1

Amortization of excess deferred taxes	(0.8)		(0.9)		—
All other differences	(0.3)		0.1		1.6
Consolidated effective income tax rate	21.6	%	20.5	%	25.3	%

Deferred tax assets and liabilities consist of the following:


	December 31,
(Thousands of dollars)	2020		2019
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	$	104,314	$	105,077
Employee benefits	39,907		37,439
Alternative minimum tax credit	—		4,409
Federal net operating losses	4,118		7,467
Deferred payroll taxes	9,055		—
Lease-related item	20,890		21,226
Other	14,350		21,536
Valuation allowance	(22)		(25)
	192,612		197,129
Deferred tax liabilities:
Property-related items, including accelerated depreciation	785,734		732,798
Regulatory balancing accounts	4,205		9,931

Debt-related costs	2,585		2,818
Intangibles	13,511		10,611
Lease-related item	19,789		20,386
Other	13,786		19,569
	839,610		796,113
Net noncurrent deferred tax liabilities	$	646,998	$	598,984

Net noncurrent deferred tax liabilities above at December 31, 2020 and 2019 are reflected net of $455,000 and $856,000 of noncurrent deferred tax assets associated with the Company’s Canadian operations, which are shown separately on the Company’s Consolidated Balance Sheets.

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:


	December 31,
(Thousands of dollars)	2020		2019
Unrecognized tax benefits at beginning of year	$	1,056	$	971
Gross increases – tax positions in prior period	641		85
Gross decreases – tax positions in prior period	—		—
Gross increases – current period tax positions	231		—
Gross decreases – current period tax positions	—		—
Settlements	—		—
Lapse in statute of limitations	—		—
Unrecognized tax benefits at end of year	$	1,928	$	1,056

Southwest Gas Corporation:

The following is a summary of income before taxes:


	Year ended December 31,
(Thousands of dollars)	2020		2019		2018
Total income before income taxes	$	194,873	$	198,144	$	182,833

Income tax expense (benefit) consists of the following:


	Year Ended December 31,
(Thousands of dollars)	2020		2019		2018
Current:
Federal	$	(4,678)	$	4,109	$	(17,584)
State	(179)		250		(6,783)
	(4,857)		4,359		(24,367)
Deferred:
Federal	38,561		29,543		58,136
State	2,051		1,071		10,222
	40,612		30,614		68,358
Total income tax expense	$	35,755	$	34,973	$	43,991

Deferred income tax expense (benefit) consists of the following significant components:


	Year Ended December 31,
(Thousands of dollars)	2020		2019		2018
Deferred federal and state:
Property-related items	$	36,029	$	34,398	$	67,576
Purchased gas cost adjustments	(5,726)		3,834		(3,507)
Employee benefits	11,437		6,493		2,156
Regulatory adjustments	(9,885)		(11,962)		2,412
Deferred payroll taxes	(1,810)		—		—
Alternative minimum tax	4,409		441		849
All other deferred	6,210		(2,344)		(515)
Total deferred federal and state	40,664		30,860		68,971
Deferred ITC, net	(52)		(246)		(613)
Total deferred income tax expense	$	40,612	$	30,614	$	68,358

A reconciliation of the U.S. federal statutory rate to the consolidated effective tax rate (and the sources of these differences and the effect of each) are summarized as follows:


	Year Ended December 31,
	2020		2019		2018
U.S. federal statutory income tax rate	21.0	%	21.0	%	21.0	%
Net state taxes	1.7		0.7		2.1
Tax credits	(0.7)		(0.4)		(0.4)
Company-owned life insurance	(1.0)		(1.9)		0.3

Amortization of excess deferred taxes	(1.3)		(1.2)		—
All other differences	(1.4)		(0.5)		1.1
Effective income tax rate	18.3	%	17.7	%	24.1	%

Deferred tax assets and liabilities consist of the following:


	December 31,
(Thousands of dollars)	2020		2019
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	$	104,314	$	105,077
Employee benefits	4,806		13,574
Alternative minimum tax credit	—		4,409
Deferred payroll taxes	1,810		—
Other	7,790		12,193
Valuation allowance	(22)		(25)
	118,698		135,228
Deferred tax liabilities:
Property-related items, including accelerated depreciation	680,294		644,046
Regulatory balancing accounts	4,205		9,931

Debt-related costs	2,585		2,818
Other	12,714		17,483
	699,798		674,278
Net deferred tax liabilities	$	581,100	$	539,050

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:


	December 31,
(Thousands of dollars)	2020		2019
Unrecognized tax benefits at beginning of year	$	1,056	$	971
Gross increases – tax positions in prior period	506		85
Gross decreases – tax positions in prior period	—		—
Gross increases – current period tax positions	231		—
Gross decreases – current period tax positions	—		—
Settlements	—		—
Lapse in statute of limitations	—		—
Unrecognized tax benefits at end of year	$	1,793	$	1,056

In assessing whether uncertain tax positions should be recognized in its financial statements, management first determines whether it is more-likely-than-not that a tax position will be sustained upon examination, including resolution of any related appeals or litigation processes, based on the technical merits of the position. In evaluations of whether a tax position has met the more-likely-than-not recognition threshold, management presumes that the position will be examined by the appropriate taxing authority that would have full knowledge of all relevant information. For tax positions that meet the more-likely-than-not recognition threshold, management measures the amount of benefit recognized in the financial statements at the largest amount of benefit that is greater than 50% likely of being realized upon ultimate settlement. Unrecognized tax benefits are recognized in the first financial reporting period in which information becomes available indicating that such benefits will more-likely-than-not be realized. For each reporting period, management applies a consistent methodology to measure unrecognized tax benefits, and all unrecognized tax benefits are reviewed periodically and adjusted as circumstances warrant. Measurement of unrecognized tax benefits is based on management’s assessment of all relevant information, including prior audit experience,

the status of audits, conclusions of tax audits, lapsing of applicable statutes of limitation, identification of new issues, and any administrative guidance or developments.

At December 31, 2020, the total amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate was $1.9 million for the Company and $1.8 million for Southwest. No significant increases or decreases in unrecognized tax benefits are expected within the next 12 months.

The Company and Southwest recognize interest expense and income and penalties related to income tax matters in income tax expense. There was $523,000 of tax-related interest income for 2020, and none in 2019 or 2018.

The Company’s regulated operations accounting for income taxes is impacted by the FASB’s ASC 980 – Regulated Operations. Reductions in accumulated deferred income tax balances due to the reduction in the corporate income tax rates to 21% under the provisions of the Tax Cuts and Jobs Act (the “TCJA”), enacted in December 2017, may continue to result in a refund of excess deferred taxes to customers, generally through reductions in future rates. The TCJA included provisions that stipulate how these excess deferred taxes may be passed back to customers for certain accelerated tax depreciation benefits. Potential refunds of other deferred taxes will be determined in conjunction with appropriate regulatory commissions. Southwest began refunding excess deferred taxes to Nevada customers starting in January 2019 and to Arizona customers in January 2021. Refunding to California customers will begin with the effective date of new rates for the most recent general rate case. Paiute began refunding excess deferred taxes to its customers starting in December 2019. The December 31, 2020 Consolidated Balance Sheets of Southwest and the Company reflect the impact of the TCJA and the remaining unamortized balance of the regulatory liability (including a gross-up), barring further changes to income tax rates. See also Note 5 - Regulatory Assets and Liabilities.

The Company and its subsidiaries file a consolidated federal income tax return in the U.S. and in various states, as well as separate returns in Canada. With few exceptions, the Company is no longer subject to U.S. federal, state and local, or Canadian income tax examinations for years before 2016.

The Company and each of its subsidiaries, including Southwest, participate in a tax sharing agreement to establish the method for allocating tax benefits and losses among members of the consolidated group. The consolidated federal income tax is apportioned among the subsidiaries using a separate return method.

At December 31, 2020, the Company has no U.S. federal net operating loss carryforward. The Company also has no general business credit carryforwards. The Company has a net capital loss carryforward of $97,000, which will begin to expire in 2021. At December 31, 2020, the Company has an income tax net operating loss carryforward related to Canadian operations of $12.7 million, which begins to expire in 2034. At December 31, 2020, the Company has $50.9 million of state net operating loss carryforwards, with a tax effect (net of federal benefit) of $3.2 million. Depending on the jurisdiction in which the state net operating loss was generated, the carryforwards will begin to expire in 2025.

Management intends to continue to permanently reinvest any future foreign earnings in Canada.

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Segment Information

12 Months Ended

Dec. 31, 2020

Segment Reporting [Abstract]

Segment Information

Note 13 - Segment Information

The Company’s operating segments are determined based on the nature of their activities. The natural gas operations segment is engaged in the business of purchasing, distributing, and transporting natural gas. Revenues are generated from the distribution and transportation of natural gas. The utility infrastructure services segment is primarily engaged in the business of providing utility companies with trenching and installation, replacement, and maintenance services for energy distribution systems. Although our utility infrastructure services operations are geographically dispersed, they are aggregated and reported as a single segment as each reporting unit has similar economic characteristics. Over 99% of the total Company’s long-lived assets are in the U.S.

The accounting policies of the reported segments are the same as those described within Note 1 - Background, Organization, and Summary of Significant Accounting Policies. Centuri accounts for the services provided to Southwest at contractual prices at contract inception. Accounts receivable for these services, which are not eliminated during consolidation, are presented in the table below:


	December 31,
(Thousands of dollars)	2020		2019
Accounts receivable for Centuri services	$	13,956	$	15,235

The following table presents the amount of revenues for both segments by geographic area:


	December 31,
(Thousands of dollars)	2020		2019		2018
Revenues (a)
United States	$	3,057,041	$	2,893,201	$	2,664,670
Canada	241,832		226,716		215,343
Total	$	3,298,873	$	3,119,917	$	2,880,013

(a)Revenues are attributed to countries based on the location of customers.

The Company has two reportable segments: natural gas operations and utility infrastructure services. Southwest has a single reportable segment that is referred to herein as the natural gas operations segment of the Company. In order to reconcile to net income as disclosed in the Consolidated Statements of Income, an Other column is included associated with impacts of corporate and administrative activities related to Southwest Gas Holdings, Inc. The financial information pertaining to the natural gas operations and utility infrastructure services segments for each of the three years in the period ended December 31, 2020 is as follows:


	Year Ended December 31, 2020
(Thousands of dollars)	Natural Gas Operations		Utility Infrastructure Services		Other		Total
Revenues from external customers	$	1,350,585	$	1,813,429	$	—	$	3,164,014
Intersegment sales	—		134,859		—		134,859
Total	$	1,350,585	$	1,948,288	$	—	$	3,298,873
Interest income	$	4,015	$	—	$	—	$	4,015
Interest expense	$	101,148	$	9,269	$	1,060	$	111,477
Depreciation and amortization	$	235,295	$	96,732	$	—	$	332,027
Income tax expense	$	35,755	$	31,128	$	(1,130)	$	65,753
Segment net income	$	159,118	$	74,862	$	(1,656)	$	232,324
Segment assets	$	7,256,636	$	1,475,237	$	3,980	$	8,735,853
Capital expenditures	$	692,216	$	132,889	$	—	$	825,105


	Year Ended December 31, 2019
(Thousands of dollars)	Natural Gas Operations		Utility Infrastructure Services		Other		Total
Revenues from external customers	$	1,368,939	$	1,592,252	$	—	$	2,961,191
Intersegment sales	—		158,726		—		158,726
Total	$	1,368,939	$	1,750,978	$	—	$	3,119,917
Interest income	$	6,356	$	—	$	—	$	6,356
Interest expense	$	95,026	$	14,086	$	114	$	109,226
Depreciation and amortization	$	215,620	$	87,617	$	—	$	303,237
Income tax expense	$	34,973	$	21,399	$	(349)	$	56,023
Segment net income	$	163,171	$	52,404	$	(1,639)	$	213,936
Segment assets	$	6,798,746	$	1,365,194	$	6,108	$	8,170,048
Capital expenditures	$	778,748	$	159,400	$	—	$	938,148


	Year Ended December 31, 2018
(Thousands of dollars)	Natural Gas Operations		Utility Infrastructure Services		Other		Total
Revenues from external customers	$	1,357,728	$	1,386,371	$	—	$	2,744,099
Intersegment sales	—		135,914		—		135,914
Total	$	1,357,728	$	1,522,285	$	—	$	2,880,013
Interest income	$	6,020	$	88	$	—	$	6,108
Interest expense	$	81,740	$	14,190	$	741	$	96,671
Depreciation and amortization	$	191,816	$	57,396	$	—	$	249,212
Income tax expense	$	43,991	$	18,420	$	(727)	$	61,684
Segment net income	$	138,842	$	44,977	$	(1,542)	$	182,277
Segment assets	$	6,141,584	$	1,215,573	$	572	$	7,357,729
Capital expenditures	$	682,869	$	83,045	$	—	$	765,914

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

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Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Data (Unaudited)

Note 14 - Quarterly Financial Data (Unaudited)

The following table presents summarized quarterly financial data for 2020 and 2019:


	Quarter Ended
	March 31		June 30		September 30		December 31
(Thousands of dollars, except per share amounts)
2020
Southwest Gas Holdings, Inc.:
Operating revenues	$	836,320	$	757,247	$	791,226	$	914,080
Operating income	148,373		67,572		54,264		152,795
Net income	73,005		39,881		21,063		105,036
Net income attributable to Southwest Gas Holdings, Inc.	72,542		37,965		18,273		103,544
Basic earnings per common share (1)	$	1.31	$	0.68	$	0.32	$	1.82
Diluted earnings per common share (1)	$	1.31	$	0.68	$	0.32	$	1.82

Southwest Gas Corporation:
Operating revenues	$	502,827	$	262,434	$	210,834	$	374,490
Operating income	157,815		27,101		1,625		116,070
Net income (loss)	83,599		11,942		(15,973)		79,550

2019
Southwest Gas Holdings, Inc.:
Operating revenues	$	833,539	$	713,011	$	725,230	$	848,137
Operating income	140,480		54,869		38,258		138,204
Net income	95,384		22,832		6,525		91,906
Net income attributable to Southwest Gas Holdings, Inc.	94,809		22,056		5,353		91,718
Basic earnings per common share (1)	$	1.78	$	0.41	$	0.10	$	1.67
Diluted earnings per common share (1)	$	1.77	$	0.41	$	0.10	$	1.67

Southwest Gas Corporation:
Operating revenues	$	520,677	$	258,711	$	209,980	$	379,571
Operating income (loss)	148,713		24,069		(1,807)		112,678
Net income (loss)	103,389		3,369		(20,012)		76,425

(1)The sum of quarterly earnings (loss) per average common share may not equal the annual earnings (loss) per share due to the ongoing change in the weighted-average number of common shares.

The demand for natural gas is seasonal, and it is the opinion of management that comparisons of earnings for interim periods do not reliably reflect overall trends and changes in operations. Also, the timing of general rate relief can have a significant impact on earnings for interim periods.

Redeemable Noncontrolling Interest

12 Months Ended

Dec. 31, 2020

Noncontrolling Interest [Abstract]

Redeemable Noncontrolling Interest

Note 15 - Redeemable Noncontrolling Interest

In connection with the acquisition of Linetec in November 2018, the previous owner retained a 20% equity interest in Linetec, the reduction of which is subject to certain rights based on the passage of time or upon the occurrence of certain triggering events. Effective January 2022, the Company, by means of Centuri, has the right, but not the obligation, to purchase at fair value (subject to a floor) a portion of the interest held by the noncontrolling party, and in incremental amounts each year thereafter. The shares subject to the election accumulate (if earlier elections are not made) such that 100% of the interest retained by the noncontrolling party is subject to the election beginning in 2024. If the Company does not exercise its rights at each or any of the specified intervals, the noncontrolling party has the ability, but not the obligation, to exit their investment retained by requiring Centuri to purchase a similar portion of their interest up to the maximum cumulative amounts specified and at each interval discussed above. The outstanding noncontrolling interest is not subject to minimum purchase provisions and following the eligibility dates for the elections, they do not expire. The redemption price represents the greater of fair value

of the ownership interest to be redeemed on the redemption date or a floor amount under the terms of the agreement. The Company has determined that this noncontrolling interest is a redeemable noncontrolling interest and, in accordance with SEC guidance, is classified as mezzanine equity (temporary equity) in the Company’s Consolidated Balance Sheets.

Significant changes in the value of the redeemable noncontrolling interest, above a floor established at the acquisition date, are recognized as they occur, and the carrying value is adjusted as necessary at each reporting date. The fair value is estimated using a market approach that utilizes certain financial metrics from guideline public companies of similar industry and operating characteristics. Based on the fair value model employed, the estimated redemption value of the redeemable noncontrolling interest increased by approximately $74.5 million during the year ended December 31, 2020. Adjustment to the redemption value also impacted retained earnings, as reflected in the Company’s Consolidated Statement of Equity, but did not impact net income.

The following depicts changes to the balance of the redeemable noncontrolling interest:


	Redeemable Noncontrolling Interest
(Thousands of dollars)
Balance, December 31, 2018	$	81,831
Net income attributable to redeemable noncontrolling interest	2,711
Balance, December 31, 2019	84,542
Net income attributable to redeemable noncontrolling interest	6,661
Redemption value adjustment	74,513
Balance, December 31, 2020	$	165,716

