Southwest Gas 10-Q 6 May 21 2021 Q1 Quarterly report Financial data

Cover

Cover - shares	3 Months Ended
	Mar. 31, 2021	Apr. 30, 2021
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	001-37976
Entity Registrant Name	Southwest Gas Holdings, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	81-3881866
Entity Address, Address Line One	8360 S. Durango Drive
Entity Address, Address Line Two	Post Office Box 98510
Entity Address, City or Town	Las Vegas,
Entity Address, State or Province	NV
Entity Address, Postal Zip Code	89193-8510
City Area Code	(702)
Local Phone Number	876-7237
Title of 12(b) Security	Southwest Gas Holdings, Inc. Common Stock, $1 Par Value
Trading Symbol	SWX
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		58,001,396
Entity Central Index Key	0001692115
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Southwest Gas Corporation
Entity Information [Line Items]
Document Type	10-Q
Entity File Number	1-7850
Entity Registrant Name	Southwest Gas Corporation
Entity Incorporation, State or Country Code	CA
Entity Tax Identification Number	88-0085720
Entity Address, Address Line One	8360 S. Durango Drive
Entity Address, Address Line Two	Post Office Box 98510
Entity Address, City or Town	Las Vegas,
Entity Address, State or Province	NV
Entity Address, Postal Zip Code	89193-8510
City Area Code	(702)
Local Phone Number	876-7237
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Central Index Key	0000092416
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false
Current Fiscal Year End Date	--12-31

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Utility plant:
Gas plant	$ 8,479,295	$ 8,384,000
Less: accumulated depreciation	(2,453,924)	(2,419,348)
Construction work in progress	215,395	211,429
Net utility plant	6,240,766	6,176,081
Other property and investments	842,672	834,245
Current assets:
Cash and cash equivalents	92,345	83,352
Accounts receivable, net of allowances	479,184	522,172
Accrued utility revenue	50,500	82,400
Income taxes receivable, net	6,523	10,884
Deferred purchased gas costs	238,886	2,053
Prepaid and other current assets	133,466	170,152
Total current assets	1,000,904	871,013
Noncurrent assets:
Goodwill	346,553	345,184
Deferred income taxes	563	455
Deferred charges and other assets	497,187	508,875
Total noncurrent assets	844,303	854,514
Total assets	8,928,645	8,735,853
Capitalization:
Common stock, $1 par (authorized - 120,000,000 shares; issued and outstanding - 57,995,563 and 57,192,925 shares)	59,625	58,823
Additional paid-in capital	1,660,108	1,609,155
Accumulated other comprehensive loss, net	(58,388)	(61,003)
Retained earnings	1,112,377	1,067,978
Total equity	2,773,722	2,674,953
Redeemable noncontrolling interest	205,286	165,716
Long-term debt, less current maturities	2,696,570	2,732,200
Total capitalization	5,675,578	5,572,869
Current liabilities:
Current maturities of long-term debt	67,334	40,433
Short-term debt	310,000	107,000
Accounts payable	182,805	231,301
Customer deposits	67,121	67,920
Income taxes payable, net	19,356	12,556
Accrued general taxes	72,103	48,640
Accrued interest	34,571	20,536
Deferred purchased gas costs	0	54,636
Other current liabilities	282,745	328,945
Total current liabilities	1,036,035	911,967
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits, net	671,574	647,453
Accumulated removal costs	407,000	404,000
Other deferred credits and other long-term liabilities	1,138,458	1,199,564
Total deferred income taxes and other credits	2,217,032	2,251,017
Total capitalization and liabilities	8,928,645	8,735,853
Southwest Gas Corporation
Utility plant:
Gas plant	8,479,295	8,384,000
Less: accumulated depreciation	(2,453,924)	(2,419,348)
Construction work in progress	215,395	211,429
Net utility plant	6,240,766	6,176,081
Other property and investments	146,338	143,611
Current assets:
Cash and cash equivalents	49,795	41,070
Accounts receivable, net of allowances	174,492	146,861
Accrued utility revenue	50,500	82,400
Income taxes receivable, net	0	11,155
Deferred purchased gas costs	238,886	2,053
Prepaid and other current assets	118,397	152,748
Total current assets	632,070	436,287
Noncurrent assets:
Goodwill	10,095	10,095
Deferred charges and other assets	481,184	490,562
Total noncurrent assets	491,279	500,657
Total assets	7,510,453	7,256,636
Capitalization:
Common stock, $1 par (authorized - 120,000,000 shares; issued and outstanding - 57,995,563 and 57,192,925 shares)	49,112	49,112
Additional paid-in capital	1,458,344	1,410,345
Accumulated other comprehensive loss, net	(59,343)	(61,135)
Retained earnings	926,011	835,146
Total equity	2,374,124	2,233,468
Long-term debt, less current maturities	2,413,588	2,438,206
Total capitalization	4,787,712	4,671,674
Current liabilities:
Current maturities of long-term debt	25,000	0
Short-term debt	267,000	57,000
Accounts payable	117,471	161,646
Customer deposits	67,121	67,920
Income taxes payable, net	7,232	0
Accrued general taxes	72,103	48,640
Accrued interest	34,532	20,495
Deferred purchased gas costs	0	54,636
Payable to parent	90	142
Other current liabilities	141,167	146,046
Total current liabilities	731,716	556,525
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits, net	596,617	581,100
Accumulated removal costs	407,000	404,000
Other deferred credits and other long-term liabilities	987,408	1,043,337
Total deferred income taxes and other credits	1,991,025	2,028,437
Total capitalization and liabilities	$ 7,510,453	$ 7,256,636

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Common stock, par (in USD per share)	$ 1	$ 1
Common stock, authorized (in shares)	120,000,000	120,000,000
Common stock, issued (in shares)	57,995,563	57,192,925
Common stock, outstanding (in shares)	57,995,563	57,192,925

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Operating revenues:
Gas operating revenues	$ 521,932	$ 502,827	$ 1,369,690	$ 1,351,089
Utility infrastructure services revenues	535,520	504,793	1,369,878	1,337,907
Total operating revenues	885,907	836,320	3,348,460	3,122,698
Operating expenses:
Net cost of gas sold	156,021	160,821	338,037	353,381
Operations and maintenance	106,690	103,781	411,025	421,686
Depreciation and amortization	93,442	87,653	337,816	313,351
Taxes other than income taxes	20,687	16,378	67,769	62,500
Utility infrastructure services expenses	335,614	319,314	1,745,729	1,592,076
Total operating expenses	712,454	687,947	2,900,376	2,742,994
Operating income	173,453	148,373	448,084	379,704
Other income and (expenses):
Net interest deductions	(23,964)	(28,380)	(107,061)	(111,209)
Other income (deductions)	448	(20,770)	14,429	(17,524)
Total other income and (expenses)	(23,516)	(49,150)	(92,632)	(128,733)
Income before income taxes	149,937	99,223	355,452	250,971
Income tax expense	31,092	26,218	70,627	56,703
Net income	118,845	73,005	284,825	194,268
Net income attributable to noncontrolling interest	1,552	463	7,750	2,599
Net income attributable to Southwest Gas Holdings, Inc.	$ 117,293	$ 72,542	$ 277,075	$ 191,669
Earnings per share:
Basic (in USD per share)	$ 2.04	$ 1.31	$ 4.90	$ 3.50
Diluted (in USD per share)	$ 2.03	$ 1.31	$ 4.89	$ 3.50
Weighted average shares:
Basic (in shares)	57,600	55,310	56,564	54,726
Diluted (in shares)	57,679	55,363	56,649	54,792
Southwest Gas Corporation
Operating revenues:
Gas operating revenues	$ 521,932	$ 502,827	$ 1,369,690	$ 1,351,089
Operating expenses:
Net cost of gas sold	156,021	160,821	338,037	353,381