Background, Organization, and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations. This is a combined annual report of Southwest Gas Holdings, Inc. and its subsidiaries (the “Company”) and Southwest Gas Corporation and its subsidiaries (“Southwest” or the “natural gas operations” segment). The notes to the consolidated financial statements apply to both entities. Southwest Gas Holdings, Inc., a Delaware corporation, is a holding company, owning all of the shares of common stock of Southwest and all of the shares of common stock of Centuri Group, Inc. (“Centuri” or the “utility infrastructure services” segment). Southwest is engaged in the business of purchasing, distributing, and transporting natural gas for customers in portions of Arizona, Nevada, and California. Public utility rates, practices, facilities, and service territories of Southwest are subject to regulatory oversight. The timing and amount of rate relief can materially impact results of operations. Natural gas purchases and the timing of related recoveries can materially impact liquidity. Results for the natural gas operations segment are higher during winter periods due to the seasonality incorporated in its regulatory rate structures. Centuri is a comprehensive utility infrastructure services enterprise dedicated to delivering a diverse array of solutions to North America’s gas and electric providers. Centuri derives revenue primarily from installation, replacement, repair, and maintenance of energy distribution systems. Centuri operations are generally conducted under the business names of NPL Construction Co. (“NPL”), NPL Canada Ltd. (“NPL Canada”), New England Utility Constructors, Inc. (“Neuco”), and Linetec Services, LLC (“Linetec”). Utility infrastructure services activity is seasonal in many of Centuri’s operating areas. Peak periods are the summer and fall months in colder climate areas, such as the northeastern and midwestern United States (“U.S.”) and in Canada. In warmer climate areas, such as the southwestern and southeastern U.S., utility infrastructure services activity continues year round.
Basis of Presentation	Basis of Presentation. The Company follows accounting principles generally accepted in the United States (“U.S. GAAP”) in accounting for all of its businesses. Unless specified otherwise, all amounts are in U.S. dollars. Accounting for natural gas utility operations conforms with U.S. GAAP as applied to rate-regulated companies and as prescribed by federal agencies and commissions of the various states in which the utility operates. The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Consolidation	Consolidation. The accompanying financial statements are presented on a consolidated basis for Southwest Gas Holdings, Inc. and all subsidiaries and Southwest Gas Corporation and all subsidiaries as of December 31, 2020 (except those accounted for using the equity method as discussed below). All significant intercompany balances and transactions have been eliminated with the exception of transactions between Southwest and Centuri in accordance with accounting treatment for rate-regulated entities.Centuri, through its subsidiaries, holds a 50% interest in W.S. Nicholls Western Construction Ltd. (“Western”), a Canadian infrastructure services company that is a variable interest entity. Centuri determined that it is not the primary beneficiary of the entity due to a shared-power structure; therefore, Centuri does not consolidate the entity and has recorded its investment, and results related thereto, using the equity method. The investment in Western, related earnings, and dividends received from Western in 2020 and 2019 were not significant. Centuri’s maximum exposure to loss as a result of its involvement with Western was estimated at $12.2 million as of December 31, 2020.
Fair Value Measurements	Fair Value Measurements. Certain assets and liabilities are reported at fair value, which is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. U.S. GAAP states that a fair value measurement should be based on the assumptions that market participants would use in pricing the asset or liability and establishes a fair value hierarchy that ranks the inputs used to measure fair value by their reliability. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements) and the lowest priority to fair values derived from unobservable inputs (Level 3 measurements). Financial assets and liabilities are categorized in their entirety based on the lowest level of input that is significant to the fair value measurement. The three levels of the fair value hierarchy are as follows: Level 1 – quoted prices (unadjusted) in active markets for identical assets or liabilities that a company has the ability to access at the measurement date. Level 2 – inputs other than quoted prices included within Level 1 that are observable for similar assets or liabilities, either directly or indirectly. Level 3 – unobservable inputs for the asset or liability. Unobservable inputs are used to measure fair value to the extent that observable inputs are not available, thereby allowing for situations in which there is little, if any, market activity for the asset or liability at the measurement date. The Company primarily used quoted market prices and other observable market pricing information in valuing cash and cash equivalents, derivatives, long-term debt outstanding, and assets of the qualified pension plan and the postretirement benefits other than pensions required to be recorded and/or disclosed at fair value. The Company uses prices and inputs that are current as of the measurement date, and recognizes transfers between levels at either the actual date of an event or a change in circumstance that caused the transfer.
Net Utility Plant	Net Utility Plant. Net utility plant includes gas plant at original cost, less the accumulated provision for depreciation and amortization, plus any unamortized balance of acquisition adjustments. Original cost generally includes contracted services, material, payroll, and related costs such as taxes and certain benefits, general and administrative expenses, and an allowance for funds used during construction, less contributions in aid of construction. See also Depreciation and Amortization below.
Intangible Assets	Intangible Assets. Intangible assets (other than goodwill) are amortized using the straight-line method to reflect the pattern of economic benefits consumed over the estimated periods benefited. The recoverability of intangible assets is evaluated when events or circumstances indicate that a revision of estimated useful lives is warranted or that an intangible asset may be impaired. Non-utility intangible assets are associated with utility infrastructure services businesses previously acquired. All have finite lives. These intangible assets are included in Other property and investments on the Company’s Consolidated Balance Sheets.
Cash and Cash Equivalents	Cash and Cash Equivalents. For purposes of reporting consolidated cash flows, cash and cash equivalents include cash on hand and financial instruments with original maturities of three months or less. Such investments are carried at cost, which approximates market value. Cash and cash equivalents for Southwest and the Company also include money market fund investments totaling approximately $40,000 for both entities at December 31, 2020, and $23.5 million and $26.7 million, for each, respectively, at December 31, 2019, which fall within Level 2 of the fair value hierarchy, due to the asset valuation methods used by money market funds. Typical non-cash investing activities for Southwest include customer advances applied as contributions toward utility construction activity and capital expenditures that were not paid as of year end that are included in accounts payable. Amounts related to such activities were not significant for the periods presented herein. Also, see Note 2 - Utility Plant and Leases for information related to right-of-use (“ROU”) assets obtained in exchange for lease liabilities, which are non-cash investing and financing activities. ROU assets and lease liabilities are also subject to non-cash impacts as a result of other factors, such as lease terminations and modifications.
Income Taxes	Income Taxes. The asset and liability method of accounting is utilized for the recognition of income taxes. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are anticipated to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the period that includes the enactment date. For regulatory and financial reporting purposes, investment tax credits (“ITC”) related to gas utility operations are deferred and amortized over the life of related fixed assets. As of December 31, 2020, the Company had cumulative book earnings of approximately $47 million in its foreign jurisdiction. Management previously asserted and continues to assert that all the earnings of Centuri’s Canadian subsidiaries will be permanently reinvested in Canada. As a result, no U.S. deferred income taxes have been recorded related to cumulative foreign earnings. The Financial Accounting Standards Board (the “FASB”) issued guidance to allow an accounting policy election of either (i) treating taxes attributable to future taxable income related to Global Intangible Low-Taxed Income (“GILTI”) as a current period expense when incurred or (ii) recognizing deferred taxes for temporary differences expected to reverse as GILTI in future years. The Company has elected to treat GILTI as a current period cost when incurred and has considered the estimated 2020 GILTI impact, which was immaterial, in its 2020 tax expense.
Deferred Purchased Gas Costs	Deferred Purchased Gas Costs. The various regulatory commissions have established procedures to enable Southwest to adjust its billing rates for changes in the cost of natural gas purchased. The difference between the current cost of gas purchased and the cost of gas recovered in billed rates is deferred. Generally, these deferred amounts are recovered or refunded within one year. In mid-February 2021, the central U.S. (from south Texas to North Dakota and the eastern Rocky Mountains) experienced extreme cold temperatures, which increased natural gas demand and caused temporary shortages due to wellhead freeze-off. These conditions caused daily natural gas prices to reach unprecedented levels. During this time, Southwest secured natural gas supplies, albeit at substantially higher prices, and was able to maintain service to its customers. The incremental costs for these natural gas supplies (estimated at $200 million to $300 million) are expected to be funded using existing cash on hand and short-term borrowings, including current credit facilities. It is anticipated that these incremental gas costs will be recoverable from customers through the purchased gas adjustment (“PGA”) mechanisms in each jurisdiction.
Prepaids and Other Current Assets	Prepaid and other current assets. Prepaid and other current assets for Southwest and the Company include, among other things, gas pipe materials and operating supplies of $50 million in 2020 and $57 million in 2019 (carried at weighted average cost), as well as $1 million in 2020 and $33 million in 2019 related to a regulatory asset associated with the Arizona decoupling mechanism (an alternative revenue program).
Goodwill	Goodwill. As required by U.S. GAAP, goodwill is assessed for impairment annually, or more frequently, if circumstances indicate impairment to the carrying value of goodwill may have occurred. The goodwill impairment analysis was conducted as of October 1st using a qualitative assessment, as permitted by U.S. GAAP. Management of the Company and Southwest considered its reporting units and segments and determined that its segments and reporting units remain consistent between periods presented below, and that no change was necessary with regard to the level at which goodwill is assessed for impairment.
Other Current Liabilities	Other Current Liabilities. Management recognizes in its balance sheets various liabilities that are expected to be settled through future cash payment within the next twelve months, including certain regulatory liabilities (refer to Note 5 - Regulatory Assets and Liabilities), customary accrued expenses for employee compensation and benefits, and declared but unpaid dividends. Other current liabilities for the Company includes $32.6 million and $30 million of dividends declared as of December 31, 2020 and 2019, respectively.
Accumulated Removal Costs	Accumulated Removal Costs. Approved regulatory practices allow Southwest to include in depreciation expense a component intended to recover removal costs associated with utility plant retirements. In accordance with the Securities and Exchange Commission (“SEC”) position on presentation of these amounts, management reclassifies estimated removal costs from Accumulated depreciation to Accumulated removal costs within the liabilities section of the Consolidated Balance Sheets. Management regularly updates the estimated accumulated removal costs as amounts fluctuate between periods depending on the level of replacement work performed, the estimated cost of removal in rates, and the actual cost of removal experienced.
Gas Operating Revenues	Gas Operating Revenues. Southwest recognizes revenue when it satisfies its performance by transferring gas to the customer. Natural gas is delivered and “consumed” by the customer simultaneously. Revenues are recorded when customers are billed. Customer billings are substantially based on monthly meter reads and include certain other charges assessed monthly, and are calculated in accordance with applicable tariffs and state and local laws, regulations, and related agreements. An estimate of the margin associated with natural gas service provided, but not yet billed, to residential and commercial customers from the latest meter read date to the end of the reporting period is also recognized as accrued utility revenue. Revenues also include the net impacts of margin tracker/decoupling accruals based on criteria in U.S. GAAP for rate-regulated entities associated with alternative revenue programs. All of Southwest’s service territories have decoupled rate structures, which are designed to eliminate the direct link between volumetric sales and revenue, thereby mitigating the impacts of unusual weather variability and conservation on margin. See Note 3 - Revenue. Utility Infrastructure Services Revenues. The majority of Centuri contracts are performed under unit-price contracts. Generally, these contracts state prices per unit of installation. Typical installations are accomplished in a few weeks or less. Revenues are recorded as installations are completed. Revenues are recorded for long-term fixed-price contracts in a pattern that reflects the transfer of control of promised goods and services to the customer over time. The amount of revenue recognized on fixed-price contracts is based on costs expended to date relative to anticipated final contract costs. Changes in job performance, job conditions, and final contract settlements are factors that influence management’s assessment of total contract value and the total estimated costs to complete those contracts. Revisions in estimates of costs and earnings during the course of work are reflected in the accounting period in which the facts requiring revision become known. If a loss on a contract becomes known or is anticipated, the entire amount of the estimated ultimate loss is recognized at that time in the financial statements. Some unit-price contracts contain caps that if encroached, trigger revenue and loss recognition similar to a fixed-price contract model. See Note 3 - Revenue. Utility Infrastructure Services Expenses. Centuri’s utility infrastructure services expenses in the Consolidated Statements of Income includes payroll expenses, office and equipment rental costs, subcontractor expenses, training, job-related materials, gains and losses on equipment sales, and professional fees. Net Cost of Gas Sold. Components of net cost of gas sold include natural gas commodity costs (fixed-price and variable-rate), pipeline capacity/transportation costs, and actual settled costs of natural gas derivative instruments, where relevant. Also included are the net impacts of PGA deferrals and recoveries, which by their inclusion, result in net cost of gas sold overall that is comparable to amounts included in billed gas operating revenues. Differences between amounts incurred with suppliers, transmission pipelines, etc. and amounts already included in customer rates, are temporarily deferred in PGA accounts pending inclusion in customer rates.
Operations and Maintenance Expense	Operations and Maintenance Expense. Operations and maintenance expense includes Southwest’s operating and maintenance costs associated with serving utility customers and maintaining its distribution and transmission systems, uncollectible expense, administrative and general salaries and expense, employee benefits expense excluding relevant non-service cost components, and legal expense (including injuries and damages).
Depreciation and Amortization	Depreciation and Amortization. Utility plant depreciation is computed on the straight-line remaining life method at composite rates considered sufficient to amortize costs over estimated service lives, including components which compensate for removal costs (net of salvage value), and retirements, as approved by the appropriate regulatory agency. When plant is retired from service, the original cost of plant, including cost of removal, less salvage, is charged to the accumulated provision for depreciation. See also discussion regarding Accumulated Removal Costs above. Other regulatory assets, including acquisition adjustments, are amortized when appropriate, over time periods authorized by regulators. Non-utility and utility infrastructure services-related property and equipment are depreciated on a straight-line method based on the estimated useful lives of the related assets. Costs and gains related to refunding utility debt and debt issuance expenses are deferred and amortized over the weighted-average lives of the new issues and become a component of interest expense.
Allowance for Funds Used During Construction ("AFUDC")	Allowance for Funds Used During Construction (“AFUDC”). AFUDC represents the cost of both debt and equity funds used to finance utility construction. AFUDC is capitalized as part of the cost of utility plant. The debt portion of AFUDC is reported in the Company’s and Southwest’s Consolidated Statements of Income as an offset to Net interest deductions and the equity portion is reported as Other income. Utility plant construction costs, including AFUDC, are recovered in authorized rates through depreciation when completed projects are placed into operation, and general rate relief is requested and granted. AFUDC, disaggregated by type, included in the Company’s and Southwest’s Consolidated Statements of Income are presented in the table below:
Derivatives	Derivatives. In managing its natural gas supply portfolios, Southwest has historically entered into fixed- and variable-price contracts, which qualify as derivatives. Additionally, Southwest has utilized fixed-for-floating swap contracts (“Swaps”) to supplement its fixed-price contracts. The fixed-price contracts, firm commitments to purchase a fixed amount of gas in the future at a fixed price, qualify for the normal purchases and normal sales exception that is allowed for contracts that are probable of delivery in the normal course of business, and are exempt from fair value reporting. The variable-price contracts qualify as derivative instruments; however, because the contract price is the prevailing price at the future transaction date, no fair value adjustment is required. In consultation with its regulators, management does not currently anticipate entering into new Swaps in the near term and the recently remaining Swaps matured in October 2020. Southwest does not utilize derivative financial instruments for speculative purposes, nor does it have trading operations.
Foreign Currency Translation	Foreign Currency Translation. Foreign currency-denominated assets and liabilities of consolidated subsidiaries are translated into U.S. dollars at exchange rates existing at the respective balance sheet dates. Translation adjustments resulting from fluctuations in exchange rates are recorded as a separate component of accumulated other comprehensive income within stockholders’ equity. Results of operations of foreign subsidiaries are translated using the monthly weighted-average exchange rates during the respective periods. Gains and losses resulting from foreign currency transactions are included in Other income and (expenses) of the Company. Gains and losses resulting from intercompany foreign currency transactions that are of a long-term investment nature are reported in Other comprehensive income, if applicable.
Earnings Per Share	Earnings Per Share. Basic earnings per share (“EPS”) in each period of this report were calculated by dividing net income attributable to Southwest Gas Holdings, Inc. by the weighted-average number of shares during those periods. Diluted EPS includes additional weighted-average common stock equivalents (performance shares and restricted stock units). Unless otherwise noted, the term “Earnings Per Share” refers to Basic EPS.
Recently Accounting Standards Updates	Recent Accounting Standards Updates. Accounting pronouncements adopted in 2020: In June 2016, the FASB issued ASU 2016-13 “Financial Instruments — Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” The update requires the measurement of all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. The Company and Southwest adopted the update in the first quarter of 2020, and concluded the impact was not material to the consolidated financial statements of the Company or Southwest. See Note 4 - Receivables and Related Allowances. In January 2017, the FASB issued ASU 2017-04 “Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment.” Under the update, an entity will apply a one-step quantitative test as opposed to a two-step test as previously required, and record the amount of goodwill impairment as the excess of a reporting unit's carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit. The new guidance does not amend the optional qualitative assessment of goodwill impairment. The Company and Southwest adopted the update in the first quarter of 2020. In August 2018, the FASB issued ASU 2018-14 “Compensation—Retirement Benefits—Defined Benefit Plans—General (Subtopic 715-20): Disclosure Framework—Changes to the Disclosure Requirements for Defined Benefit Plans.” This update removes disclosures that are no longer considered cost-beneficial, clarifies the specific requirements of disclosures, and adds disclosure requirements identified as relevant. The update applies to all employers that sponsor defined benefit pension or other postretirement plans. The Company and Southwest modified their disclosures to conform to the requirements, where applicable. See Note 11 - Pension and Other Postretirement Benefits. In August 2018, the FASB issued ASU 2018-15 “Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract.” The update generally aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement (that is a service contract) with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software, with the exception that such costs are to be included in the same line item in the balance sheet that a prepayment of the fees associated with the arrangement would be presented. Once capitalized, the update also requires the entity to expense the amount capitalized over the term of the hosting arrangement, including reasonably certain renewal periods. The Company and Southwest adopted the update in the first quarter of 2020 using the prospective transition method, which did not result in a material impact to the Company’s or Southwest’s consolidated financial statements. Recently issued accounting pronouncements that will be effective in 2021: In December 2019, the FASB issued ASU 2019-12 “Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes.” The update simplifies the accounting for income taxes by removing certain exceptions to the general principles, as well as improving consistent application in Topic 740 by clarifying and amending existing guidance. The update is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020. Management does not believe the update will have a material impact on the Company’s and Southwest’s consolidated financial statements and disclosures. Recently issued accounting pronouncements that will be effective after 2021: In March 2020, the FASB issued ASU 2020-04 “Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting.” The update provides optional guidance for a limited time to ease the potential burden in accounting for, or recognizing the effects of, reference rate reform on financial reporting, including when modifying a contract (during the eligibility period covered by the update to the topic) to replace a reference rate affected by reference rate reform. The update applies only to contracts and hedging relationships that reference the London Interbank Offered Rate (“LIBOR”) or another reference rate expected to be discontinued due to reference rate reform. The guidance was eligible to be applied upon issuance on March 12, 2020, and can generally be applied through December 31, 2022. Management will monitor the impacts this update might have on the Company’s and Southwest’s consolidated financial statements and disclosures, and will reflect such appropriately, in the event that the optional guidance is elected. See also LIBOR discussion in Note 8 - Debt. In August 2020, the FASB issued ASU 2020-06 “Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging— Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity.” The update, amongst other amendments, improves the guidance related to the disclosures and earnings per share for convertible instruments and contracts in an entity’s own equity. The update is effective for fiscal years beginning after December 15, 2021, including interim periods within those fiscal years; early adoption is permitted. Management is evaluating the impacts this update might have on the Company’s consolidated financial statements and disclosures.
Subsequent Events	Subsequent Events. Management monitors events occurring after the balance sheet date and prior to the issuance of the financial statements to determine the impacts, if any, of events on the financial statements to be issued or disclosures to be made, and has reflected them where appropriate.
Receivables and Related Allowances	Business activity with respect to natural gas utility operations is conducted with customers located within the three-state region of Arizona, Nevada, and California. Southwest’s accounts receivable are short-term in nature with no billing due dates customarily extending beyond one month, with customers’ credit worthiness assessed upon account creation by evaluation of other utility service and related payment history. Although Southwest seeks generally to minimize its credit risk related to utility operations by requiring security deposits from new customers, imposing late fees, and actively pursuing collection on overdue accounts, some accounts are ultimately not collected. Customer accounts are subject to collection procedures that vary by jurisdiction (late fee assessment, noticing requirements for disconnection of service, and procedures for actual disconnection and/or reestablishment of service). After disconnection of service, accounts are customarily written off approximately two months after inactivation. Dependent upon the jurisdiction, reestablishment of service requires both payment of previously unpaid balances and additional deposit requirements. Provisions for uncollectible accounts are recorded monthly based on experience, consideration of current and expected future conditions, customer and rate composition, and write-off processes. They are included in the ratemaking process as a cost of service. The Nevada jurisdictions have a regulatory mechanism associated with the gas-cost-related portion of uncollectible accounts. Such amounts are deferred and collected through a surcharge in the ratemaking process. Due to the ongoing COVID-19 pandemic, Southwest initiated a moratorium in March 2020 on disconnection of natural gas service for non-payment and also ceased charging late fees until further notice. While the moratorium continues to be in place, Southwest is actively working with customers experiencing financial hardship by means of flexible payment options and by coordinating with certain governmental and nonprofit entities for customer payment assistance. Management continues to monitor expected credit losses in light of the evolving financial impact of COVID-19. The allowance as of December 31, 2020 reflects the expected impact from the pandemic on balances as of that date, including consideration of customers’ ability to pay currently and once the moratorium is lifted. Utility infrastructure services contracts receivable are recorded at face amounts less an allowance for doubtful accounts. Centuri’s customers are generally investment-grade gas and electric utility companies for which Centuri has historically recognized an insignificant amount of write-offs. Centuri’s trade accounts receivable balances carry standard payment terms of up to 60 days. Centuri maintains an allowance that is an estimate based on historical collection experience, current and estimated future economic and market conditions, and a review of the current status of each customer's trade accounts receivable balance. Account balances are monitored at least monthly, and are charged off against the allowance when management determines it is probable the balance will not be recovered. Centuri has not been significantly impacted, nor does it anticipate it will experience significant difficulty in collecting amounts due, given the nature of its customers, as a result of the current environment surrounding COVID-19.