Operations and maintenance	106,135	103,088	409,429	419,720
Depreciation and amortization	68,698	64,725	239,268	222,733
Taxes other than income taxes	20,687	16,378	67,769	62,500
Total operating expenses	351,541	345,012	1,054,503	1,058,334
Operating income	170,391	157,815	315,187	292,755
Other income and (expenses):
Net interest deductions	(22,166)	(25,058)	(98,256)	(96,985)
Other income (deductions)	550	(20,536)	14,496	(16,965)
Total other income and (expenses)	(21,616)	(45,594)	(83,760)	(113,950)
Income before income taxes	148,775	112,221	231,427	178,805
Income tax expense	30,060	28,622	37,193	35,424
Net income	118,715	83,599	194,234	143,381
Net income attributable to Southwest Gas Holdings, Inc.	118,715	83,599	194,234	143,381
Centuri
Operating revenues:
Utility infrastructure services revenues	$ 363,975	$ 333,493	$ 1,978,770	$ 1,771,609

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Net income	$ 118,845	$ 73,005	$ 284,825	$ 194,268
Defined benefit pension plans:
Net actuarial loss	0	0	(43,730)	(54,026)
Amortization of prior service cost	182	220	840	945
Amortization of net actuarial loss	8,474	7,188	30,037	20,513
Prior service cost	0	0	0	(1,426)
Regulatory adjustment	(7,277)	(6,380)	4,753	25,760
Net defined benefit pension plans	1,379	1,028	(8,100)	(8,234)
Forward-starting interest rate swaps (“FSIRS”):
Amounts reclassified into net income	413	636	2,244	2,542
Net forward-starting interest rate swaps	413	636	2,244	2,542
Foreign currency translation adjustments	823	(4,005)	6,541	(2,758)
Total other comprehensive income (loss), net of tax	2,615	(2,341)	685	(8,450)
Comprehensive income	121,460	70,664	285,510	185,818
Comprehensive income attributable to noncontrolling interest	1,552	463	7,750	2,599
Comprehensive income attributable to Southwest Gas Holdings, Inc.	119,908	70,201	277,760	183,219
Southwest Gas Corporation
Net income	118,715	83,599	194,234	143,381
Defined benefit pension plans:
Net actuarial loss	0	0	(43,730)	(54,026)
Amortization of prior service cost	182	220	840	945
Amortization of net actuarial loss	8,474	7,188	30,037	20,513
Prior service cost	0	0	0	(1,426)
Regulatory adjustment	(7,277)	(6,380)	4,753	25,760
Net defined benefit pension plans	1,379	1,028	(8,100)	(8,234)
Forward-starting interest rate swaps (“FSIRS”):
Amounts reclassified into net income	413	636	2,244	2,542
Net forward-starting interest rate swaps	413	636	2,244	2,542
Total other comprehensive income (loss), net of tax	1,792	1,664	(5,856)	(5,692)
Comprehensive income	$ 120,507	$ 85,263	$ 188,378	$ 137,689

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$ 118,845	$ 73,005	$ 284,825	$ 194,268
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation and amortization	93,442	87,653	337,816	313,351
Deferred income taxes	23,326	25,309	48,734	54,548
Changes in current assets and liabilities:
Accounts receivable, net of allowances	42,892	45,837	(51,717)	7,154
Accrued utility revenue	31,900	31,000	(2,400)	(1,100)
Deferred purchased gas costs	(291,469)	10,155	(265,385)	19,527
Accounts payable	(41,147)	(60,723)	11,882	(49,945)
Accrued taxes	34,636	30,377	19,430	10,220
Other current assets and liabilities	(5,255)	76,453	25,719	102,742
Gains on sale of equipment	(1,509)	(28)	(3,329)	(5,268)
Changes in undistributed stock compensation	3,658	2,816	7,956	6,524
Equity AFUDC	(981)	(1,061)	(4,644)	(4,262)
Changes in deferred charges and other assets	(10,379)	6,495	(49,465)	1,772
Changes in other liabilities and deferred credits	(50,416)	(55,722)	(57,365)	(65,704)
Net cash provided by (used in) operating activities	(52,457)	271,566	302,057	583,827
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(152,709)	(210,655)	(767,159)	(938,141)
Acquisition of businesses, net of cash acquired	0	0	0	(47,638)