Background, Organization, and Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Schedule of Other Property and Investments

Other property and investments on Southwest’s and the Company’s Consolidated Balance Sheets includes:


	December 31,
(Thousands of dollars)	2020		2019
Southwest Gas Corporation:
Net cash surrender value of COLI policies	$	140,874	$	132,072
Other property	2,737		1,715
Total Southwest Gas Corporation	143,611		133,787
Centuri property, equipment, and intangibles	1,089,414		983,905
Centuri accumulated provision for depreciation and amortization	(422,741)		(352,333)
Other property	23,961		18,814
Total Southwest Gas Holdings, Inc.	$	834,245	$	784,173

Summary of Intangible Assets

These intangible assets are included in Other property and investments on the Company’s Consolidated Balance Sheets. Centuri’s intangible assets, not including goodwill, at December 31, 2020 and 2019, respectively, were as follows:


	December 31, 2020
(Thousands of dollars)	Gross Carrying Amount		Accumulated Amortization		Net Carrying Amount
Customer relationships	$	154,757	$	(29,237)	$	125,520
Trade names and trademarks	23,618		(8,954)		14,664
Customer contracts backlog	270		(270)		—
Noncompete agreements	1,931		(1,931)		—
Total	$	180,576	$	(40,392)	$	140,184

	December 31, 2019
Customer relationships	$	154,186	$	(20,735)	$	133,451
Trade names and trademarks	23,353		(6,754)		16,599
Customer contracts backlog	270		(252)		18
Noncompete agreements	2,045		(1,602)		443
Total	$	179,854	$	(29,343)	$	150,511

Schedule of Estimated Future Amortization of Intangible Assets

The estimated future amortization of the intangible assets for the next five years and thereafter is as follows:


(Thousands of dollars)
2021	$	10,343
2022	10,255
2023	10,255
2024	10,255
2025	10,211
Thereafter	88,865
Total	$	140,184

Schedule of Goodwill

Goodwill on Southwest’s and the Company’s Consolidated Balance Sheets includes:


(Thousands of dollars)	Natural Gas Operations		Utility Infrastructure Services		Total Company
Balance, December 31, 2018	$	10,095	$	348,950	$	359,045
Measurement-period adjustments - Linetec acquisition	—		(21,172)		(21,172)
Foreign currency translation adjustment	—		5,150		5,150
Balance, December 31, 2019	10,095		332,928		343,023
Foreign currency translation adjustment	—		2,161		2,161
Balance, December 31, 2020	$	10,095	$	335,089	$	345,184

Schedule of Capitalized and Debt Portion of AFUDC

Utility plant construction costs, including AFUDC, are recovered in authorized rates through depreciation when completed projects are placed into operation, and general rate relief is requested and granted. AFUDC, disaggregated by type, included in the Company’s and Southwest’s Consolidated Statements of Income are presented in the table below:


(Thousands of dollars)	2020			2019			2018
AFUDC:
Debt portion	$	3,202		$	4,558		$	3,264
Equity portion	4,724			4,161			3,627
AFUDC capitalized as part of utility plant	$	7,926		$	8,719		$	6,891
AFUDC rate	5.51		%	5.36		%	5.85		%

Schedule of Other Income (Deductions)

The following table provides the composition of significant items included in Other income (deductions) on the Consolidated Statements of Income:


(Thousands of dollars)	2020		2019		2018
Southwest Gas Corporation – natural gas operations segment:
Change in COLI policies	$	9,200	$	17,400	$	(3,200)
Interest income	4,015		6,356		6,020
Equity AFUDC	4,724		4,161		3,627
Other components of net periodic benefit cost	(20,022)		(15,059)		(21,059)
Miscellaneous income and (expense)	(4,507)		(3,341)		(2,628)
Southwest Gas Corporation – total other income (deductions)	(6,590)		9,517		(17,240)
Utility infrastructure services segment:
Interest income	—		—		88
Foreign transaction gain (loss)	(16)		546		(222)
Equity in earnings of unconsolidated investment – Western	80		439		531
Miscellaneous income and (expense)	(271)		(519)		(635)
Centuri – total other income (deductions)	(207)		466		(238)
Corporate and administrative	8		102		52
Consolidated Southwest Gas Holdings, Inc. - total other income (deductions)	$	(6,789)	$	10,085	$	(17,426)

Schedule of Earnings Per Share, Basic and Diluted

A reconciliation of the denominator used in Basic and Diluted EPS calculations is shown in the following table:


(In thousands)	2020	2019	2018
Weighted average basic shares	55,998	54,245	49,419
Effect of dilutive securities:
Management Incentive Plan shares	—	12	25
Restricted stock units (1)	78	55	32
Weighted average diluted shares	56,076	54,312	49,476

Utility Plant and Leases (Tables)

12 Months Ended

Dec. 31, 2020

Schedule of Net Utility Plant

Net utility plant as of December 31, 2020 and 2019 was as follows:


	December 31,
(Thousands of dollars)	2020		2019
Gas plant:
Storage	$	101,203	$	100,908
Transmission	400,657		391,864
Distribution	7,078,656		6,581,043
General	515,879		467,274
Software and software-related intangibles	270,883		256,299
Other	16,722		15,833
	8,384,000		7,813,221
Less: accumulated depreciation and amortization	(2,419,348)		(2,313,050)
Construction work in progress	211,429		185,026
Net utility plant	$	6,176,081	$	5,685,197

Schedule of Depreciation and Amortization Expense

Depreciation and amortization expense on gas plant, including intangibles, was as follows:


(Thousands of dollars)	2020		2019		2018
Depreciation and amortization expense	$	215,636	$	197,358	$	185,719

Schedule of Components of Lease Expense, and Supplemental Cash Flow Information Related to Leases

The components of lease expense were as follows:


(Thousands of dollars)	2020		2019
Southwest:
Operating lease cost	$	1,251	$	1,531

Centuri:
Operating lease cost	$	14,294	$	12,235

Finance lease cost:
Amortization of ROU assets	$	140	$	137
Interest on lease liabilities	37		34
Total finance lease cost	177		171
Short-term lease cost	19,806		16,217
Total lease cost - Southwest Gas Holdings, Inc.	$	35,528	$	30,154

Supplemental cash flow information related to leases for the years ended December 31, 2020 and 2019 was as follows:


	2020
(Thousands of dollars)	Southwest		Centuri		Total
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	766	$	12,889	$	13,655
Operating cash flows from finance leases	—		36		36
Financing cash flows from finance leases	—		199		199

ROU assets obtained in exchange for lease obligations:
Operating leases	$	1,547	$	19,372	$	20,919
Finance leases	—		361		361


	2019
(Thousands of dollars)	Southwest		Centuri		Total
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	1,278	$	11,166	$	12,444
Operating cash flows from finance leases	—		33		33
Financing cash flows from finance leases	—		212		212

ROU assets obtained in exchange for lease obligations:
Operating leases	$	862	$	23,825	$	24,687
Finance leases	—		13,839		13,839

Schedule of Supplemental Information Related to Leases Included in Balance Sheet

Supplemental information related to leases, including location in the Consolidated Balance Sheets, is as follows:


(Thousands of dollars)	December 31,
	2020			2019
Southwest:
Operating leases:
Net utility plant	$	2,195		$	1,443

Other current liabilities	$	656		$	723
Other deferred credits and other long-term liabilities	1,586			730
Total operating lease liabilities	$	2,242		$	1,453

Weighted average remaining lease term (in years)	4.24			2.88
Weighted average discount rate	4.49		%	3.18		%

Centuri:
Operating leases:
Other property and investments	$	81,010		$	78,954

Other current liabilities	$	10,032		$	8,851
Other deferred credits and other long-term liabilities	75,247			73,323
Total operating lease liabilities	$	85,279		$	82,174

Finance leases:
Other property and investments	$	752		$	14,264

Other current liabilities	$	202		$	13,769
Other deferred credits and other long-term liabilities	490			355
Total finance lease liabilities	$	692		$	14,124

Weighted average remaining lease term (in years)
Operating leases	10.08			10.25
Finance leases	2.12			2.13

Weighted average discount rate
Operating leases	4.05		%	4.03		%
Finance leases	5.55		%	6.10		%

Schedule of Maturities of Operating Lease Liabilities

The following are schedules of maturities of lease liabilities as of December 31, 2020:


(Thousands of dollars)	Operating Leases
Southwest:
2021	$	735
2022	543
2023	426
2024	404
2025	330
Thereafter	36
Total lease payments	2,474
Less imputed interest	232
Total	$	2,242


(Thousands of dollars)	Operating Leases		Finance Leases
Centuri:
2021	$	13,144	$	235
2022	12,153		241
2023	10,864		275
2024	9,904		1
2025	8,051		—
Thereafter	50,605		—
Total lease payments	104,721		752
Less imputed interest	19,442		60
Total	$	85,279	$	692

Schedule of Maturities of Finance Lease Liabilities

The following are schedules of maturities of lease liabilities as of December 31, 2020:


(Thousands of dollars)	Operating Leases
Southwest:
2021	$	735
2022	543
2023	426
2024	404
2025	330
Thereafter	36
Total lease payments	2,474
Less imputed interest	232
Total	$	2,242


(Thousands of dollars)	Operating Leases		Finance Leases
Centuri:
2021	$	13,144	$	235
2022	12,153		241
2023	10,864		275
2024	9,904		1
2025	8,051		—
Thereafter	50,605		—
Total lease payments	104,721		752
Less imputed interest	19,442		60
Total	$	85,279	$	692

Revenue (Tables)

12 Months Ended

Dec. 31, 2020

Revenue from Contract with Customer [Abstract]

Summary of Revenue Disaggregated by Service Type and Contract Type


	December 31,
(Thousands of dollars)	2020		2019		2018
Residential	$	958,520	$	972,788	$	887,220
Small commercial	221,541		249,117		255,083
Large commercial	44,633		48,935		53,192
Industrial/other	26,242		22,074		23,489
Transportation	88,215		92,380		86,990
Revenue from contracts with customers	1,339,151		1,385,294		1,305,974
Alternative revenue program deferrals	12,140		(25,112)		45,979
Other revenues (a)	(706)		8,757		5,775
Total Gas operating revenues	$	1,350,585	$	1,368,939	$	1,357,728

The following tables display Centuri’s revenue from contracts with customers disaggregated by service type and contract type:


	December 31,
(Thousands of dollars)	2020		2019		2018
Service Types:
Gas infrastructure services	$	1,261,160	$	1,238,974	$	1,123,682
Electric power infrastructure services	411,826		247,717		32,629
Other	275,302		264,287		365,974
Total Utility infrastructure services revenues	$	1,948,288	$	1,750,978	$	1,522,285


	December 31,
(Thousands of dollars)	2020		2019		2018
Contract Types:
Master services agreement	$	1,490,009	$	1,383,377	$	1,102,412
Bid contract	458,279		367,601		419,873
Total Utility infrastructure services revenues	$	1,948,288	$	1,750,978	$	1,522,285

Unit price contracts	$	1,356,640	$	1,380,256	$	1,258,419
Fixed price contracts	157,701		112,924		117,298
Time and materials contracts	433,947		257,798		146,568
Total Utility infrastructure services revenues	$	1,948,288	$	1,750,978	$	1,522,285

Summary of Information about Receivables, Revenue Earned on Contracts in Progress in Excess of Billings, Which are Included Within Accounts Receivable, Net of Allowances, and Amounts Billed in Excess of Revenue Earned on Contracts


	December 31,
(Thousands of dollars)	2020		2019
Contracts receivable, net	$	278,316	$	223,904
Revenue earned on contracts in progress in excess of billings	96,996		99,399
Amounts billed in excess of revenue earned on contracts	4,507		4,525

Schedule of Utility Infrastructure Services Contracts Receivable

Utility infrastructure services contracts receivable consists of the following:


	December 31,
(Thousands of dollars)	2020		2019
Billed on completed contracts and contracts in progress	$	273,778	$	216,268
Other receivables	6,692		8,456
Contracts receivable, gross	280,470		224,724
Allowance for doubtful accounts	(2,154)		(820)
Contracts receivable, net	$	278,316	$	223,904

Receivables and Related Allowances (Tables)

12 Months Ended

Dec. 31, 2020

Receivables [Abstract]

Schedule of Accounts Receivable


	December 31,
(Thousands of dollars)	2020		2019
Gas utility customer accounts receivable balance	$	145,108	$	148,173

Schedule of Percent of Customers by State

The following table represents customers by state at December 31, 2020:


Percent of customers by state:
Arizona	53	%
Nevada	37	%
California	10	%

Schedule of Allowance for Uncollectibles

Activity in the allowance account for uncollectibles is summarized as follows:


(Thousands of dollars)	Allowance for Uncollectibles
Balance, December 31, 2017	$	2,111
Additions charged to expense	2,959
Accounts written off, less recoveries	(2,902)
Balance, December 31, 2018	2,168
Additions charged to expense	3,507
Accounts written off, less recoveries	(3,580)
Balance, December 31, 2019	2,095
Additions charged to expense	4,693
Accounts written off, less recoveries	(2,454)
Balance, December 31, 2020	$	4,334

Regulatory Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2020

Schedule of Regulatory Assets

The following table represents existing regulatory assets and liabilities:


	December 31,
(Thousands of dollars)	2020		2019
Regulatory assets:
Accrued pension and other postretirement benefit costs (1)	$	427,550	$	420,114
Unrealized net loss on non-trading derivatives (Swaps) (2)	—		10,951
Deferred purchased gas costs (3)	2,053		44,412
Accrued purchased gas costs (4)	29,000		8,000
Unamortized premium on reacquired debt (5)	17,124		18,249
Accrued absence time (8)	15,565		14,519
Margin, interest- and property tax-tracking (9)	4,759		33,380
Other (10)	45,596		33,134
	541,647		582,759
Regulatory liabilities:
Deferred purchased gas costs (3)	(54,636)		(60,755)
Accumulated removal costs	(404,000)		(395,000)

Unamortized gain on reacquired debt (6)	(7,644)		(8,181)
Regulatory excess deferred/other taxes and gross-up (7)	(461,023)		(455,625)
Margin, interest- and property tax-tracking (9)	(17,132)		(22,650)
Other (10)	(3,503)		(4,438)
Net regulatory liabilities	$	(406,291)	$	(363,890)

(1)Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovery period is greater than five years. (See Note 11 - Pension and Other Postretirement Benefits).

(3)Balance recovered or refunded on an ongoing basis with interest.

(4)Included in Prepaid and other current assets on the Consolidated Balance Sheets. Balance recovered or refunded on an ongoing basis.

(5)Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovered over life of debt instruments.

(6)Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. Amortized over life of debt instruments.

(8)Regulatory recovery occurs on a one-year lag basis through the labor loading process. Included in Prepaid and other current assets on the Consolidated Balance Sheets.


	December 31,
(Thousands of dollars)	2020		2019
Other Regulatory Assets:
State mandated public purpose programs (including low income and conservation programs) (a) (e)	$	11,527	$	9,172
Infrastructure replacement programs and similar (b) (e)	7,731		8,236
Environmental compliance programs (c) (e)	5,235		5,768
Pension tracking mechanism (b)	3,075		—
Other (d)	18,028		9,958
	$	45,596	$	33,134

a)Included in Prepaid and other current assets on the Consolidated Balance Sheets.

b)Included in Deferred charges and other assets on the Consolidated Balance Sheets.

e)Balance recovered or refunded on an ongoing basis, generally with interest.


	December 31,
(Thousands of dollars)	2020		2019
Other Regulatory Liabilities:
State mandated public purpose programs (including low income and conservation programs) (a) (c)	$	(834)	$	(308)
Environmental compliance programs (c) (d)	(405)		(527)
Pension tracking mechanism (b)	—		(2,476)
Other (b) (c)	(2,264)		(1,127)
	$	(3,503)	$	(4,438)

a)Included in Other current liabilities, except for $339,000 which is included in Other deferred credits and long-term liabilities on the Consolidated Balance Sheets.

b)Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.

c)Balance typically recovered or refunded on an ongoing basis, generally with interest.

d)In 2020 and 2019, included in Other current liabilities on the Consolidated Balance Sheet.

Schedule of Regulatory Liabilities

The following table represents existing regulatory assets and liabilities:


	December 31,
(Thousands of dollars)	2020		2019
Regulatory assets:
Accrued pension and other postretirement benefit costs (1)	$	427,550	$	420,114
Unrealized net loss on non-trading derivatives (Swaps) (2)	—		10,951
Deferred purchased gas costs (3)	2,053		44,412
Accrued purchased gas costs (4)	29,000		8,000
Unamortized premium on reacquired debt (5)	17,124		18,249
Accrued absence time (8)	15,565		14,519
Margin, interest- and property tax-tracking (9)	4,759		33,380
Other (10)	45,596		33,134
	541,647		582,759
Regulatory liabilities:
Deferred purchased gas costs (3)	(54,636)		(60,755)
Accumulated removal costs	(404,000)		(395,000)

Unamortized gain on reacquired debt (6)	(7,644)		(8,181)
Regulatory excess deferred/other taxes and gross-up (7)	(461,023)		(455,625)
Margin, interest- and property tax-tracking (9)	(17,132)		(22,650)
Other (10)	(3,503)		(4,438)
Net regulatory liabilities	$	(406,291)	$	(363,890)

(1)Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovery period is greater than five years. (See Note 11 - Pension and Other Postretirement Benefits).

(3)Balance recovered or refunded on an ongoing basis with interest.

(4)Included in Prepaid and other current assets on the Consolidated Balance Sheets. Balance recovered or refunded on an ongoing basis.

(5)Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovered over life of debt instruments.

(6)Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. Amortized over life of debt instruments.

(8)Regulatory recovery occurs on a one-year lag basis through the labor loading process. Included in Prepaid and other current assets on the Consolidated Balance Sheets.


	December 31,
(Thousands of dollars)	2020		2019
Other Regulatory Assets:
State mandated public purpose programs (including low income and conservation programs) (a) (e)	$	11,527	$	9,172
Infrastructure replacement programs and similar (b) (e)	7,731		8,236
Environmental compliance programs (c) (e)	5,235		5,768
Pension tracking mechanism (b)	3,075		—
Other (d)	18,028		9,958
	$	45,596	$	33,134

a)Included in Prepaid and other current assets on the Consolidated Balance Sheets.

b)Included in Deferred charges and other assets on the Consolidated Balance Sheets.

e)Balance recovered or refunded on an ongoing basis, generally with interest.


	December 31,
(Thousands of dollars)	2020		2019
Other Regulatory Liabilities:
State mandated public purpose programs (including low income and conservation programs) (a) (c)	$	(834)	$	(308)
Environmental compliance programs (c) (d)	(405)		(527)
Pension tracking mechanism (b)	—		(2,476)
Other (b) (c)	(2,264)		(1,127)
	$	(3,503)	$	(4,438)

a)Included in Other current liabilities, except for $339,000 which is included in Other deferred credits and long-term liabilities on the Consolidated Balance Sheets.

b)Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.

c)Balance typically recovered or refunded on an ongoing basis, generally with interest.

d)In 2020 and 2019, included in Other current liabilities on the Consolidated Balance Sheet.

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") (Tables)

12 Months Ended

Dec. 31, 2020

Schedule of Rollforward of Accumulated Other Comprehensive Income

Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss)


	Year Ended December 31,
	2020						2019						2018
(Thousands of dollars)	Before- Tax Amount		Tax (Expense) or Benefit (1)		Net-of- Tax Amount		Before- Tax Amount		Tax (Expense) or Benefit (1)		Net-of- Tax Amount		Before- Tax Amount		Tax (Expense) or Benefit (1)		Net-of- Tax Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(57,539)	$	13,809	$	(43,730)	$	(71,087)	$	17,061	$	(54,026)	$	(20,426)	$	4,902	$	(15,524)
Amortization of prior service cost	1,155		(277)		878		1,271		(305)		966		1,335		(320)		1,015
Amortization of net actuarial (gain)/loss	37,830		(9,079)		28,751		23,376		(5,610)		17,766		33,617		(8,068)		25,549
Prior service cost	—		—		—		(1,878)		452		(1,426)		—		—		—
Regulatory adjustment	7,435		(1,785)		5,650		36,944		(8,867)		28,077		(8,233)		1,976		(6,257)
Pension plans other comprehensive income (loss)	(11,119)		2,668		(8,451)		(11,374)		2,731		(8,643)		6,293		(1,510)		4,783
FSIRS (designated hedging activities):
Amounts reclassified into net income	3,247		(780)		2,467		3,344		(803)		2,541		3,345		(804)		2,541
FSIRS other comprehensive income (loss)	3,247		(780)		2,467		3,344		(803)		2,541		3,345		(804)		2,541
Total other comprehensive income (loss) –Southwest Gas Corporation	(7,872)		1,888		(5,984)		(8,030)		1,928		(6,102)		9,638		(2,314)		7,324
Foreign currency translation adjustments:
Translation adjustments	1,713		—		1,713		2,038		—		2,038		(3,010)		—		(3,010)
Foreign currency other comprehensive income (loss)	1,713		—		1,713		2,038		—		2,038		(3,010)		—		(3,010)
Total other comprehensive income (loss) – Southwest Gas Holdings, Inc.	$	(6,159)	$	1,888	$	(4,271)	$	(5,992)	$	1,928	$	(4,064)	$	6,628	$	(2,314)	$	4,314

The following table represents a rollforward of AOCI, presented on the Company’s Consolidated Balance Sheets and its Consolidated Statements of Equity:


	Defined Benefit Plans						FSIRS						Foreign Currency Items
(Thousands of dollars)	Before-Tax		Tax (Expense) Benefit (4)		After-Tax		Before- Tax		Tax (Expense) Benefit (4)		After-Tax		Before- Tax		Tax (Expense) Benefit		After-Tax		AOCI
Beginning Balance AOCI December 31, 2019	$	(66,601)	$	15,985	$	(50,616)	$	(5,966)	$	1,431	$	(4,535)	$	(1,581)	$	—	$	(1,581)	$	(56,732)
Net actuarial gain/(loss)	(57,539)		13,809		(43,730)		—		—		—		—		—		—		(43,730)
Translation adjustments	—		—		—		—		—		—		1,713		—		1,713		1,713
Other comprehensive income before reclassifications	(57,539)		13,809		(43,730)		—		—		—		1,713		—		1,713		(42,017)
FSIRS amounts reclassified from AOCI (1)	—		—		—		3,247		(780)		2,467		—		—		—		2,467
Amortization of prior service cost (2)	1,155		(277)		878		—		—		—		—		—		—		878
Amortization of net actuarial loss (2)	37,830		(9,079)		28,751		—		—		—		—		—		—		28,751

Regulatory adjustment (3)	7,435		(1,785)		5,650		—		—		—				—		—		5,650
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.	(11,119)		2,668		(8,451)		3,247		(780)		2,467		1,713		—		1,713		(4,271)
Ending Balance AOCI December 31, 2020	$	(77,720)	$	18,653	$	(59,067)	$	(2,719)	$	651	$	(2,068)	$	132	$	—	$	132	$	(61,003)

(1)The FSIRS reclassification amounts are included in Net interest deductions on the Company’s Consolidated Statements of Income.

(2)These AOCI components are included in the computation of net periodic benefit cost (see Note 11 - Pension and Other Postretirement Benefits for additional details).

(4) Tax amounts are calculated using a 24% rate.

The following table represents a rollforward of AOCI, presented on Southwest’s Consolidated Balance Sheets:


	Defined Benefit Plans						FSIRS
(Thousands of dollars)	Before-Tax		Tax (Expense) Benefit (9)		After- Tax		Before- Tax		Tax (Expense) Benefit (9)		After- Tax		AOCI
Beginning Balance AOCI December 31, 2019	$	(66,601)	$	15,985	$	(50,616)	$	(5,966)	$	1,431	$	(4,535)	$	(55,151)
Net actuarial gain/(loss)	(57,539)		13,809		(43,730)		—		—		—		(43,730)
Other comprehensive loss before reclassifications	(57,539)		13,809		(43,730)		—		—		—		(43,730)
FSIRS amounts reclassified from AOCI (6)	—		—		—		3,247		(780)		2,467		2,467
Amortization of prior service cost (7)	1,155		(277)		878		—		—		—		878
Amortization of net actuarial loss (7)	37,830		(9,079)		28,751		—		—		—		28,751

Regulatory adjustment (8)	7,435		(1,785)		5,650		—		—		—		5,650
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation	(11,119)		2,668		(8,451)		3,247		(780)		2,467		(5,984)
Ending Balance AOCI December 31, 2020	$	(77,720)	$	18,653	$	(59,067)	$	(2,719)	$	651	$	(2,068)	$	(61,135)

(6) The FSIRS reclassification amounts are included in Net interest deductions on Southwest’s Consolidated Statements of Income.

(7) These AOCI components are included in the computation of net periodic benefit cost (see Note 11 - Pension and Other Postretirement Benefits for additional details).

(9) Tax amounts are calculated using a 24% rate.

Schedule of Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income

The following table represents amounts (before income tax impacts) included in AOCI (in the tables above), that have not yet been recognized in net periodic benefit cost:


	Year Ended December 31,
(Thousands of dollars)	2020		2019
Net actuarial loss	$	(502,783)	$	(483,074)
Prior service cost	(2,487)		(3,641)
Less: amount recognized in regulatory assets	427,550		420,114
Recognized in AOCI	$	(77,720)	$	(66,601)

Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income


	Year Ended December 31,
	2020								2019								2018
(Thousands of dollars)	Total		Qualified Retirement Plan		SERP		PBOP		Total		Qualified Retirement Plan		SERP		PBOP		Total		Qualified Retirement Plan		SERP		PBOP
Net actuarial loss (gain) (a)	$	57,539	$	45,665	$	7,240	$	4,634	$	71,087	$	66,557	$	7,975	$	(3,445)	$	20,426	$	23,607	$	(3,940)	$	759
Amortization of prior service cost (b)	(1,155)		—		—		(1,155)		(1,271)		—		—		(1,271)		(1,335)		—		—		(1,335)
Amortization of net actuarial loss (b)	(37,830)		(36,025)		(1,805)		—		(23,376)		(22,356)		(1,020)		—		(33,617)		(32,115)		(1,502)		—
Prior service cost	—		—		—		—		1,878		—		—		1,878		—		—		—		—
Regulatory adjustment	(7,435)		(3,956)		—		(3,479)		(36,944)		(39,782)		—		2,838		8,233		7,657		—		576
Recognized in other comprehensive (income) loss	11,119		5,684		5,435		—		11,374		4,419		6,955		—		(6,293)		(851)		(5,442)		—
Net periodic benefit costs recognized in net income	56,291		50,583		3,798		1,910		42,465		36,982		3,046		2,437		51,332		46,089		3,405		1,838
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$	67,410	$	56,267	$	9,233	$	1,910	$	53,839	$	41,401	$	10,001	$	2,437	$	45,039	$	45,238	$	(2,037)	$	1,838

Common Stock (Tables)

12 Months Ended

Dec. 31, 2020

Schedule of Common Stock Activity

The following table provides the activity in the Equity Shelf Program for the three-month and life-to-date periods ended December 31, 2020:


	Three Months Ended		Life-To-Date Ended
	December 31, 2020
Gross proceeds	$	46,425,623	$	253,551,490
Less: agent commissions	(464,256)		(2,535,515)
Net proceeds	$	45,961,367	$	251,015,975
Number of shares sold	691,081		3,396,457
Weighted average price per share	$	67.18	$	74.65

Debt (Tables)

12 Months Ended

Dec. 31, 2020

Debt Disclosure [Abstract]

Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt


	December 31,
	2020				2019
	Carrying Amount		Fair Value		Carrying Amount		Fair Value
(Thousands of dollars)
Southwest Gas Corporation:
Debentures:
Notes, 4.45%, due 2020	$	—	$	—	$	125,000	$	126,673
Notes, 6.1%, due 2041	125,000		174,858		125,000		162,666
Notes, 3.875%, due 2022	250,000		258,825		250,000		258,550
Notes, 4.875%, due 2043	250,000		317,190		250,000		291,928
Notes, 3.8%, due 2046	300,000		347,046		300,000		308,307
Notes, 3.7%, due 2028	300,000		344,553		300,000		320,685
Notes, 4.15%, due 2049	300,000		370,278		300,000		330,138
Notes, 2.2%, due 2030	450,000		474,552		—		—
8% Series, due 2026	75,000		99,723		75,000		96,905
Medium-term notes, 7.78% series, due 2022	25,000		26,663		25,000		27,500
Medium-term notes, 7.92% series, due 2027	25,000		33,802		25,000		32,543
Medium-term notes, 6.76% series, due 2027	7,500		9,613		7,500		9,156
Unamortized discount and debt issuance costs	(17,822)				(14,450)
	2,089,678				1,768,050
Revolving credit facility and commercial paper	150,000		150,000		150,000		150,000
Industrial development revenue bonds:
Tax-exempt Series A, due 2028	50,000		50,000		50,000		50,000
2003 Series A, due 2038	50,000		50,000		50,000		50,000
2008 Series A, due 2038	50,000		50,000		50,000		50,000
2009 Series A, due 2039	50,000		50,000		50,000		50,000
Unamortized discount and debt issuance costs	(1,472)				(1,717)
	198,528				198,283
Less: current maturities	—				(125,000)
Long-term debt, less current maturities – Southwest Gas Corporation	$	2,438,206			$	1,991,333
Centuri:
Centuri term loan facility	$	226,648	230,824		$	244,812	252,182
Unamortized debt issuance costs	(820)				(1,101)
	225,828				243,711
Centuri secured revolving credit facility	26,626		26,645		60,021		60,057
Centuri other debt obligations	81,973		84,246		43,929		44,787
Less: current maturities	(40,433)				(38,512)
Long-term debt, less current maturities – Centuri	$	293,994			$	309,149
Consolidated Southwest Gas Holdings, Inc.:
Southwest Gas Corporation long-term debt	$	2,438,206			$	2,116,333
Centuri long-term debt	334,427				347,661
Less: current maturities	(40,433)				(163,512)
Long-term debt, less current maturities – Southwest Gas Holdings, Inc.	$	2,732,200			$	2,300,482

Summary of Effective Interest Rates on Variable-Rate IDRBs

The effective interest rates on Southwest’s variable-rate IDRBs are included in the table below:


	December 31,
	2020		2019
2003 Series A	0.80	%	2.51	%
2008 Series A	0.83	%	2.46	%
2009 Series A	0.76	%	2.37	%
Tax-exempt Series A	0.87	%	2.32	%

Estimated Maturities of Long-Term Debt

Estimated maturities of long-term debt for the next five years are:


(Thousands of dollars)	Southwest		Centuri		Total
2021	$	—	$	40,433	$	40,433
2022	275,000		45,433		320,433
2023	—		219,638		219,638
2024	—		11,284		11,284
2025	150,000		9,076		159,076

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

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Share-based Payment Arrangement [Abstract]

Share-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Schedule of Share-Based Plan Compensation Expense, Including Cash Award

The table below shows total share-based plan compensation expense which was recognized in the Consolidated Statements of Income:


	Year Ended December 31,
(Thousands of dollars)	2020		2019		2018
Share-based compensation plan expense, net of related tax benefits	$	4,816	$	5,154	$	4,644
Share-based compensation plan related tax benefits	1,521		1,627		1,467

Schedule of Nonvested Performance and Restricted Stock Unit Plans

The following table summarizes the activity of the management incentive plan shares and restricted stock/units as of December 31, 2020:


(thousands of shares)	Management Incentive Plan Shares	Weighted- average grant date fair value		Restricted Stock/ Units (1)	Weighted- average grant date fair value
Nonvested/unissued at December 31, 2019	29	$	79.16	365	$	60.94
Granted	—	—		129	76.85
Dividends	—			8
Forfeited or expired	—	—		—	—
Vested and issued (2)	(29)	79.16		(101)	71.06
Nonvested/unissued at December 31, 2020	—	$	—	401	$	62.23

(2)Includes shares for retiree payouts and those converted for taxes.

Pension and Other Postretirement Benefits (Tables)

12 Months Ended

Dec. 31, 2020

Retirement Benefits [Abstract]

Schedule of Assumptions Used

The rates are presented in the table below:


	December 31,
	2020		2019
Discount rate	2.75	%	3.50	%
Weighted-average rate of compensation increase	3.00	%	3.25	%
Asset return assumption	6.50	%	6.75	%

Schedule of Defined Benefit Plans Disclosures

The following table sets forth the retirement plan, SERP, and PBOP funded statuses and amounts recognized on the Consolidated Balance Sheets and Consolidated Statements of Income.


	Year Ended December 31,
	2020									2019
(Thousands of dollars)	Qualified Retirement Plan			SERP			PBOP			Qualified Retirement Plan			SERP			PBOP
Change in benefit obligations:
Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$	1,329,577		$	47,397		$	76,111		$	1,116,014		$	40,603		$	69,956
Service cost	34,299			389			1,581			25,864			266			1,276
Interest cost	45,555			1,604			2,582			49,006			1,760			3,046
Plan amendments	—			—			—			—			—			1,878
Actuarial loss (gain)	145,440			7,240			6,547			192,416			7,974			3,156
Benefits paid	(55,632)			(2,999)			(4,616)			(53,723)			(3,206)			(3,201)
Benefit obligation at end of year (PBO/PBO/APBO)	1,499,239			53,631			82,205			1,329,577			47,397			76,111
Change in plan assets:
Market value of plan assets at beginning of year	974,993			—			52,838			790,614			—			47,341
Actual return on plan assets	165,072			—			5,320			186,102			—			9,757
Employer contributions	102,000			2,999			—			52,000			3,206			—
Benefits paid	(55,632)			(2,999)			(5,872)			(53,723)			(3,206)			(4,260)
Market value of plan assets at end of year	1,186,433			—			52,286			974,993			—			52,838
Funded status at year end	$	(312,806)		$	(53,631)		$	(29,919)		$	(354,584)		$	(47,397)		$	(23,273)
Weighted-average assumptions (benefit obligation):
Discount rate	2.75		%	2.75		%	2.75		%	3.50		%	3.50		%	3.50		%
Weighted-average rate of compensation increase	3.00		%	3.00		%	N/A			3.25		%	3.25		%	N/A

Schedule of Accumulated Benefit Obligation

The accumulated benefit obligation for the retirement plan and the SERP is presented below:


	December 31,
(Thousands of dollars)	2020		2019
Retirement plan	$	1,367,179	$	1,219,989
SERP	50,471		46,067

Schedule of Expected Benefit Payments

Benefits expected to be paid for pension, SERP, and PBOP over the next 10 years are as follows:


(Millions of dollars)	2021		2022		2023		2024		2025		2026-2030
Pension	$	58.0	$	60.0	$	61.0	$	62.0	$	63.0	$	339.0
SERP	3.2		3.2		3.2		3.2		3.1		15.1
PBOP	4.9		5.0		4.9		4.9		4.9		24.0

Schedule of Net Benefit Cost

Components of net periodic benefit cost:


	Qualified Retirement Plan									SERP									PBOP
(Thousands of dollars)	2020			2019			2018			2020			2019			2018			2020			2019			2018
Service cost	$	34,299		$	25,864		$	28,555		$	389		$	266		$	245		$	1,581		$	1,276		$	1,473
Interest cost	45,555			49,006			44,174			1,604			1,760			1,658			2,582			3,046			2,748
Expected return on plan assets	(65,296)			(60,244)			(58,755)			—			—			—			(3,408)			(3,156)			(3,718)
Amortization of prior service cost	—			—			—			—			—			—			1,155			1,271			1,335
Amortization of net actuarial loss	36,025			22,356			32,115			1,805			1,020			1,502			—			—			—
Net periodic benefit cost	$	50,583		$	36,982		$	46,089		$	3,798		$	3,046		$	3,405		$	1,910		$	2,437		$	1,838
Weighted-average assumptions (net benefit cost)
Discount rate	3.50		%	4.50		%	3.75		%	3.50		%	4.50		%	3.75		%	3.50		%	4.50		%	3.75		%
Expected return on plan assets	6.75		%	7.00		%	7.00		%	N/A			N/A			N/A			6.75		%	7.00		%	7.00		%
Weighted-average rate of compensation increase	3.25		%	3.25		%	3.25		%	3.25		%	3.25		%	3.25		%	N/A			N/A			N/A

Schedule of Amounts Recognized in Other Comprehensive Income

The following table represents amounts (before income tax impacts) included in AOCI (in the tables above), that have not yet been recognized in net periodic benefit cost:


	Year Ended December 31,
(Thousands of dollars)	2020		2019
Net actuarial loss	$	(502,783)	$	(483,074)
Prior service cost	(2,487)		(3,641)
Less: amount recognized in regulatory assets	427,550		420,114
Recognized in AOCI	$	(77,720)	$	(66,601)

Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income


	Year Ended December 31,
	2020								2019								2018
(Thousands of dollars)	Total		Qualified Retirement Plan		SERP		PBOP		Total		Qualified Retirement Plan		SERP		PBOP		Total		Qualified Retirement Plan		SERP		PBOP
Net actuarial loss (gain) (a)	$	57,539	$	45,665	$	7,240	$	4,634	$	71,087	$	66,557	$	7,975	$	(3,445)	$	20,426	$	23,607	$	(3,940)	$	759
Amortization of prior service cost (b)	(1,155)		—		—		(1,155)		(1,271)		—		—		(1,271)		(1,335)		—		—		(1,335)
Amortization of net actuarial loss (b)	(37,830)		(36,025)		(1,805)		—		(23,376)		(22,356)		(1,020)		—		(33,617)		(32,115)		(1,502)		—
Prior service cost	—		—		—		—		1,878		—		—		1,878		—		—		—		—
Regulatory adjustment	(7,435)		(3,956)		—		(3,479)		(36,944)		(39,782)		—		2,838		8,233		7,657		—		576
Recognized in other comprehensive (income) loss	11,119		5,684		5,435		—		11,374		4,419		6,955		—		(6,293)		(851)		(5,442)		—
Net periodic benefit costs recognized in net income	56,291		50,583		3,798		1,910		42,465		36,982		3,046		2,437		51,332		46,089		3,405		1,838
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$	67,410	$	56,267	$	9,233	$	1,910	$	53,839	$	41,401	$	10,001	$	2,437	$	45,039	$	45,238	$	(2,037)	$	1,838

Schedule of Fair Value of Plan Assets


	December 31,
	2020						2019
(Thousands of dollars)	Qualified Retirement Plan		PBOP		Total		Qualified Retirement Plan		PBOP		Total
Assets at fair value:
Level 1 – Quoted prices in active markets for identical financial assets
Mutual funds	$	—	$	30,358	$	30,358	$	—	$	29,188	$	29,188
Total Level 1 Assets (1)	—		30,358		30,358		—		29,188		29,188
Level 2 – Significant other observable inputs
Private commingled equity funds (2)
Global	324,084		5,878		329,962		266,908		6,338		273,246
International	141,290		2,563		143,853		117,086		2,780		119,866
U.S. equity securities	223,374		4,051		227,425		184,642		4,386		189,028
Emerging markets	76,679		1,391		78,070		62,943		1,494		64,437
Private commingled fixed income funds (3)	412,230		7,476		419,706		335,138		7,959		343,097
Pooled funds and mutual funds	5,990		565		6,555		5,359		689		6,048
Government fixed income and mortgage backed securities	201		4		205		181		4		185
Total Level 2 assets (4)	1,183,848		21,928		1,205,776		972,257		23,650		995,907
Total Plan assets at fair value	1,183,848		52,286		1,236,134		972,257		52,838		1,025,095
Insurance company general account contracts (5)	2,585		—		2,585		2,736		—		2,736
Total Plan assets	$	1,186,433	$	52,286	$	1,238,719	$	974,993	$	52,838	$	1,027,831

▪Global equities, including domestic equities

▪International developed countries equities

▪Domestic equities

▪Emerging markets equities

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Summary of Income Before Taxes and Noncontrolling Interest for Domestic and Foreign Operations

The following is a summary of income before taxes and noncontrolling interest for domestic and foreign operations:


	Year ended December 31,
(Thousands of dollars)	2020		2019		2018
U.S.	$	282,489	$	261,525	$	235,120
Foreign	22,249		11,145		8,216
Total income before income taxes	$	304,738	$	272,670	$	243,336

Summary of Income Tax Expense (Benefit)

Income tax expense (benefit) consists of the following:


	Year Ended December 31,
(Thousands of dollars)	2020		2019		2018
Current:
Federal	$	6,287	$	622	$	(13,476)
State	8,617		(1,510)		(3,219)
Foreign	4,666		5,013		2,563
	19,570		4,125		(14,132)
Deferred:
Federal	44,547		45,593		67,784
State	414		8,212		8,901
Foreign	1,222		(1,907)		(869)
	46,183		51,898		75,816
Total income tax expense	$	65,753	$	56,023	$	61,684

Income tax expense (benefit) consists of the following:


	Year Ended December 31,
(Thousands of dollars)	2020		2019		2018
Current:
Federal	$	(4,678)	$	4,109	$	(17,584)
State	(179)		250		(6,783)
	(4,857)		4,359		(24,367)
Deferred:
Federal	38,561		29,543		58,136
State	2,051		1,071		10,222
	40,612		30,614		68,358
Total income tax expense	$	35,755	$	34,973	$	43,991

Significant Components of Deferred Income Tax Expense (Benefit)

Deferred income tax expense (benefit) consists of the following significant components:


	Year Ended December 31,
(Thousands of dollars)	2020		2019		2018
Deferred federal and state:
Property-related items	$	50,504	$	60,449	$	94,899
Purchased gas cost adjustments	(5,726)		3,834		(3,507)
Employee benefits	459		7,680		(7,334)
Regulatory adjustments	(9,885)		(11,962)		2,412
Deferred payroll taxes	(9,055)		—		—
Alternative minimum tax	4,409		441		849
All other deferred	15,529		(8,298)		(10,890)
Total deferred federal and state	46,235		52,144		76,429
Deferred ITC, net	(52)		(246)		(613)
Total deferred income tax expense	$	46,183	$	51,898	$	75,816

Deferred income tax expense (benefit) consists of the following significant components:


	Year Ended December 31,
(Thousands of dollars)	2020		2019		2018
Deferred federal and state:
Property-related items	$	36,029	$	34,398	$	67,576
Purchased gas cost adjustments	(5,726)		3,834		(3,507)
Employee benefits	11,437		6,493		2,156
Regulatory adjustments	(9,885)		(11,962)		2,412
Deferred payroll taxes	(1,810)		—		—
Alternative minimum tax	4,409		441		849
All other deferred	6,210		(2,344)		(515)
Total deferred federal and state	40,664		30,860		68,971
Deferred ITC, net	(52)		(246)		(613)
Total deferred income tax expense	$	40,612	$	30,614	$	68,358

Reconciliation of U.S Federal Statutory Rate to Consolidated Effective Tax Rate

A reconciliation of the U.S. federal statutory rate to the consolidated effective tax rate (and the sources of these differences and the effect of each) are summarized as follows:


	Year Ended December 31,
	2020		2019		2018
U.S. federal statutory income tax rate	21.0	%	21.0	%	21.0	%
Net state taxes	3.0		2.1		2.9
Tax credits	(0.5)		(0.3)		(0.3)
Company-owned life insurance	(0.8)		(1.5)		0.1

Amortization of excess deferred taxes	(0.8)		(0.9)		—
All other differences	(0.3)		0.1		1.6
Consolidated effective income tax rate	21.6	%	20.5	%	25.3	%

A reconciliation of the U.S. federal statutory rate to the consolidated effective tax rate (and the sources of these differences and the effect of each) are summarized as follows:


	Year Ended December 31,
	2020		2019		2018
U.S. federal statutory income tax rate	21.0	%	21.0	%	21.0	%
Net state taxes	1.7		0.7		2.1
Tax credits	(0.7)		(0.4)		(0.4)
Company-owned life insurance	(1.0)		(1.9)		0.3

Amortization of excess deferred taxes	(1.3)		(1.2)		—
All other differences	(1.4)		(0.5)		1.1
Effective income tax rate	18.3	%	17.7	%	24.1	%

Deferred Tax Assets and Liabilities

Deferred tax assets and liabilities consist of the following:


	December 31,
(Thousands of dollars)	2020		2019
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	$	104,314	$	105,077
Employee benefits	39,907		37,439
Alternative minimum tax credit	—		4,409
Federal net operating losses	4,118		7,467
Deferred payroll taxes	9,055		—
Lease-related item	20,890		21,226
Other	14,350		21,536
Valuation allowance	(22)		(25)
	192,612		197,129
Deferred tax liabilities:
Property-related items, including accelerated depreciation	785,734		732,798
Regulatory balancing accounts	4,205		9,931

Debt-related costs	2,585		2,818
Intangibles	13,511		10,611
Lease-related item	19,789		20,386
Other	13,786		19,569
	839,610		796,113
Net noncurrent deferred tax liabilities	$	646,998	$	598,984

Deferred tax assets and liabilities consist of the following:


	December 31,
(Thousands of dollars)	2020		2019
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	$	104,314	$	105,077
Employee benefits	4,806		13,574
Alternative minimum tax credit	—		4,409
Deferred payroll taxes	1,810		—
Other	7,790		12,193
Valuation allowance	(22)		(25)
	118,698		135,228
Deferred tax liabilities:
Property-related items, including accelerated depreciation	680,294		644,046
Regulatory balancing accounts	4,205		9,931

Debt-related costs	2,585		2,818
Other	12,714		17,483
	699,798		674,278
Net deferred tax liabilities	$	581,100	$	539,050

Reconciliation of Unrecognized Tax Benefits

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:


	December 31,
(Thousands of dollars)	2020		2019
Unrecognized tax benefits at beginning of year	$	1,056	$	971
Gross increases – tax positions in prior period	641		85
Gross decreases – tax positions in prior period	—		—
Gross increases – current period tax positions	231		—
Gross decreases – current period tax positions	—		—
Settlements	—		—
Lapse in statute of limitations	—		—
Unrecognized tax benefits at end of year	$	1,928	$	1,056

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:


	December 31,
(Thousands of dollars)	2020		2019
Unrecognized tax benefits at beginning of year	$	1,056	$	971
Gross increases – tax positions in prior period	506		85
Gross decreases – tax positions in prior period	—		—
Gross increases – current period tax positions	231		—
Gross decreases – current period tax positions	—		—
Settlements	—		—
Lapse in statute of limitations	—		—
Unrecognized tax benefits at end of year	$	1,793	$	1,056

Summary of Income Before Taxes for Continuing and Discontinued Operations

The following is a summary of income before taxes:


	Year ended December 31,
(Thousands of dollars)	2020		2019		2018
Total income before income taxes	$	194,873	$	198,144	$	182,833

Segment Information (Tables)

12 Months Ended

Dec. 31, 2020

Segment Reporting [Abstract]

Schedule of Accounts Receivable for Services

Accounts receivable for these services, which are not eliminated during consolidation, are presented in the table below:


	December 31,
(Thousands of dollars)	2020		2019
Accounts receivable for Centuri services	$	13,956	$	15,235

Schedule of Revenues by Geographic Area

The following table presents the amount of revenues for both segments by geographic area:


	December 31,
(Thousands of dollars)	2020		2019		2018
Revenues (a)
United States	$	3,057,041	$	2,893,201	$	2,664,670
Canada	241,832		226,716		215,343
Total	$	3,298,873	$	3,119,917	$	2,880,013

(a)Revenues are attributed to countries based on the location of customers.

Schedule of Segment Reporting Information

The financial information pertaining to the natural gas operations and utility infrastructure services segments for each of the three years in the period ended December 31, 2020 is as follows:


	Year Ended December 31, 2020
(Thousands of dollars)	Natural Gas Operations		Utility Infrastructure Services		Other		Total
Revenues from external customers	$	1,350,585	$	1,813,429	$	—	$	3,164,014
Intersegment sales	—		134,859		—		134,859
Total	$	1,350,585	$	1,948,288	$	—	$	3,298,873
Interest income	$	4,015	$	—	$	—	$	4,015
Interest expense	$	101,148	$	9,269	$	1,060	$	111,477
Depreciation and amortization	$	235,295	$	96,732	$	—	$	332,027
Income tax expense	$	35,755	$	31,128	$	(1,130)	$	65,753
Segment net income	$	159,118	$	74,862	$	(1,656)	$	232,324
Segment assets	$	7,256,636	$	1,475,237	$	3,980	$	8,735,853
Capital expenditures	$	692,216	$	132,889	$	—	$	825,105


	Year Ended December 31, 2019
(Thousands of dollars)	Natural Gas Operations		Utility Infrastructure Services		Other		Total
Revenues from external customers	$	1,368,939	$	1,592,252	$	—	$	2,961,191
Intersegment sales	—		158,726		—		158,726
Total	$	1,368,939	$	1,750,978	$	—	$	3,119,917
Interest income	$	6,356	$	—	$	—	$	6,356
Interest expense	$	95,026	$	14,086	$	114	$	109,226
Depreciation and amortization	$	215,620	$	87,617	$	—	$	303,237
Income tax expense	$	34,973	$	21,399	$	(349)	$	56,023
Segment net income	$	163,171	$	52,404	$	(1,639)	$	213,936
Segment assets	$	6,798,746	$	1,365,194	$	6,108	$	8,170,048
Capital expenditures	$	778,748	$	159,400	$	—	$	938,148


	Year Ended December 31, 2018
(Thousands of dollars)	Natural Gas Operations		Utility Infrastructure Services		Other		Total
Revenues from external customers	$	1,357,728	$	1,386,371	$	—	$	2,744,099
Intersegment sales	—		135,914		—		135,914
Total	$	1,357,728	$	1,522,285	$	—	$	2,880,013
Interest income	$	6,020	$	88	$	—	$	6,108
Interest expense	$	81,740	$	14,190	$	741	$	96,671
Depreciation and amortization	$	191,816	$	57,396	$	—	$	249,212
Income tax expense	$	43,991	$	18,420	$	(727)	$	61,684
Segment net income	$	138,842	$	44,977	$	(1,542)	$	182,277
Segment assets	$	6,141,584	$	1,215,573	$	572	$	7,357,729
Capital expenditures	$	682,869	$	83,045	$	—	$	765,914

Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Data

The following table presents summarized quarterly financial data for 2020 and 2019:


	Quarter Ended
	March 31		June 30		September 30		December 31
(Thousands of dollars, except per share amounts)
2020
Southwest Gas Holdings, Inc.:
Operating revenues	$	836,320	$	757,247	$	791,226	$	914,080
Operating income	148,373		67,572		54,264		152,795
Net income	73,005		39,881		21,063		105,036
Net income attributable to Southwest Gas Holdings, Inc.	72,542		37,965		18,273		103,544
Basic earnings per common share (1)	$	1.31	$	0.68	$	0.32	$	1.82
Diluted earnings per common share (1)	$	1.31	$	0.68	$	0.32	$	1.82

Southwest Gas Corporation:
Operating revenues	$	502,827	$	262,434	$	210,834	$	374,490
Operating income	157,815		27,101		1,625		116,070
Net income (loss)	83,599		11,942		(15,973)		79,550

2019
Southwest Gas Holdings, Inc.:
Operating revenues	$	833,539	$	713,011	$	725,230	$	848,137
Operating income	140,480		54,869		38,258		138,204
Net income	95,384		22,832		6,525		91,906
Net income attributable to Southwest Gas Holdings, Inc.	94,809		22,056		5,353		91,718
Basic earnings per common share (1)	$	1.78	$	0.41	$	0.10	$	1.67
Diluted earnings per common share (1)	$	1.77	$	0.41	$	0.10	$	1.67

Southwest Gas Corporation:
Operating revenues	$	520,677	$	258,711	$	209,980	$	379,571
Operating income (loss)	148,713		24,069		(1,807)		112,678
Net income (loss)	103,389		3,369		(20,012)		76,425

(1)The sum of quarterly earnings (loss) per average common share may not equal the annual earnings (loss) per share due to the ongoing change in the weighted-average number of common shares.

Redeemable Noncontrolling Interest (Tables)

12 Months Ended

Dec. 31, 2020

Noncontrolling Interest [Abstract]

Summary of Redeemable Noncontrolling Interest

The following depicts changes to the balance of the redeemable noncontrolling interest:


	Redeemable Noncontrolling Interest
(Thousands of dollars)
Balance, December 31, 2018	$	81,831
Net income attributable to redeemable noncontrolling interest	2,711
Balance, December 31, 2019	84,542
Net income attributable to redeemable noncontrolling interest	6,661
Redemption value adjustment	74,513
Balance, December 31, 2020	$	165,716

Background, Organization, and Summary of Significant Accounting Policies - Narrative (Details)	1 Months Ended	12 Months Ended
	Feb. 25, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2020 instrument	Nov. 30, 2018
Significant Accounting Policies [Line Items]
Amortization expenses		$ 10,800,000	$ 10,700,000	$ 7,600,000
Cumulative foreign earnings		$ 47,000,000
Deferred purchased gas costs, recovered or refunded period		1 year
Goodwill impairment charges		$ 0	0
Dividends declared		$ 32,600,000	30,000,000
Minimum \| Subsequent Event
Significant Accounting Policies [Line Items]
Deferred purchased gas costs, estimated incremental costs	$ 200,000,000
Maximum \| Subsequent Event
Significant Accounting Policies [Line Items]
Deferred purchased gas costs, estimated incremental costs	$ 300,000,000
Interest Rate Swap
Significant Accounting Policies [Line Items]
Derivative, number of instruments held \| instrument					2
Centuri \| W.S. Nicholls Western Construction LTD
Significant Accounting Policies [Line Items]
VIR ownership percentage		50.00%
Exposure to loss as a result of its investment		$ 12,200,000
Money Market Funds
Significant Accounting Policies [Line Items]
Money market fund investments		40,000	26,700,000
Money Market Funds \| Southwest Gas Corporation
Significant Accounting Policies [Line Items]
Money market fund investments		40,000	23,500,000
Gas Pipe Materials and Supplies \| Prepaids and other current assets
Significant Accounting Policies [Line Items]
Gas pipe materials and operating supplies		50,000,000	57,000,000
Prepaid and other current assets		$ 1,000,000	$ 33,000,000
Linetec
Significant Accounting Policies [Line Items]
Ownership percentage by noncontrolling owners						20.00%

Background, Organization, and Summary of Significant Accounting Policies - Schedule of Other Property and Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Other property	$ 23,961	$ 18,814
Total	834,245	784,173
Southwest Gas Corporation
Property, Plant and Equipment [Line Items]
Net cash surrender value of COLI policies	140,874	132,072
Other property	2,737	1,715
Total	143,611	133,787
Centuri
Property, Plant and Equipment [Line Items]
Centuri property, equipment, and intangibles	1,089,414	983,905
Centuri accumulated provision for depreciation and amortization	$ (422,741)	$ (352,333)

Background, Organization, and Summary of Significant Accounting Policies - Summary of Intangible Assets (Details) - Centuri - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 180,576	$ 179,854
Accumulated Amortization	(40,392)	(29,343)
Net Carrying Amount	140,184	150,511
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	154,757	154,186
Accumulated Amortization	(29,237)	(20,735)
Net Carrying Amount	125,520	133,451
Trade names and trademarks
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	23,618	23,353
Accumulated Amortization	(8,954)	(6,754)
Net Carrying Amount	14,664	16,599
Customer contracts backlog
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	270	270
Accumulated Amortization	(270)	(252)
Net Carrying Amount	0	18
Noncompete agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,931	2,045
Accumulated Amortization	(1,931)	(1,602)
Net Carrying Amount	$ 0	$ 443

Background, Organization, and Summary of Significant Accounting Policies - Schedule of Estimated Future Amortization of Intangible Assets (Details) - Centuri - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
2021	$ 10,343
2022	10,255
2023	10,255
2024	10,255
2025	10,211
Thereafter	88,865
Net Carrying Amount	$ 140,184	$ 150,511

Background, Organization, and Summary of Significant Accounting Policies - Schedule of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Goodwill, Beginning balance	$ 343,023	$ 359,045
Foreign currency translation adjustment	2,161	5,150
Goodwill, Ending balance	345,184	343,023
Linetec
Goodwill [Roll Forward]
Goodwill from acquisition		(21,172)
Natural Gas Operations
Goodwill [Roll Forward]
Goodwill, Beginning balance	10,095	10,095
Foreign currency translation adjustment	0	0
Goodwill, Ending balance	10,095	10,095
Natural Gas Operations \| Linetec
Goodwill [Roll Forward]
Goodwill from acquisition		0
Utility Infrastructure Services
Goodwill [Roll Forward]
Goodwill, Beginning balance	332,928	348,950
Foreign currency translation adjustment	2,161	5,150
Goodwill, Ending balance	$ 335,089	332,928
Utility Infrastructure Services \| Linetec
Goodwill [Roll Forward]
Goodwill from acquisition		$ (21,172)

Background, Organization, and Summary of Significant Accounting Policies - Schedule of Capitalized and Debt Portion of AFUDC (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Debt portion	$ 3,202	$ 4,558	$ 3,264
Equity portion	4,724	4,161	3,627
AFUDC capitalized as part of utility plant	$ 7,926	$ 8,719	$ 6,891
AFUDC rate	5.51%	5.36%	5.85%

Background, Organization, and Summary of Significant Accounting Policies - Other Income (Deductions) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other income (deductions)
Interest income	$ 4,015	$ 6,356	$ 6,108
Equity AFUDC	4,724	4,161	3,627
Total other income (deductions)	(6,789)	10,085	(17,426)
Southwest Gas Corporation
Other income (deductions)
Total other income (deductions)	(6,590)	9,517	(17,240)
Corporate and administrative
Other income (deductions)
Total other income (deductions)	8	102	52
Natural Gas Operations
Other income (deductions)
Interest income	4,015	6,356	6,020
Natural Gas Operations \| Operating Segments \| Southwest Gas Corporation
Other income (deductions)
Change in COLI policies	9,200	17,400	(3,200)
Interest income	4,015	6,356	6,020
Equity AFUDC	4,724	4,161	3,627
Other components of net periodic benefit cost	(20,022)	(15,059)	(21,059)
Miscellaneous income and (expense)	(4,507)	(3,341)	(2,628)
Total other income (deductions)	(6,590)	9,517	(17,240)
Utility Infrastructure Services
Other income (deductions)
Interest income	0	0	88
Utility Infrastructure Services \| Operating Segments \| Centuri
Other income (deductions)
Interest income	0	0	88
Foreign transaction gain (loss)	(16)	546	(222)
Equity in earnings of unconsolidated investment – Western	80	439	531
Miscellaneous income and (expense)	(271)	(519)	(635)
Total other income (deductions)	$ (207)	$ 466	$ (238)

Background, Organization, and Summary of Significant Accounting Policies - Schedule of Earnings Per Share, Basic and Diluted (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Weighted average basic shares (in shares)	55,998,000	54,245,000	49,419,000
Effect of dilutive securities:
Management Incentive Plan shares (in shares)	0	12,000	25,000
Restricted stock units (in shares)	78,000	55,000	32,000
Average diluted shares (in shares)	56,076,000	54,312,000	49,476,000
Number of performance share units granted (in shares)	69,000	46,000	23,000

Utility Plant and Leases - Schedule of Net Utility Plant (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	$ 8,384,000	$ 7,813,221
Less: accumulated depreciation	(2,419,348)	(2,313,050)
Construction work in progress	211,429	185,026
Net utility plant	6,176,081	5,685,197
Software and software-related intangibles
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	270,883	256,299
Storage
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	101,203	100,908
Transmission
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	400,657	391,864
Distribution
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	7,078,656	6,581,043
General
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	515,879	467,274
Other
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	$ 16,722	$ 15,833

Utility Plant and Leases - Narrative (Details) - USD ($) $ in Millions	12 Months Ended			36 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Public Utility, Property, Plant and Equipment [Line Items]
Annual utility depreciation and amortization expense percentage	2.70%	2.70%	2.70%
Amortization of intangibles	$ 10.8	$ 10.7	$ 7.6
Gas, Transmission and Distribution Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual utility depreciation and amortization expense percentage				2.30%
Gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Amortization of intangibles	$ 13.7	$ 13.2	$ 13.6
Southwest Gas Corporation
Public Utility, Property, Plant and Equipment [Line Items]
Lease remaining lease term (up to)	6 years			6 years
Centuri
Public Utility, Property, Plant and Equipment [Line Items]
Lease remaining lease term (up to)	18 years			18 years
Lease renewal term (up to)	5 years			5 years
Lease termination period	1 year
Short-term lease total future lease payments	$ 1.0			$ 1.0

Utility Plant and Leases - Schedule of Depreciation and Amortization Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 332,027	$ 303,237	$ 249,212
Gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 215,636	$ 197,358	$ 185,719

Utility Plant and Leases - Components of Lease Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Total lease cost - Southwest Gas Holdings, Inc.	$ 35,528	$ 30,154
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Operating lease cost	1,251	1,531
Centuri
Lessee, Lease, Description [Line Items]
Operating lease cost	14,294	12,235
Amortization of ROU assets	140	137
Interest on lease liabilities	37	34
Total finance lease cost	177	171
Short-term lease cost	$ 19,806	$ 16,217

Utility Plant and Leases - Supplemental Cash Flow Information Related to Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Operating cash flows from operating leases	$ 13,655	$ 12,444
Operating cash flows from finance leases	36	33
Financing cash flows from finance leases	199	212
Operating leases	20,919	24,687
Finance leases	361	13,839
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Operating cash flows from operating leases	766	1,278
Operating cash flows from finance leases	0	0
Financing cash flows from finance leases	0	0
Operating leases	1,547	862
Finance leases	0	0
Centuri
Lessee, Lease, Description [Line Items]
Operating cash flows from operating leases	12,889	11,166
Operating cash flows from finance leases	36	33
Financing cash flows from finance leases	199	212
Operating leases	19,372	23,825
Finance leases	$ 361	$ 13,839

Utility Plant and Leases - Supplemental Information Related to Leases Included in Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Operating lease ROU assets	$ 2,195	$ 1,443
Operating lease liabilities, current	656	723
Operating lease liabilities, noncurrent	1,586	730
Total	$ 2,242	$ 1,453
Operating lease, weighted average remaining lease term (in years)	4 years 2 months 26 days	2 years 10 months 17 days
Finance lease, weighted average discount rate	4.49%	3.18%
Centuri
Lessee, Lease, Description [Line Items]
Operating lease ROU assets	$ 81,010	$ 78,954
Operating lease liabilities, current	10,032	8,851
Operating lease liabilities, noncurrent	75,247	73,323
Total	$ 85,279	$ 82,174
Operating lease, weighted average remaining lease term (in years)	10 years 29 days	10 years 3 months
Finance lease, weighted average remaining lease term (in years)	2 years 1 month 13 days	2 years 1 month 17 days
Finance lease, weighted average discount rate	4.05%	4.03%
Finance lease ROU assets	$ 752	$ 14,264
Finance lease liabilities, current	202	13,769
Finance lease liabilities, noncurrent	490	355
Finance lease liabilities	$ 692	$ 14,124
Finance lease, weighted average discount rate	5.55%	6.10%

Utility Plant and Leases - Schedule of Maturities of Lease Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
2021	$ 735
2022	543
2023	426
2024	404
2025	330
Thereafter	36
Total lease payments	2,474
Less imputed interest	232
Operating lease liabilities	2,242	$ 1,453
Centuri
Lessee, Lease, Description [Line Items]
2021	13,144
2022	12,153
2023	10,864
2024	9,904
2025	8,051
Thereafter	50,605
Total lease payments	104,721
Less imputed interest	19,442
Operating lease liabilities	85,279	82,174
Finance Leases
2021	235
2022	241
2023	275
2024	1
2025	0
Thereafter	0
Total lease payments	752
Less imputed interest	60
Finance lease liabilities	$ 692	$ 14,124