Changes in customer advances	4,286	5,434	12,885	21,357
Other	3,563	4,430	8,136	19,321
Net cash used in investing activities	(144,860)	(200,791)	(746,138)	(945,101)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	48,990	3,148	185,087	135,215
Dividends paid	(32,619)	(30,006)	(128,117)	(118,531)
Issuance of long-term debt, net	10,659	99,978	573,058	601,908
Retirement of long-term debt	(21,228)	(75,168)	(302,466)	(257,797)
Change in short-term debt	203,000	(54,000)	153,000	(31,000)
Withholding remittance - share-based compensation	(1,242)	(2,736)	(1,242)	(2,756)
Other	(1,353)	(250)	(4,505)	(1,615)
Net cash provided by (used in) financing activities	206,207	(59,034)	474,815	325,424
Effects of currency translation on cash and cash equivalents	103	(315)	646	(222)
Change in cash and cash equivalents	8,993	11,426	31,380	(36,072)
Cash and cash equivalents at beginning of period	83,352	49,539	60,965	97,037
Cash and cash equivalents at end of period	92,345	60,965	92,345	60,965
SUPPLEMENTAL INFORMATION:
Interest paid, net of amounts capitalized	8,303	13,073	100,412	99,481
Income taxes paid (received), net	1,651	(20,064)	10,764	(17,766)
Southwest Gas Corporation
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	118,715	83,599	194,234	143,381
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation and amortization	68,698	64,725	239,268	222,733
Deferred income taxes	14,952	7,707	52,242	15,118
Changes in current assets and liabilities:
Accounts receivable, net of allowances	(27,631)	1,975	(25,673)	21,652
Accrued utility revenue	31,900	31,000	(2,400)	(1,100)
Deferred purchased gas costs	(291,469)	10,155	(265,385)	19,527
Accounts payable	(33,076)	(41,899)	18,441	(29,798)
Accrued taxes	41,851	53,335	(13,011)	33,526
Other current assets and liabilities	41,018	99,257	(9,694)	106,803
Changes in undistributed stock compensation	2,908	2,496	5,706	5,045
Equity AFUDC	(981)	(1,061)	(4,644)	(4,262)
Changes in deferred charges and other assets	(13,535)	3,658	(61,484)	(9,871)
Changes in other liabilities and deferred credits	(48,782)	(55,910)	(58,008)	(66,294)
Net cash provided by (used in) operating activities	(95,432)	259,037	69,592	456,460
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(128,544)	(173,353)	(647,407)	(788,465)
Changes in customer advances	4,285	5,433	12,885	21,356
Other	(121)	(31)	681	(48)
Net cash used in investing activities	(124,380)	(167,951)	(633,841)	(767,157)
CASH FLOW FROM FINANCING ACTIVITIES:
Contributions from parent	45,984	50,000	173,906	187,094
Dividends paid	(26,000)	(25,200)	(105,300)	(97,600)
Issuance of long-term debt, net	0	0	446,508	297,222
Retirement of long-term debt	0	0	(125,000)	0
Change in short-term debt	210,000	(97,000)	170,000	(91,000)
Withholding remittance - share-based compensation	(1,242)	(2,736)	(1,242)	(2,756)
Other	(205)	(115)	(1,352)	(887)
Net cash provided by (used in) financing activities	228,537	(75,051)	557,520	292,073
Change in cash and cash equivalents	8,725	16,035	(6,729)	(18,624)
Cash and cash equivalents at beginning of period	41,070	40,489	56,524	75,148
Cash and cash equivalents at end of period	49,795	56,524	49,795	56,524
SUPPLEMENTAL INFORMATION:
Interest paid, net of amounts capitalized	6,952	10,204	93,474	87,277
Income taxes paid (received), net	$ 0	$ (22,962)	$ 3,359	$ 700

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Thousands	Total	Southwest Gas Corporation	Common Stock	Common StockSouthwest Gas Corporation	Additional paid-in capital	Additional paid-in capitalSouthwest Gas Corporation	Accumulated other comprehensive loss	Accumulated other comprehensive lossSouthwest Gas Corporation	Retained earnings	Retained earningsSouthwest Gas Corporation	Southwest Gas Holdings, Inc.
Beginning balance (in shares) at Dec. 31, 2019			55,007,000
Beginning balance at Dec. 31, 2019			$ 56,637		$ 1,466,937	$ 1,229,083	$ (56,732)	$ (55,151)	$ 1,039,072	$ 782,108
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			119,000
Common stock issuances			$ 119		3,474
Foreign currency exchange translation adjustment							(4,005)
Net actuarial gain arising during period, less amortization of unamortized benefit plan cost, net of tax							1,028	1,028
FSIRS amounts reclassified to net income, net of tax	$ 636	$ 636					636	636
Net income									72,542	83,599
Share-based compensation						125				(364)
Contributions from Southwest Gas Holdings, Inc.						50,000
Dividends declared									(31,813)	(25,900)
Redemption value adjustments									0
Ending balance (in shares) at Mar. 31, 2020			55,126,000	47,482,000
Ending balance at Mar. 31, 2020		2,114,276	$ 56,756	$ 49,112	1,470,411	1,279,208	(59,073)	(53,487)	1,079,801	839,443	$ 2,547,895
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 0.57
FSIRS amounts reclassified to net income, net of tax	$ 2,244	2,244
Ending balance (in shares) at Mar. 31, 2021	57,995,563		57,995,000	47,482,000
Ending balance at Mar. 31, 2021		2,374,124	$ 59,625	$ 49,112	1,660,108	1,458,344	(58,388)	(59,343)	1,112,377	926,011	2,773,722
Beginning balance (in shares) at Dec. 31, 2020	57,192,925		57,193,000
Beginning balance at Dec. 31, 2020			$ 58,823		1,609,155	1,410,345	(61,003)	(61,135)	1,067,978	835,146
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			802,000
Common stock issuances			$ 802		50,953
Foreign currency exchange translation adjustment							823
Net actuarial gain arising during period, less amortization of unamortized benefit plan cost, net of tax							1,379	1,379
FSIRS amounts reclassified to net income, net of tax	$ 413	413					413	413
Net income									117,293	118,715
Share-based compensation						2,015				(350)
Contributions from Southwest Gas Holdings, Inc.						45,984
Dividends declared									(34,876)	(27,500)
Redemption value adjustments	$ 38,018								(38,018)
Ending balance (in shares) at Mar. 31, 2021	57,995,563		57,995,000	47,482,000
Ending balance at Mar. 31, 2021		$ 2,374,124	$ 59,625	$ 49,112	$ 1,660,108	$ 1,458,344	$ (58,388)	$ (59,343)	$ 1,112,377	$ 926,011	$ 2,773,722
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 0.595

Background, Organization, and S

Background, Organization, and Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Background, Organization, and Summary of Significant Accounting Policies	Note 1 – Background, Organization, and Summary of Significant Accounting Policies Nature of Operations. Southwest Gas Holdings, Inc. is a holding company, owning all of the shares of common stock of Southwest Gas Corporation (“Southwest” or the “natural gas operations” segment) 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 most 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. The condensed consolidated financial statements of Southwest Gas Holdings, Inc. and subsidiaries (the “Company”) and Southwest included herein have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) have been condensed or omitted pursuant to such rules and regulations. No substantive change has occurred with regard to the Company’s business segments on the whole. The preparation of the condensed consolidated financial statements 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. In the opinion of management, all adjustments, consisting of normal recurring items and estimates necessary for a fair depiction of results for the interim periods, have been made. In association with the novel Coronavirus (“COVID-19”) pandemic environment, utility operations, and to a large extent, utility infrastructure services, have been deemed “essential services.” Management has considered the impact of the pandemic and adjusted certain estimates, where relevant, used in the preparation of the condensed consolidated financial statements. These condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the 2020 Annual Report to Stockholders, which is incorporated by reference into the 2020 Form 10-K. 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, long-term debt outstanding, and assets of the qualified pension plan and postretirement benefit plans required to be recorded and/or disclosed at fair value. Other Property and Investments. Other property and investments on the Condensed Consolidated Balance Sheets includes: (Thousands of dollars) March 31, 2021 December 31, 2020 Southwest Gas Corporation: Net cash surrender value of COLI policies $ 143,618 $ 140,874 Other property 2,720 2,737 Total Southwest Gas Corporation 146,338 143,611 Centuri property, equipment, and intangibles 1,106,855 1,089,414 Centuri accumulated provision for depreciation and amortization (439,126) (422,741) Other property and investments 28,605 23,961 Total Southwest Gas Holdings, Inc. $ 842,672 $ 834,245 Included in the table above are the net cash surrender values of company-owned life insurance (“COLI”) policies. These life insurance policies on members of management and other key employees are used by Southwest to indemnify itself against the loss of talent, expertise, and knowledge, as well as to provide indirect funding for certain nonqualified benefit plans. Balances reflect impacts of equity and fixed-income securities underlying the cash surrender values at each reporting date; however, ultimately, only the insurance proceeds are ever actually received, due to management’s intent to hold the policies to maturity. 