Revenue - Narrative (Details) $ in Millions	12 Months Ended
Revenue - Narrative (Details) $ in Millions	Dec. 31, 2020 USD ($) Type Customer Contract
Segment Reporting Information [Line Items]
Number of customers party to contracts with rate components subject to negotiation (less than) \| Customer	24
Change in contract liability, revenue recognized \| $	$ 4.5
Southwest Gas Corporation
Segment Reporting Information [Line Items]
Number of types of services provided to customers	2
Number of customers \| Customer	2,000,000
Centuri
Segment Reporting Information [Line Items]
Number of types of services provided to customers	3
Number of types of agreements with customers	2
Number of contracts with original duration more than one year \| Contract	16

Revenue - Schedule of Gas Operating Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 1,339,151	$ 1,385,294	$ 1,305,974
Alternative revenue program deferrals	12,140	(25,112)	45,979
Other revenues	(706)	8,757	5,775
Total Gas operating revenues	1,350,585	1,368,939	1,357,728
Other revenue related to tax reform savings reserves/adjustments	(5,000)	(4,900)	(13,500)
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	958,520	972,788	887,220
Small commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	221,541	249,117	255,083
Large commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	44,633	48,935	53,192
Industrial/other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	26,242	22,074	23,489
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 88,215	$ 92,380	$ 86,990

Revenue - Summary of Revenue Disaggregated by Service Type, and Contract Type (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 1,339,151	$ 1,385,294	$ 1,305,974
Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,948,288	1,750,978	1,522,285
Master services agreement \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,490,009	1,383,377	1,102,412
Bid contract \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	458,279	367,601	419,873
Unit price contracts \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,356,640	1,380,256	1,258,419
Fixed price contracts \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	157,701	112,924	117,298
Time and materials contracts \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	433,947	257,798	146,568
Gas infrastructure services \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,261,160	1,238,974	1,123,682
Electric power infrastructure services \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	411,826	247,717	32,629
Other \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 275,302	$ 264,287	$ 365,974

Revenue - Summary of Information about Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Contracts receivable, net	$ 278,316	$ 223,904
Revenue earned on contracts in progress in excess of billings	96,996	99,399
Amounts billed in excess of revenue earned on contracts	$ 4,507	$ 4,525

Revenue - Revenue Performance Obligation (Details) - Centuri - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01 $ in Millions	Dec. 31, 2020 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to unsatisfied performance obligations of contracts	$ 76.4
Revenue, remaining performance obligation, expected timing of satisfaction, period	2 years

Revenue - Schedule of Utility Infrastructure Services Contracts Receivable (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Contracts receivable, gross	$ 280,470	$ 224,724
Allowance for doubtful accounts	(2,154)	(820)
Contracts receivable, net	278,316	223,904
Billed on completed contracts and contracts in progress
Disaggregation of Revenue [Line Items]
Contracts receivable, gross	273,778	216,268
Other Receivables [Member]
Disaggregation of Revenue [Line Items]
Contracts receivable, gross	$ 6,692	$ 8,456

Receivables and Related Allowances - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) state	Dec. 31, 2019 USD ($)
Receivables And Related Allowances [Line Items]
Number of states in which entity operates \| state	3
Accounts receivable, extending period	1 month
Threshold period after inactivation for write-off of accounts receivable	2 months
Customer accounts receivable	$ 522,172	$ 474,097
Centuri
Receivables And Related Allowances [Line Items]
Threshold period after inactivation for write-off of accounts receivable	60 days
Customer accounts receivable	$ 375,300

Receivables and Related Allowances - Schedule of Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Receivables [Abstract]
Gas utility customer accounts receivable balance	$ 145,108	$ 148,173

Receivables and Related Allowances - Schedule of Percent of Customers by State (Details)	12 Months Ended
	Dec. 31, 2020
Arizona
Receivables And Related Allowances [Line Items]
Percent of customers by state:	53.00%
Nevada
Receivables And Related Allowances [Line Items]
Percent of customers by state:	37.00%
California
Receivables And Related Allowances [Line Items]
Percent of customers by state:	10.00%

Receivables and Related Allowances - Schedule of Allowance for Uncollectibles (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 2,095	$ 2,168	$ 2,111
Additions charged to expense	4,693	3,507	2,959
Accounts written off, less recoveries	(2,454)	(3,580)	(2,902)
Ending Balance	$ 4,334	$ 2,095	$ 2,168

Regulatory Assets and Liabilities - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$ 541,647	$ 582,759
Net regulatory liabilities	$ (406,291)	(363,890)
Accrued pension and other postretirement benefit costs, recovery period	5 years
Regulatory assets, accrued absence time, lag period	1 year
Regulatory assets, margin, interest and property tax-tracking, recovery period	24 months
Other current liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ (15,000)
Deferred purchased gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	(54,636)	(60,755)
Accumulated removal costs
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	(404,000)	(395,000)
Unamortized gain on reacquired debt
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	(7,644)	(8,181)
Regulatory excess deferred/other taxes and gross-up
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	(461,023)	(455,625)
Margin, interest - and property tax-tracking
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	(17,132)	(22,650)
Other regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	(3,503)	(4,438)
Gross-up related to contributions in aid of construction
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	(2,600)
Accrued pension and other postretirement benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	427,550	420,114
Unrealized net loss on non-trading derivatives (Swaps)
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	0	10,951
Deferred purchased gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	2,053	44,412
Accrued purchased gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	29,000	8,000
Unamortized premium on reacquired debt
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	17,124	18,249
Accrued absence time
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	15,565	14,519
Margin, interest - and property tax-tracking
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	4,759	33,380
Other regulatory assets
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$ 45,596	$ 33,134

Regulatory Assets and Liabilities - Schedule of Components of Other Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Regulatory assets	$ 541,647	$ 582,759
State mandated public purpose programs (including low income and conservation programs)
Regulatory Assets [Line Items]
Regulatory assets	11,527	9,172
Infrastructure replacement programs and similar
Regulatory Assets [Line Items]
Regulatory assets	7,731	8,236
Environmental compliance programs
Regulatory Assets [Line Items]
Regulatory assets	5,235	5,768
Pension tracking mechanism
Regulatory Assets [Line Items]
Regulatory assets	3,075	0
Other
Regulatory Assets [Line Items]
Regulatory assets	18,028	9,958
Other regulatory assets
Regulatory Assets [Line Items]
Regulatory assets	45,596	33,134
Prepaids and other current assets \| Environmental compliance programs
Regulatory Assets [Line Items]
Regulatory assets	4,200	5,000
Prepaids and other current assets \| Other
Regulatory Assets [Line Items]
Regulatory assets	3,600	1,600
Deferred charges and other assets \| Environmental compliance programs
Regulatory Assets [Line Items]
Regulatory assets	998	782
Deferred charges and other assets \| Other
Regulatory Assets [Line Items]
Regulatory assets	$ 14,400	$ 8,300

Regulatory Assets and Liabilities - Schedule of Components of Other Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Other current liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ (15,000)
Other deferred credits and other long-term liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	(339)
State mandated public purpose programs (including low income and conservation programs)
Regulatory Liabilities [Line Items]
Regulatory liabilities	(834)	$ (308)
Environmental compliance programs
Regulatory Liabilities [Line Items]
Regulatory liabilities	(405)	(527)
Pension tracking mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	0	(2,476)
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	(2,264)	(1,127)
Other regulatory liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ (3,503)	$ (4,438)

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Related Tax Effects Allocated to OCI (Detail) - USD ($) $ in Thousands	12 Months Ended			36 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, net of tax	$ (42,017)
Other comprehensive income (loss), before tax	(6,159)	$ (5,992)	$ 6,628
Other comprehensive income (loss), tax	1,888	1,928	(2,314)
Total other comprehensive income (loss), net of tax	$ (4,271)	$ (4,064)	$ 4,314
Effective income tax rate	21.60%	20.50%	25.30%	24.00%
Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, net of tax	$ (43,730)
Other comprehensive income (loss), before tax	(7,872)	$ (8,030)	$ 9,638
Other comprehensive income (loss), tax	1,888	1,928	(2,314)
Total other comprehensive income (loss), net of tax	$ (5,984)	$ (6,102)	$ 7,324
Effective income tax rate	18.30%	17.70%	24.10%	24.00%
Net actuarial gain/(loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	$ (57,539)	$ (71,087)	$ (20,426)
Other comprehensive income (loss) before reclassifications, tax	13,809	17,061	4,902
Other comprehensive income (loss) before reclassifications, net of tax	(43,730)	(54,026)	(15,524)
Reclassification from AOCI, before Tax	37,830	23,376	33,617
Reclassification from AOCI, tax	(9,079)	(5,610)	(8,068)
Reclassification from AOCI, net of tax	28,751	17,766	25,549
Other comprehensive income (loss), before tax	37,830
Other comprehensive income (loss), tax	(9,079)
Total other comprehensive income (loss), net of tax	28,751
Net actuarial gain/(loss) \| Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	37,830
Other comprehensive income (loss), tax	(9,079)
Total other comprehensive income (loss), net of tax	28,751
Amortization of prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	1,155	1,271	1,335
Reclassification from AOCI, tax	(277)	(305)	(320)
Reclassification from AOCI, net of tax	878	966	1,015
Other comprehensive income (loss), before tax	1,155
Other comprehensive income (loss), tax	(277)
Total other comprehensive income (loss), net of tax	878
Amortization of prior service cost \| Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	1,155
Other comprehensive income (loss), tax	(277)
Total other comprehensive income (loss), net of tax	878
Prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	0	(1,878)	0
Other comprehensive income (loss) before reclassifications, tax	0	452	0
Other comprehensive income (loss) before reclassifications, net of tax	0	(1,426)	0
Regulatory adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	7,435	36,944	(8,233)
Reclassification from AOCI, tax	(1,785)	(8,867)	1,976
Reclassification from AOCI, net of tax	5,650	28,077	(6,257)
Other comprehensive income (loss), before tax	7,435
Other comprehensive income (loss), tax	(1,785)
Total other comprehensive income (loss), net of tax	5,650
Regulatory adjustment \| Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	7,435
Other comprehensive income (loss), tax	(1,785)
Total other comprehensive income (loss), net of tax	5,650
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	(57,539)
Other comprehensive income (loss) before reclassifications, tax	13,809
Other comprehensive income (loss) before reclassifications, net of tax	(43,730)
Other comprehensive income (loss), before tax	(11,119)	(11,374)	6,293
Other comprehensive income (loss), tax	2,668	2,731	(1,510)
Total other comprehensive income (loss), net of tax	(8,451)	(8,643)	4,783
Defined Benefit Plans \| Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	(57,539)
Other comprehensive income (loss) before reclassifications, tax	13,809
Other comprehensive income (loss) before reclassifications, net of tax	(43,730)
Other comprehensive income (loss), before tax	(11,119)
Other comprehensive income (loss), tax	2,668
Total other comprehensive income (loss), net of tax	(8,451)
FSIRS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	3,247	3,344	3,345
Reclassification from AOCI, tax	(780)	(803)	(804)
Reclassification from AOCI, net of tax	2,467	2,541	2,541
Other comprehensive income (loss), before tax	3,247	3,344	3,345
Other comprehensive income (loss), tax	(780)	(803)	(804)
Total other comprehensive income (loss), net of tax	2,467	2,541	2,541
FSIRS \| Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	3,247
Reclassification from AOCI, tax	(780)
Reclassification from AOCI, net of tax	2,467
Other comprehensive income (loss), before tax	3,247
Other comprehensive income (loss), tax	(780)
Total other comprehensive income (loss), net of tax	2,467
Foreign currency items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	1,713	2,038	(3,010)
Other comprehensive income (loss) before reclassifications, tax	0	0	0
Other comprehensive income (loss) before reclassifications, net of tax	$ 1,713	$ 2,038	$ (3,010)

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Narrative (Detail) $ in Millions	Dec. 31, 2020 USD ($)
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$ 1.7

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - AOCI Rollforward (Details) - USD ($) $ in Thousands	12 Months Ended			36 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 2,505,914	$ 2,251,590
Beginning balance, attributable to parent	2,505,914
Other comprehensive income (loss) before reclassifications, net of tax	(42,017)
Other comprehensive income (loss), before tax	(6,159)	(5,992)	$ 6,628
Other comprehensive income (loss), tax	1,888	1,928	(2,314)
Total other comprehensive income (loss), net of tax	(4,271)	(4,064)	4,314
Ending balance, attributable to parent	2,674,953	2,505,914		$ 2,674,953
Ending balance	$ 2,674,953	$ 2,505,914	$ 2,251,590	$ 2,674,953
Effective income tax rate	21.60%	20.50%	25.30%	24.00%
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	$ (66,601)
Beginning balance, tax	15,985
Beginning balance	(50,616)
Other comprehensive income (loss) before reclassifications, before tax	(57,539)
Other comprehensive income (loss) before reclassifications, tax	13,809
Other comprehensive income (loss) before reclassifications, net of tax	(43,730)
Other comprehensive income (loss), before tax	(11,119)	$ (11,374)	$ 6,293
Other comprehensive income (loss), tax	2,668	2,731	(1,510)
Total other comprehensive income (loss), net of tax	(8,451)	(8,643)	4,783
Ending balance, before tax	(77,720)	(66,601)		$ (77,720)
Ending balance, tax	18,653	15,985		18,653
Ending balance	(59,067)	(50,616)		(59,067)
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	1,155	1,271	1,335
Reclassification from AOCI, tax	(277)	(305)	(320)
Reclassification from AOCI, net of tax	878	966	1,015
Other comprehensive income (loss), before tax	1,155
Other comprehensive income (loss), tax	(277)
Total other comprehensive income (loss), net of tax	878
Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications, before tax	(57,539)	(71,087)	(20,426)
Other comprehensive income (loss) before reclassifications, tax	13,809	17,061	4,902
Other comprehensive income (loss) before reclassifications, net of tax	(43,730)	(54,026)	(15,524)
Reclassification from AOCI, before Tax	37,830	23,376	33,617
Reclassification from AOCI, tax	(9,079)	(5,610)	(8,068)
Reclassification from AOCI, net of tax	28,751	17,766	25,549
Other comprehensive income (loss), before tax	37,830
Other comprehensive income (loss), tax	(9,079)
Total other comprehensive income (loss), net of tax	28,751
Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	7,435	36,944	(8,233)
Reclassification from AOCI, tax	(1,785)	(8,867)	1,976
Reclassification from AOCI, net of tax	5,650	28,077	(6,257)
Other comprehensive income (loss), before tax	7,435
Other comprehensive income (loss), tax	(1,785)
Total other comprehensive income (loss), net of tax	5,650
FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	(5,966)
Beginning balance, tax	1,431
Beginning balance	(4,535)
Reclassification from AOCI, before Tax	3,247	3,344	3,345
Reclassification from AOCI, tax	(780)	(803)	(804)
Reclassification from AOCI, net of tax	2,467	2,541	2,541
Other comprehensive income (loss), before tax	3,247	3,344	3,345
Other comprehensive income (loss), tax	(780)	(803)	(804)
Total other comprehensive income (loss), net of tax	2,467	2,541	2,541
Ending balance, before tax	(2,719)	(5,966)		(2,719)
Ending balance, tax	651	1,431		651
Ending balance	(2,068)	(4,535)		(2,068)
Foreign Currency Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	(1,581)
Beginning balance	(1,581)
Other comprehensive income (loss) before reclassifications, before tax	1,713
Other comprehensive income (loss) before reclassifications, net of tax	1,713
Ending balance, before tax	132	(1,581)		132
Ending balance	132	(1,581)		132
AOCI Including Portion Attributable to Noncontrolling Interest
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications, net of tax	(4,271)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(56,732)	(52,668)	(47,682)
Ending balance	(61,003)	(56,732)	(52,668)	(61,003)
Southwest Gas Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	2,005,152	1,782,460
Beginning balance, attributable to parent	2,005,152
Other comprehensive income (loss) before reclassifications, net of tax	(43,730)
Other comprehensive income (loss), before tax	(7,872)	(8,030)	9,638
Other comprehensive income (loss), tax	1,888	1,928	(2,314)
Total other comprehensive income (loss), net of tax	(5,984)	(6,102)	7,324
Ending balance, attributable to parent	2,233,468	2,005,152		2,233,468
Ending balance	$ 2,233,468	$ 2,005,152	$ 1,782,460	$ 2,233,468
Effective income tax rate	18.30%	17.70%	24.10%	24.00%
Southwest Gas Corporation \| Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax	$ (66,601)
Beginning balance, attributable to parent, tax	15,985
Beginning balance, attributable to parent	(50,616)
Other comprehensive income (loss) before reclassifications, before tax	(57,539)
Other comprehensive income (loss) before reclassifications, tax	13,809
Other comprehensive income (loss) before reclassifications, net of tax	(43,730)
Other comprehensive income (loss), before tax	(11,119)
Other comprehensive income (loss), tax	2,668
Total other comprehensive income (loss), net of tax	(8,451)
Ending balance, attributable to parent, before tax	(77,720)	$ (66,601)		$ (77,720)
Ending balance, attributable to parent, tax	18,653	15,985		18,653
Ending balance, attributable to parent	(59,067)	(50,616)		(59,067)
Southwest Gas Corporation \| Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	1,155
Other comprehensive income (loss), tax	(277)
Total other comprehensive income (loss), net of tax	878
Southwest Gas Corporation \| Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	37,830
Other comprehensive income (loss), tax	(9,079)
Total other comprehensive income (loss), net of tax	28,751
Southwest Gas Corporation \| Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	7,435
Other comprehensive income (loss), tax	(1,785)
Total other comprehensive income (loss), net of tax	5,650
Southwest Gas Corporation \| FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax	(5,966)
Beginning balance, attributable to parent, tax	1,431
Beginning balance, attributable to parent	(4,535)
Reclassification from AOCI, before Tax	3,247
Reclassification from AOCI, tax	(780)
Reclassification from AOCI, net of tax	2,467
Other comprehensive income (loss), before tax	3,247
Other comprehensive income (loss), tax	(780)
Total other comprehensive income (loss), net of tax	2,467
Ending balance, attributable to parent, before tax	(2,719)	(5,966)		(2,719)
Ending balance, attributable to parent, tax	651	1,431		651
Ending balance, attributable to parent	(2,068)	(4,535)		(2,068)
Southwest Gas Corporation \| AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(55,151)	(49,049)	$ (47,073)
Beginning balance, attributable to parent	(55,151)
Ending balance, attributable to parent	(61,135)	(55,151)		(61,135)
Ending balance	$ (61,135)	$ (55,151)	$ (49,049)	$ (61,135)

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Amount Recognized Before Income Tax Associated with Defined Benefit Plans in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Equity [Abstract]
Net actuarial loss	$ (502,783)	$ (483,074)
Prior service cost	(2,487)	(3,641)
Less: amount recognized in regulatory assets	427,550	420,114
Recognized in AOCI	$ (77,720)	$ (66,601)

Common Stock (Detail) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended			20 Months Ended
Common Stock (Detail) - USD ($)	May 31, 2019	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Class of Stock [Line Items]
Net proceeds			$ 139,245,000	$ 157,946,000	$ 354,402,000
Shares registered and available for issuance (in shares)		4,500,000	4,500,000			4,500,000
Restricted Stock/Unit Plan and Management Incentive Plan
Class of Stock [Line Items]
Common stock issued (in shares)			96,000
Dividend Reinvestment and Stock Purchase Plan
Class of Stock [Line Items]
Common stock issued (in shares)			172,000
Common stock issuances			$ 11,000,000
Equity Shelf Program
Class of Stock [Line Items]
Maximum value of common stock issuance	$ 300,000,000
Gross proceeds		$ 46,425,623				$ 253,551,490
Less: agent commissions		(464,256)				(2,535,515)
Net proceeds		$ 45,961,367				$ 251,015,975
Number of shares sold (in shares)		691,081				3,396,457
Weighted average price per share (USD per share)		$ 67.18				$ 74.65
Common stock available for sale under the program		$ 46,448,510	$ 46,448,510			$ 46,448,510

Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Less: current maturities	$ (40,433)		$ (163,512)
Long-term debt, less current maturities	2,732,200		2,300,482
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(40,433)		(163,512)
Long-term debt, less current maturities	2,732,200		2,300,482
Southwest Gas Corporation
Debt Instrument [Line Items]
Less: current maturities	0		(125,000)
Long-term debt, less current maturities	2,438,206		1,991,333
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	0		(125,000)
Long-term debt, less current maturities	$ 2,438,206		1,991,333
Debentures \| Notes, 4.45%, due 2020 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	4.45%	4.45%
Debentures \| Notes, 6.1%, due 2041 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	6.10%
Debentures \| Notes, 3.875%, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.875%
Debentures \| Notes, 4.875%, due 2043 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	4.875%
Debentures \| Notes, 3.8%, due 2046 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.80%
Debentures \| Notes, 3.7%, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.70%
Debentures \| Notes, 4.15%, due 2049 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	4.15%
Debentures \| Notes, 2.2%, due 2030 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	2.20%	2.20%
Debentures \| 8% Series, due 2026 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	8.00%
Debentures \| Medium-term notes, 7.78% series, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	7.78%
Debentures \| Medium-term notes, 7.92% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	7.92%
Debentures \| Medium-term notes, 6.76% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument, stated interest rate	6.76%
Carrying Amount
Debt Instrument [Line Items]
Less: current maturities	$ (40,433)		(163,512)
Long-term debt, less current maturities	2,732,200		2,300,482
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(40,433)		(163,512)
Long-term debt, less current maturities	2,732,200		2,300,482
Carrying Amount \| Southwest Gas Corporation
Debt Instrument [Line Items]
Less: current maturities	0		(125,000)
Long-term debt, less current maturities	2,438,206		1,991,333
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	2,438,206		2,116,333
Less: current maturities	0		(125,000)
Long-term debt, less current maturities	2,438,206		1,991,333
Carrying Amount \| Centuri
Debt Instrument [Line Items]
Less: current maturities	(40,433)		(38,512)
Long-term debt, less current maturities	293,994		309,149
Unamortized debt issuance costs	(820)		(1,101)
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	334,427		347,661
Less: current maturities	(40,433)		(38,512)
Long-term debt, less current maturities	293,994		309,149
Centuri term loan/secured revolving credit facility	225,828		243,711
Carrying Amount \| Centuri secured revolving credit facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	26,626		60,021
Carrying Amount \| Revolving credit facility and commercial paper \| Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit facility	150,000		150,000
Carrying Amount \| Centuri term loan facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	226,648		244,812
Carrying Amount \| Centuri other debt obligations \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	81,973		43,929
Carrying Amount \| Debentures \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	2,089,678		1,768,050
Unamortized discount and debt issuance costs	(17,822)		(14,450)
Carrying Amount \| Debentures \| Notes, 4.45%, due 2020 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	0		125,000
Carrying Amount \| Debentures \| Notes, 6.1%, due 2041 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	125,000		125,000
Carrying Amount \| Debentures \| Notes, 3.875%, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	250,000		250,000
Carrying Amount \| Debentures \| Notes, 4.875%, due 2043 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	250,000		250,000
Carrying Amount \| Debentures \| Notes, 3.8%, due 2046 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000		300,000
Carrying Amount \| Debentures \| Notes, 3.7%, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000		300,000
Carrying Amount \| Debentures \| Notes, 4.15%, due 2049 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000		300,000
Carrying Amount \| Debentures \| Notes, 2.2%, due 2030 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	450,000
Carrying Amount \| Debentures \| 8% Series, due 2026 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	75,000		75,000
Carrying Amount \| Debentures \| Medium-term notes, 7.78% series, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	25,000		25,000
Carrying Amount \| Debentures \| Medium-term notes, 7.92% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	25,000		25,000
Carrying Amount \| Debentures \| Medium-term notes, 6.76% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	7,500		7,500
Carrying Amount \| IDRBs \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(1,472)		(1,717)
Unsecured debt	198,528		198,283
Carrying Amount \| IDRBs \| Tax-exempt Series A, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount \| IDRBs \| 2003 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount \| IDRBs \| 2008 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount \| IDRBs \| 2009 Series A, due 2039 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Fair Value \| Centuri secured revolving credit facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	26,645		60,057
Fair Value \| Revolving credit facility and commercial paper \| Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit facility, fair value	150,000		150,000
Fair Value \| Centuri term loan facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	230,824		252,182
Fair Value \| Centuri other debt obligations \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	84,246		44,787
Fair Value \| Debentures \| Notes, 4.45%, due 2020 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	0		126,673
Fair Value \| Debentures \| Notes, 6.1%, due 2041 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	174,858		162,666
Fair Value \| Debentures \| Notes, 3.875%, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	258,825		258,550
Fair Value \| Debentures \| Notes, 4.875%, due 2043 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	317,190		291,928
Fair Value \| Debentures \| Notes, 3.8%, due 2046 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	347,046		308,307
Fair Value \| Debentures \| Notes, 3.7%, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	344,553		320,685
Fair Value \| Debentures \| Notes, 4.15%, due 2049 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	370,278		330,138
Fair Value \| Debentures \| Notes, 2.2%, due 2030 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	474,552
Fair Value \| Debentures \| 8% Series, due 2026 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	99,723		96,905
Fair Value \| Debentures \| Medium-term notes, 7.78% series, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	26,663		27,500
Fair Value \| Debentures \| Medium-term notes, 7.92% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	33,802		32,543
Fair Value \| Debentures \| Medium-term notes, 6.76% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	9,613		9,156
Fair Value \| IDRBs \| Tax-exempt Series A, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Fair Value \| IDRBs \| 2003 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Fair Value \| IDRBs \| 2008 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Fair Value \| IDRBs \| 2009 Series A, due 2039 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 50,000		$ 50,000

Debt - Narrative (Details)		3 Months Ended			12 Months Ended
Debt - Narrative (Details)	Apr. 10, 2020 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Short term borrowings outstanding		$ 107,000,000			$ 107,000,000		$ 211,000,000
Debt instrument percentage		12.00%			12.00%
Issuance of additional debt					$ 2,700,000,000
Equity cushion relating to minimum net worth requirement					$ 1,700,000,000
Percentage of maximum dividend payout restriction					60.00%
Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument discount rate						0.126%
Short term borrowings outstanding		$ 57,000,000			$ 57,000,000		194,000,000
Debt instrument percentage		6.00%			6.00%
Southwest Gas Corporation \| Carrying Amount
Debt Instrument [Line Items]
Long-term debt		$ 2,438,206,000			$ 2,438,206,000		2,116,333,000
Centuri
Debt Instrument [Line Items]
Issuance of additional debt					318,000,000
Equity cushion relating to minimum fixed charge ratio coverage requirement					115,000,000
Centuri \| Carrying Amount
Debt Instrument [Line Items]
Long-term debt		334,427,000			334,427,000		347,661,000
LIBOR \| Southwest Gas Corporation
Debt Instrument [Line Items]
Borrowings outstanding under facility		157,000,000			157,000,000
LIBOR \| Centuri
Debt Instrument [Line Items]
Borrowings outstanding under facility		145,000,000			145,000,000
Southwest Gas Credit Facility \| Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit designated as long term debt		150,000,000			150,000,000
Line of credit designated for working capital purposes		250,000,000			250,000,000
Long-term debt		$ 150,000,000			$ 150,000,000
Effective interest rates		0.92%			0.92%
Maximum current borrowing usage				$ 344,000,000	$ 0
Southwest Gas Credit Facility \| LIBOR \| Southwest Gas Corporation
Debt Instrument [Line Items]
Applicable margin					1.00%
Southwest Gas Credit Facility \| Alternative Base Rate \| Southwest Gas Corporation
Debt Instrument [Line Items]
Applicable margin					0.00%
Commercial Paper Program \| Southwest Gas Corporation
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 50,000,000			$ 50,000,000
Borrowings outstanding under facility		$ 50,000,000			$ 50,000,000
Notes, 4.15%, due 2049 \| Southwest Gas Corporation \| Debentures
Debt Instrument [Line Items]
Debt instrument, stated interest rate		4.15%			4.15%
Secured Revolving Credit Facility and Term Loan \| Centuri
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 590,000,000			$ 590,000,000
Borrowings outstanding under facility		$ 253,000,000			$ 253,000,000
Effective interest rates		1.30%			1.30%
Maximum current borrowing usage			$ 77,000,000
Assets amount securing the facility		$ 1,400,000,000			$ 1,400,000,000
Minimum current borrowing usage		27,000,000
Secured Revolving Credit Facility and Term Loan \| Centuri \| Minimum
Debt Instrument [Line Items]
Commitment fee percentage					0.125%
Secured Revolving Credit Facility and Term Loan \| Centuri \| Maximum
Debt Instrument [Line Items]
Commitment fee percentage					0.35%
Secured Revolving Credit Facility and Term Loan \| LIBOR \| Centuri
Debt Instrument [Line Items]
Applicable margin					0.875%
Secured Revolving Credit Facility and Term Loan \| LIBOR \| Centuri \| Maximum
Debt Instrument [Line Items]
Applicable margin					2.25%
Secured Revolving Credit Facility and Term Loan \| Alternative Base Rate \| Centuri \| Minimum
Debt Instrument [Line Items]
Applicable margin					0.00%
Secured Revolving Credit Facility and Term Loan \| Alternative Base Rate \| Centuri \| Maximum
Debt Instrument [Line Items]
Applicable margin					1.25%
Line of Credit \| Centuri
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		325,000,000			$ 325,000,000
Centuri term loan facility \| Centuri
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 265,000,000			$ 265,000,000
Notes, 6.1%, due 2041 \| Southwest Gas Corporation \| Debentures
Debt Instrument [Line Items]
Debt instrument, stated interest rate		6.10%			6.10%
Notes, 4.45%, due 2020 \| Southwest Gas Corporation \| Debentures
Debt Instrument [Line Items]
Debt instrument face amount						$ 125,000,000
Debt instrument, stated interest rate		4.45%			4.45%	4.45%
Debt redemption price, percentage					100.00%
Notes, 2.2%, due 2030 \| Southwest Gas Corporation \| Debentures
Debt Instrument [Line Items]
Debt instrument face amount						$ 450,000,000
Debt instrument, stated interest rate		2.20%			2.20%	2.20%
Equipment loans \| Centuri
Debt Instrument [Line Items]
Proceeds from debt					$ 70,000,000
$400 Million Credit Facility \| Southwest Gas Corporation
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	$ 400,000,000
Debt covenant, ratio of funded debt to total capitalization	0.70
Short term borrowings outstanding		$ 57,000,000			$ 57,000,000		$ 194,000,000
Weighted average interest rate		1.10%			1.10%		2.61%
$400 Million Credit Facility \| Southwest Gas Corporation \| Minimum
Debt Instrument [Line Items]
Commitment fee percentage	0.075%
$400 Million Credit Facility \| Southwest Gas Corporation \| Maximum
Debt Instrument [Line Items]
Commitment fee percentage	0.20%
$400 Million Credit Facility \| LIBOR \| Southwest Gas Corporation \| Minimum
Debt Instrument [Line Items]
Applicable margin	0.75%
$400 Million Credit Facility \| LIBOR \| Southwest Gas Corporation \| Maximum
Debt Instrument [Line Items]
Applicable margin	1.50%
$400 Million Credit Facility \| Alternative Base Rate \| Southwest Gas Corporation \| Minimum
Debt Instrument [Line Items]
Applicable margin	0.00%
$400 Million Credit Facility \| Alternative Base Rate \| Southwest Gas Corporation \| Maximum
Debt Instrument [Line Items]
Applicable margin	0.50%
$400 Million Credit Facility \| Southwest Gas Credit Facility \| Southwest Gas Corporation
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 400,000,000			$ 400,000,000
$100 Million Credit Facility
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		100,000,000			100,000,000
Short term borrowings outstanding		$ 50,000,000			$ 50,000,000		$ 17,000,000
Weighted average interest rate		1.225%			1.225%		2.749%
$100 Million Credit Facility \| Minimum
Debt Instrument [Line Items]
Short term borrowings outstanding				17,000,000
$100 Million Credit Facility \| Maximum
Debt Instrument [Line Items]
Short term borrowings outstanding				$ 67,000,000
$100 Million Credit Facility \| LIBOR
Debt Instrument [Line Items]
Applicable margin					1.125%
$100 Million Credit Facility \| Alternative Base Rate
Debt Instrument [Line Items]
Applicable margin					0.125%

Debt - Summary of Effective Interest Rates on Variable-Rate IDRBs (Details)	Dec. 31, 2020	Dec. 31, 2019
2003 Series A, due 2038
Debt Instrument [Line Items]
Effective interest rates	0.80%	2.51%
2008 Series A, due 2038
Debt Instrument [Line Items]
Effective interest rates	0.83%	2.46%
2009 Series A, due 2039
Debt Instrument [Line Items]
Effective interest rates	0.76%	2.37%
Tax-exempt Series A, due 2028
Debt Instrument [Line Items]
Effective interest rates	0.87%	2.32%

Debt - Estimated Maturities of Long-Term Debt (Details) $ in Thousands	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
2021	$ 40,433
2022	320,433
2023	219,638
2024	11,284
2025	159,076
Southwest Gas Corporation
Debt Instrument [Line Items]
2021	0
2022	275,000
2023	0
2024	0
2025	150,000
Centuri
Debt Instrument [Line Items]
2021	40,433
2022	45,433
2023	219,638
2024	11,284
2025	$ 9,076

Share-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 7,114	$ 6,896	$ 6,111
Compensation cost related to nonvested management incentive plan shares, performance shares, and restricted stock/units not yet recognized	$ 4,500
Compensation cost not yet recognized, period for recognition	1 year 8 months 12 days
Omnibus Incentive Plan \| Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance measurement period	3 years
Compensation expense	$ 2,800	$ 2,300	$ 2,100
Restricted Stock/Unit Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, granted (in shares)	129,000
Weighted average grant date fair value (in USD per share)	$ 76.85	$ 81.75	$ 69.16
Restricted Stock/Unit Plan \| Vesting Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	40.00%
Restricted Stock/Unit Plan \| Vesting Tranche Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	30.00%
Restricted Stock/Unit Plan \| Vesting Tranche Three
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	30.00%
Restricted Stock/Unit Plan \| Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, granted (in shares)	77,400
Legacy Restricted Stock/Unit Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, granted (in shares)	0
Management Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, granted (in shares)	0
Award vesting period	3 years
Weighted average grant date fair value (in USD per share)	$ 0

Share-Based Compensation - Schedule of Share-Based Plan Compensation Expense, Including Cash Award (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement [Abstract]
Share-based compensation plan expense, net of related tax benefits	$ 4,816	$ 5,154	$ 4,644
Share-based compensation plan related tax benefits	$ 1,521	$ 1,627	$ 1,467

Share-Based Compensation - Schedule of Nonvested Performance and Restricted Stock Unit Plans (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Management Incentive Plan
Number of nonvested/unissued shares
Nonvested/unissued at beginning of year (in shares)	29,000
Granted (in shares)	0
Dividends (in shares)	0
Forfeited or expired (in shares)	0
Vested and issued (in shares)	(29,000)
Nonvested/unissued at year end (in shares)	0	29,000
Weighted- average grant date fair value
Beginning balance (in USD per share)	$ 79.16
Granted (in USD per share)	0
Forfeited or expired (in USD per share)	0
Vested and issued (in USD per share)	79.16
Ending balance (in USD per share)	$ 0	$ 79.16
Vested and issued (in shares)	29,000
Restricted Stock/Unit Plan
Number of nonvested/unissued shares
Nonvested/unissued at beginning of year (in shares)	365,000
Granted (in shares)	129,000
Dividends (in shares)	8,000
Forfeited or expired (in shares)	0
Vested and issued (in shares)	(101,000)
Nonvested/unissued at year end (in shares)	401,000	365,000
Weighted- average grant date fair value
Beginning balance (in USD per share)	$ 60.94
Granted (in USD per share)	76.85	$ 81.75	$ 69.16
Forfeited or expired (in USD per share)	0
Vested and issued (in USD per share)	71.06
Ending balance (in USD per share)	$ 62.23	$ 60.94
Vested and issued (in shares)	101,000
Restricted Stock/Unit Plan \| Performance Shares
Number of nonvested/unissued shares
Granted (in shares)	77,400
Vested and issued (in shares)	(35,500)
Weighted- average grant date fair value
Vested and issued (in shares)	35,500

Commitments and Contingencies (Details)	Dec. 31, 2020 USD ($)
Southwest Gas Corporation
Loss Contingencies [Line Items]
Self-insured retention amount associated with general liability claims	$ 1,000,000
Additional self-insured retention amount of general liability	4,000,000
Centuri
Loss Contingencies [Line Items]
Self-insured retention amount associated with general liability claims	$ 750,000

Pension and Other Postretirement Benefits - Narrative (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) plan	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Deferred compensation plan, payment term, option one		10 years
Deferred compensation plan, payment term, option two		15 years
Deferred compensation plan, payment term, option three		20 years
Future estimated funding amount		$ 105,000,000
Multiemployer plan, special surcharges		$ 0
Multiemployer pension plans, number of critical plans \| plan		3
Subsequent Event
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plans, employer contribution amount	$ 50,000,000
Southwest Gas Corporation
Defined Benefit Plan Disclosure [Line Items]
EIP, percent of compensation deferred with match		50.00%
EIP, employer matching contribution, percent of match		3.50%
Employee investment plan cost		$ 5,900,000	$ 5,700,000	$ 5,500,000
Deferred compensation plan, maximum percentage of salary deferral		100.00%
Deferred Compensation Plan, percent of amount deferred with match		50.00%
Deferred compensation plan, matching percentage		3.50%
Period of additional option to receive payment		5 years
Deferred compensation, percentage of multiple interest rate		150.00%
Increments of change to the discount rate		0.25%
Decrease of discount rate		(0.75%)
Decrease in weighted-average rate of compensation		(0.25%)
Decrease of asset return assumption		(0.25%)
Centuri
Defined Benefit Plan Disclosure [Line Items]
Deferred compensation plan, maximum percentage of salary deferral		80.00%
Deferred compensation plan, matching percentage		5.00%
Defined contribution plans, employer contribution amount		$ 9,000,000	8,000,000	7,000,000
Deferred compensation plan, percentage of contributions with matching		100.00%
Deferred compensation plan, vesting percentage of deferral		100.00%
Multiemployer plan, contributions by employer		$ 44,300,000	$ 41,300,000	$ 38,200,000
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Future estimated funding amount		$ 102,000,000
PBOP \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Per capita cost of covered health care benefits medical rate trend assumption		6.00%
PBOP \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Per capita cost of covered health care benefits medical rate trend assumption		4.50%

Pension and Other Postretirement Benefits - Schedule of Assumptions Used (Details) - Southwest Gas Corporation	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.75%	3.50%
Weighted-average rate of compensation increase	3.00%	3.25%
Asset return assumption	6.50%	6.75%

Pension and Other Postretirement Benefits - Schedule of Amounts Recognized in Balance Sheet and Income Statement (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets:
Market value of plan assets at beginning of year	$ 1,027,831
Market value of plan assets at end of year	1,238,719	$ 1,027,831
Qualified Retirement Plan
Change in benefit obligations:
Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	1,329,577	1,116,014
Service cost	34,299	25,864	$ 28,555
Interest cost	45,555	49,006	44,174
Plan amendments	0	0
Actuarial loss (gain)	145,440	192,416
Benefits paid	(55,632)	(53,723)
Benefit obligation at end of year (PBO/PBO/APBO)	1,499,239	1,329,577	1,116,014
Change in plan assets:
Market value of plan assets at beginning of year	974,993	790,614
Actual return on plan assets	165,072	186,102
Employer contributions	102,000	52,000
Benefits paid	(55,632)	(53,723)
Market value of plan assets at end of year	1,186,433	974,993	790,614
Funded status at year end	$ (312,806)	$ (354,584)
Weighted-average assumptions (benefit obligation):
Discount rate	2.75%	3.50%
Weighted-average rate of compensation increase	3.00%	3.25%
SERP
Change in benefit obligations:
Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$ 47,397	$ 40,603
Service cost	389	266	245
Interest cost	1,604	1,760	1,658
Plan amendments	0	0
Actuarial loss (gain)	7,240	7,974
Benefits paid	(2,999)	(3,206)
Benefit obligation at end of year (PBO/PBO/APBO)	53,631	47,397	40,603
Change in plan assets:
Market value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	2,999	3,206
Benefits paid	(2,999)	(3,206)
Market value of plan assets at end of year	0	0	0
Funded status at year end	$ (53,631)	$ (47,397)
Weighted-average assumptions (benefit obligation):
Discount rate	2.75%	3.50%
Weighted-average rate of compensation increase	3.00%	3.25%
PBOP
Change in benefit obligations:
Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$ 76,111	$ 69,956
Service cost	1,581	1,276	1,473
Interest cost	2,582	3,046	2,748
Plan amendments	0	1,878
Actuarial loss (gain)	6,547	3,156
Benefits paid	(4,616)	(3,201)
Benefit obligation at end of year (PBO/PBO/APBO)	82,205	76,111	69,956
Change in plan assets:
Market value of plan assets at beginning of year	52,838	47,341
Actual return on plan assets	5,320	9,757
Employer contributions	0	0
Benefits paid	(5,872)	(4,260)
Market value of plan assets at end of year	52,286	52,838	$ 47,341
Funded status at year end	$ (29,919)	$ (23,273)
Weighted-average assumptions (benefit obligation):
Discount rate	2.75%	3.50%

Pension and Other Postretirement Benefits - Schedule of Accumulated Benefit Obligation (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Retirement plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 1,367,179	$ 1,219,989
SERP
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 50,471	$ 46,067

Pension and Other Postretirement Benefits - Schedule of Expected Benefit Payments (Details) $ in Millions	Dec. 31, 2020 USD ($)
Pension
Defined Benefit Plan Disclosure [Line Items]
2021	$ 58.0
2022	60.0
2023	61.0
2024	62.0
2025	63.0
2026-2030	339.0
SERP
Defined Benefit Plan Disclosure [Line Items]
2021	3.2
2022	3.2
2023	3.2
2024	3.2
2025	3.1
2026-2030	15.1
PBOP
Defined Benefit Plan Disclosure [Line Items]
2021	4.9
2022	5.0
2023	4.9
2024	4.9
2025	4.9
2026-2030	$ 24.0

Pension and Other Postretirement Benefits - Schedule of Net Periodic Benefit Cost and Weighted-Average Assumptions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 56,291	$ 42,465	$ 51,332
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	34,299	25,864	28,555
Interest cost	45,555	49,006	44,174
Expected return on plan assets	(65,296)	(60,244)	(58,755)
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	36,025	22,356	32,115
Net periodic benefit cost	$ 50,583	$ 36,982	$ 46,089
Weighted-average assumptions (net benefit cost)
Discount rate	3.50%	4.50%	3.75%
Expected return on plan assets	6.75%	7.00%	7.00%
Weighted-average rate of compensation increase	3.25%	3.25%	3.25%
SERP
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 389	$ 266	$ 245
Interest cost	1,604	1,760	1,658
Expected return on plan assets	0	0	0
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	1,805	1,020	1,502
Net periodic benefit cost	$ 3,798	$ 3,046	$ 3,405
Weighted-average assumptions (net benefit cost)
Discount rate	3.50%	4.50%	3.75%
Weighted-average rate of compensation increase	3.25%	3.25%	3.25%
PBOP
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,581	$ 1,276	$ 1,473
Interest cost	2,582	3,046	2,748
Expected return on plan assets	(3,408)	(3,156)	(3,718)
Amortization of prior service cost	1,155	1,271	1,335
Amortization of net actuarial loss	0	0	0
Net periodic benefit cost	$ 1,910	$ 2,437	$ 1,838
Weighted-average assumptions (net benefit cost)
Discount rate	3.50%	4.50%	3.75%
Expected return on plan assets	6.75%	7.00%	7.00%

Pension and Other Postretirement Benefits - Schedule of Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	$ 57,539	$ 71,087	$ 20,426
Amortization of prior service cost	(1,155)	(1,271)	(1,335)
Amortization of net actuarial loss	(37,830)	(23,376)	(33,617)
Prior service cost	0	1,878	0
Regulatory adjustment	(7,435)	(36,944)	8,233
Recognized in other comprehensive (income) loss	11,119	11,374	(6,293)
Net periodic benefit costs recognized in net income	56,291	42,465	51,332
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	67,410	53,839	45,039
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	45,665	66,557	23,607
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	(36,025)	(22,356)	(32,115)
Prior service cost	0	0	0
Regulatory adjustment	(3,956)	(39,782)	7,657
Recognized in other comprehensive (income) loss	5,684	4,419	(851)
Net periodic benefit costs recognized in net income	50,583	36,982	46,089
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	56,267	41,401	45,238
SERP
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	7,240	7,975	(3,940)
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	(1,805)	(1,020)	(1,502)
Prior service cost	0	0	0
Regulatory adjustment	0	0	0
Recognized in other comprehensive (income) loss	5,435	6,955	(5,442)
Net periodic benefit costs recognized in net income	3,798	3,046	3,405
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	9,233	10,001	(2,037)
PBOP
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	4,634	(3,445)	759
Amortization of prior service cost	(1,155)	(1,271)	(1,335)
Amortization of net actuarial loss	0	0	0
Prior service cost	0	1,878	0
Regulatory adjustment	(3,479)	2,838	576
Recognized in other comprehensive (income) loss	0	0	0
Net periodic benefit costs recognized in net income	1,910	2,437	1,838
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$ 1,910	$ 2,437	$ 1,838

Pension and Other Postretirement Benefits - Schedule of Fair Value of Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	$ 1,238,719	$ 1,027,831
Equity Securities \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	25.00%
Equity Securities \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	75.00%
Fair Value, Inputs, Level 1 and 2
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	$ 1,236,134	1,025,095
Level 1 – Quoted prices in active markets for identical financial assets
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	30,358	29,188
Level 1 – Quoted prices in active markets for identical financial assets \| Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	30,358	29,188
Level 2 – Significant other observable inputs
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	1,205,776	995,907
Level 2 – Significant other observable inputs \| Global
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	329,962	273,246
Level 2 – Significant other observable inputs \| International
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	143,853	119,866
Level 2 – Significant other observable inputs \| U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	227,425	189,028
Level 2 – Significant other observable inputs \| Emerging markets
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	78,070	64,437
Level 2 – Significant other observable inputs \| Private commingled fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	419,706	343,097
Level 2 – Significant other observable inputs \| Pooled funds and mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	6,555	6,048
Level 2 – Significant other observable inputs \| Government fixed income and mortgage backed securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	205	185
Insurance company general account contracts
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	2,585	2,736
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	1,186,433	974,993	$ 790,614
Qualified Retirement Plan \| Fair Value, Inputs, Level 1 and 2
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	1,183,848	972,257
Qualified Retirement Plan \| Level 1 – Quoted prices in active markets for identical financial assets
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	0	0
Qualified Retirement Plan \| Level 1 – Quoted prices in active markets for identical financial assets \| Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	0	0
Qualified Retirement Plan \| Level 2 – Significant other observable inputs
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	1,183,848	972,257
Qualified Retirement Plan \| Level 2 – Significant other observable inputs \| Global
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	324,084	266,908
Qualified Retirement Plan \| Level 2 – Significant other observable inputs \| International
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	141,290	117,086
Qualified Retirement Plan \| Level 2 – Significant other observable inputs \| U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	223,374	184,642
Qualified Retirement Plan \| Level 2 – Significant other observable inputs \| Emerging markets
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	76,679	62,943
Qualified Retirement Plan \| Level 2 – Significant other observable inputs \| Private commingled fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	412,230	335,138
Qualified Retirement Plan \| Level 2 – Significant other observable inputs \| Pooled funds and mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	5,990	5,359
Qualified Retirement Plan \| Level 2 – Significant other observable inputs \| Government fixed income and mortgage backed securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	201	181
Qualified Retirement Plan \| Insurance company general account contracts
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	2,585	2,736
PBOP
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	52,286	52,838	$ 47,341
PBOP \| Fair Value, Inputs, Level 1 and 2
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	52,286	52,838
PBOP \| Level 1 – Quoted prices in active markets for identical financial assets
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	30,358	29,188
PBOP \| Level 1 – Quoted prices in active markets for identical financial assets \| Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	30,358	29,188
PBOP \| Level 2 – Significant other observable inputs
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	21,928	23,650
PBOP \| Level 2 – Significant other observable inputs \| Global
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	5,878	6,338
PBOP \| Level 2 – Significant other observable inputs \| International
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	2,563	2,780
PBOP \| Level 2 – Significant other observable inputs \| U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	4,051	4,386
PBOP \| Level 2 – Significant other observable inputs \| Emerging markets
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	1,391	1,494
PBOP \| Level 2 – Significant other observable inputs \| Private commingled fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	7,476	7,959
PBOP \| Level 2 – Significant other observable inputs \| Pooled funds and mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	565	689
PBOP \| Level 2 – Significant other observable inputs \| Government fixed income and mortgage backed securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	4	4
PBOP \| Insurance company general account contracts
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	$ 0	$ 0

Income Taxes - Summary of Income Before Taxes and Noncontrolling Interest for Domestic and Foreign Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.S.	$ 282,489	$ 261,525	$ 235,120
Foreign	22,249	11,145	8,216
Total income before income taxes	$ 304,738	$ 272,670	$ 243,336

Income Taxes - Summary of Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ 6,287	$ 622	$ (13,476)
State	8,617	(1,510)	(3,219)
Foreign	4,666	5,013	2,563
Total current income tax expense	19,570	4,125	(14,132)
Deferred:
Federal	44,547	45,593	67,784
State	414	8,212	8,901
Foreign	1,222	(1,907)	(869)
Total deferred income tax expense	46,183	51,898	75,816
Total income tax expense	65,753	56,023	61,684
Southwest Gas Corporation
Current:
Federal	(4,678)	4,109	(17,584)
State	(179)	250	(6,783)
Total current income tax expense	(4,857)	4,359	(24,367)
Deferred:
Federal	38,561	29,543	58,136
State	2,051	1,071	10,222
Total deferred income tax expense	40,612	30,614	68,358
Total income tax expense	$ 35,755	$ 34,973	$ 43,991

Income Taxes - Significant Components of Deferred Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred federal and state:
Total deferred federal and state	$ 46,235	$ 52,144	$ 76,429
Deferred ITC, net	(52)	(246)	(613)
Total deferred income tax expense	46,183	51,898	75,816
Property-related items
Deferred federal and state:
Total deferred federal and state	50,504	60,449	94,899
Purchased gas cost adjustments
Deferred federal and state:
Total deferred federal and state	(5,726)	3,834	(3,507)
Employee benefits
Deferred federal and state:
Total deferred federal and state	459	7,680	(7,334)
Regulatory adjustments
Deferred federal and state:
Total deferred federal and state	(9,885)	(11,962)	2,412
Deferred payroll taxes
Deferred federal and state:
Total deferred federal and state	(9,055)	0	0
Alternative minimum tax
Deferred federal and state:
Total deferred federal and state	4,409	441	849
All other deferred
Deferred federal and state:
Total deferred federal and state	15,529	(8,298)	(10,890)
Southwest Gas Corporation
Deferred federal and state:
Total deferred federal and state	40,664	30,860	68,971
Deferred ITC, net	(52)	(246)	(613)
Total deferred income tax expense	40,612	30,614	68,358
Southwest Gas Corporation \| Property-related items
Deferred federal and state:
Total deferred federal and state	36,029	34,398	67,576
Southwest Gas Corporation \| Purchased gas cost adjustments
Deferred federal and state:
Total deferred federal and state	(5,726)	3,834	(3,507)
Southwest Gas Corporation \| Employee benefits
Deferred federal and state:
Total deferred federal and state	11,437	6,493	2,156
Southwest Gas Corporation \| Regulatory adjustments
Deferred federal and state:
Total deferred federal and state	(9,885)	(11,962)	2,412
Southwest Gas Corporation \| Deferred payroll taxes
Deferred federal and state:
Total deferred federal and state	(1,810)	0	0
Southwest Gas Corporation \| Alternative minimum tax
Deferred federal and state:
Total deferred federal and state	4,409	441	849
Southwest Gas Corporation \| All other deferred
Deferred federal and state:
Total deferred federal and state	$ 6,210	$ (2,344)	$ (515)

Income Taxes - Reconciliation of U.S Federal Statutory Rate to Consolidated Effective Tax Rate (Details)	12 Months Ended			36 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Income Taxes [Line Items]
U.S. federal statutory income tax rate	21.00%	21.00%	21.00%
Net state taxes	3.00%	2.10%	2.90%
Tax credits	(0.50%)	(0.30%)	(0.30%)
Company-owned life insurance	(0.80%)	(1.50%)	0.10%
Amortization of excess deferred taxes	(0.80%)	(0.90%)	0.00%
All other differences	(0.30%)	0.10%	1.60%
Consolidated effective income tax rate	21.60%	20.50%	25.30%	24.00%
Southwest Gas Corporation
Income Taxes [Line Items]
U.S. federal statutory income tax rate	21.00%	21.00%	21.00%
Net state taxes	1.70%	0.70%	2.10%
Tax credits	(0.70%)	(0.40%)	(0.40%)
Company-owned life insurance	(1.00%)	(1.90%)	0.30%
Amortization of excess deferred taxes	(1.30%)	(1.20%)	0.00%
All other differences	(1.40%)	(0.50%)	1.10%
Consolidated effective income tax rate	18.30%	17.70%	24.10%	24.00%

Income Taxes - Summary of Income Before Taxes for Continuing and Discontinued Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Southwest Gas Corporation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total income before income taxes	$ 194,873	$ 198,144	$ 182,833

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at beginning of year	$ 1,056	$ 971
Gross increases – tax positions in prior period	641	85
Gross decreases – tax positions in prior period	0	0
Gross increases – current period tax positions	231	0
Gross decreases – current period tax positions	0	0
Settlements	0	0
Lapse in statute of limitations	0	0
Unrecognized tax benefits at end of year	1,928	1,056
Southwest Gas Corporation
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at beginning of year	1,056	971
Gross increases – tax positions in prior period	506	85
Gross decreases – tax positions in prior period	0	0
Gross increases – current period tax positions	231	0
Gross decreases – current period tax positions	0	0
Settlements	0	0
Lapse in statute of limitations	0	0
Unrecognized tax benefits at end of year	$ 1,793	$ 1,056

Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	$ 104,314	$ 105,077
Employee benefits	39,907	37,439
Alternative minimum tax credit	0	4,409
Federal net operating losses	4,118	7,467
Deferred payroll taxes	9,055	0
Lease-related item	20,890	21,226
Other	14,350	21,536
Valuation allowance	(22)	(25)
Deferred tax assets, total	192,612	197,129
Deferred tax liabilities:
Property-related items, including accelerated depreciation	785,734	732,798
Regulatory balancing accounts	4,205	9,931
Debt-related costs	2,585	2,818
Intangibles	13,511	10,611
Lease-related item	19,789	20,386
Other	13,786	19,569
Deferred tax liabilities, gross	839,610	796,113
Net deferred tax liabilities	646,998	598,984
Southwest Gas Corporation
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	104,314	105,077
Employee benefits	4,806	13,574
Alternative minimum tax credit	0	4,409
Deferred payroll taxes	1,810	0
Other	7,790	12,193
Valuation allowance	(22)	(25)
Deferred tax assets, total	118,698	135,228
Deferred tax liabilities:
Property-related items, including accelerated depreciation	680,294	644,046
Regulatory balancing accounts	4,205	9,931
Debt-related costs	2,585	2,818
Other	12,714	17,483
Deferred tax liabilities, gross	699,798	674,278
Net deferred tax liabilities	$ 581,100	$ 539,050

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Unrecognized tax benefits that, if recognized, would affect the effective tax rate	$ 1,900,000
Tax credit carryforward	0
Capital Loss Carryforward
Income Tax Disclosure [Line Items]
Tax credit carryforward	97,000
Southwest Gas Corporation
Income Tax Disclosure [Line Items]
Unrecognized tax benefits that, if recognized, would affect the effective tax rate	1,800,000
Significant increases or decreases in unrecognized tax benefit within the next 12 months	0
Tax-related interest income	523,000	$ 0	$ 0
Federal
Income Tax Disclosure [Line Items]
Operating loss carryforward	0
Foreign
Income Tax Disclosure [Line Items]
Operating loss carryforward	12,700,000
State
Income Tax Disclosure [Line Items]
Operating loss carryforward	50,900,000
Operating loss carryforwards, net of federal benefit	$ 3,200,000

Segment Information - Narrative (Details)	12 Months Ended
Segment Information - Narrative (Details)	Dec. 31, 2020 Segment
Segment Reporting Information [Line Items]
Number of reportable segments	2
Net Assets, Geographic Area \| Geographic Concentration Risk \| United States
Segment Reporting Information [Line Items]
Long-lived assets	99.00%

Segment Information - Accounts Receivable for Services (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Centuri
Segment Reporting Information [Line Items]
Accounts receivable for Centuri services	$ 13,956	$ 15,235

Segment Information - Schedule of Revenues by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 914,080	$ 791,226	$ 757,247	$ 836,320	$ 848,137	$ 725,230	$ 713,011	$ 833,539	$ 3,298,873	$ 3,119,917	$ 2,880,013
United States
Segment Reporting Information [Line Items]
Revenues									3,057,041	2,893,201	2,664,670
Canada
Segment Reporting Information [Line Items]
Revenues									$ 241,832	$ 226,716	$ 215,343

Segment Information - Schedule of Segment Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues									$ 3,298,873	$ 3,119,917	$ 2,880,013
Interest income									4,015	6,356	6,108
Interest expense									111,477	109,226	96,671
Depreciation and amortization									332,027	303,237	249,212
Income tax expense									65,753	56,023	61,684
Segment net income	$ 103,544	$ 18,273	$ 37,965	$ 72,542	$ 91,718	$ 5,353	$ 22,056	$ 94,809	232,324	213,936	182,277
Segment assets	8,735,853				8,170,048				8,735,853	8,170,048	7,357,729
Capital expenditures									825,105	938,148	765,914
Operating Segments
Segment Reporting Information [Line Items]
Revenues									3,164,014	2,961,191	2,744,099
Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenues									134,859	158,726	135,914
Interest income									0	0	0
Interest expense									1,060	114	741
Depreciation and amortization									0	0	0
Income tax expense									(1,130)	(349)	(727)
Segment net income									(1,656)	(1,639)	(1,542)
Segment assets	3,980				6,108				3,980	6,108	572
Capital expenditures									0	0	0
Natural Gas Operations
Segment Reporting Information [Line Items]
Revenues									1,350,585	1,368,939	1,357,728
Interest income									4,015	6,356	6,020
Interest expense									101,148	95,026	81,740
Depreciation and amortization									235,295	215,620	191,816
Income tax expense									35,755	34,973	43,991
Segment net income									159,118	163,171	138,842
Segment assets	7,256,636				6,798,746				7,256,636	6,798,746	6,141,584
Capital expenditures									692,216	778,748	682,869
Natural Gas Operations \| Operating Segments
Segment Reporting Information [Line Items]
Revenues									1,350,585	1,368,939	1,357,728
Natural Gas Operations \| Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenues									0	0	0
Utility Infrastructure Services
Segment Reporting Information [Line Items]
Revenues									1,948,288	1,750,978	1,522,285
Interest income									0	0	88
Interest expense									9,269	14,086	14,190
Depreciation and amortization									96,732	87,617	57,396
Income tax expense									31,128	21,399	18,420
Segment net income									74,862	52,404	44,977
Segment assets	$ 1,475,237				$ 1,365,194				1,475,237	1,365,194	1,215,573
Capital expenditures									132,889	159,400	83,045
Utility Infrastructure Services \| Operating Segments
Segment Reporting Information [Line Items]
Revenues									1,813,429	1,592,252	1,386,371
Utility Infrastructure Services \| Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenues									$ 134,859	$ 158,726	$ 135,914

Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Quarterly Financial Information [Line Items]
Operating revenues	$ 914,080	$ 791,226	$ 757,247	$ 836,320	$ 848,137	$ 725,230	$ 713,011	$ 833,539	$ 3,298,873	$ 3,119,917	$ 2,880,013
Operating income	152,795	54,264	67,572	148,373	138,204	38,258	54,869	140,480	423,004	371,811	357,433
Net income	105,036	21,063	39,881	73,005	91,906	6,525	22,832	95,384	238,985	216,647	181,652
Net income attributable to Southwest Gas Holdings, Inc.	$ 103,544	$ 18,273	$ 37,965	$ 72,542	$ 91,718	$ 5,353	$ 22,056	$ 94,809	$ 232,324	$ 213,936	$ 182,277
Basic earnings per common share (in USD per share)	$ 1.82	$ 0.32	$ 0.68	$ 1.31	$ 1.67	$ 0.10	$ 0.41	$ 1.78	$ 4.15	$ 3.94	$ 3.69
Diluted earnings per common share (in USD per share)	$ 1.82	$ 0.32	$ 0.68	$ 1.31	$ 1.67	$ 0.10	$ 0.41	$ 1.77	$ 4.14	$ 3.94	$ 3.68
Southwest Gas Corporation
Schedule Of Quarterly Financial Information [Line Items]
Operating revenues	$ 374,490	$ 210,834	$ 262,434	$ 502,827	$ 379,571	$ 209,980	$ 258,711	$ 520,677
Operating income	116,070	1,625	27,101	157,815	112,678	(1,807)	24,069	148,713	$ 302,611	$ 283,653	$ 281,813
Net income	$ 79,550	$ (15,973)	$ 11,942	$ 83,599	$ 76,425	$ (20,012)	$ 3,369	$ 103,389	$ 159,118	$ 163,171	$ 138,842

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

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