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 of Southwest and the Company include money market fund investments totaling approximately $40,000 for each entity at March 31, 2021 and December 31, 2020, respectively, 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 include customer advances applied as contributions toward utility construction activity, and capital expenditures that were not paid as of period-end reporting dates, but rather included in accounts payable. Typical activities that represent aspects of both non-cash investing and non-cash financing activities relate to right-of-use assets obtained in exchange for lease liabilities (including, at times, lease terminations and modifications). Amounts related to these collective activities were immaterial for the periods presented herein. Intercompany Transactions . Centuri recognizes revenues generated from contracts with Southwest (see Note 7 – Segment Information ). The accounts receivable balance, revenues, and associated profits are included in the condensed consolidated financial statements of the Company and Southwest and were not eliminated during consolidation in accordance with accounting treatment for rate-regulated entities. Accounts Receivable, net of 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. Due to the ongoing COVID-19 pandemic, Southwest continued the moratorium initiated in March 2020 on disconnection of natural gas service for non-payment, but recommenced assessing late fees in Nevada and Arizona in April 2021, with late fees in California expected to recommence in the latter half of 2021. Southwest is actively working with customers experiencing financial hardship by means of flexible payment options. Management continues to monitor expected credit losses in light of the evolving impact of COVID-19. The allowance as of March 31, 2021 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 on disconnections 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 has not been significantly impacted, nor does it anticipate it will experience significant difficulty in collecting amounts due, as a result of the current environment surrounding COVID-19 given the nature of its customers. Activity between periods in the allowance for uncollectibles and the balances as of the periods presented within the Company’s and Southwest’s financial statements were not material to the condensed consolidated financial statements overall. 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 supply issues due to wellhead freeze-offs, power outages, or other adverse operating conditions upstream of Southwest’s distribution systems. These conditions caused daily natural gas prices to reach unprecedented levels. During this time, Southwest secured natural gas supplies, albeit at substantially higher prices, maintaining service to its customers. The incremental cost for these supplies was approximately $250 million, funded using a 364-day $250 million Bank Term Loan executed in March 2021 (see Note 5 – Debt) . The incremental gas costs are expected to be collected from customers through the purchased gas adjustment (“PGA”) mechanisms. Following the extreme weather event, an interstate transmission pipeline company billed Southwest, in addition to customary transmission costs, $65 million for pipeline imbalance charges, allegedly incurred during the period of the pipeline’s critical operation condition. However, Southwest has formally disputed these imbalance charges and believes that no amounts are due to the pipeline. Consequently, Southwest has not recognized this charge. Pipeline transmission costs, including periodic imbalance charges, are components of the cost of gas recovered from customers through the PGA and similar mechanisms. Prepaid and other current assets. Prepaid and other current assets includes gas pipe materials and operating supplies of $47 million at March 31, 2021 and $50 million at December 31, 2020 (carried at weighted average cost). Goodwill. Goodwill is assessed as of October 1 st each year for impairment, or more frequently, if circumstances indicate an impairment to the carrying value of goodwill may have occurred. 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. Since December 31, 2020, management also qualitatively assessed whether events during the first three months of 2021 may have resulted in conditions whereby the carrying value of goodwill was higher than its fair value, which if the case, could be an indication of a permanent impairment. Through this assessment, no such condition was believed to have existed and therefore, no impairment was deemed to have occurred. Goodwill on Southwest’s and the Company’s Condensed Consolidated Balance Sheets includes: (Thousands of dollars) Natural Gas Utility Infrastructure Total Company December 31, 2020 $ 10,095 $ 335,089 $ 345,184 Foreign currency translation adjustment — 1,369 1,369 March 31, 2021 $ 10,095 $ 336,458 $ 346,553 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 amounts payable under regulatory mechanisms, customary accrued expenses for employee compensation and benefits, declared but unpaid dividends, and miscellaneous other accrued liabilities. Other current liabilities for the Company includes $34.5 million and $32.6 million of dividends declared, as well as $37 million and $25.2 million of accrued property taxes, as of March 31, 2021 and December 31, 2020, respectively. Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) in the Condensed Consolidated Statements of Income: Three Months Ended March 31, Twelve Months Ended (Thousands of dollars) 2021 2020 2021 2020 Southwest Gas Corporation - natural gas operations segment: Change in COLI policies $ 2,700 $ (15,500) $ 27,400 $ (5,700) Interest income 716 1,388 3,343 6,147 Equity AFUDC 981 1,061 4,644 4,262 Other components of net periodic benefit cost (3,505) (5,005) (18,522) (16,299) Miscellaneous income and (expense) (342) (2,480) (2,369) (5,375) Southwest Gas Corporation - total other income (deductions) 550 (20,536) 14,496 (16,965) Utility infrastructure services segment: Foreign transaction gain (loss) (3) (10) (9) 5 Miscellaneous income and (expense) (99) (232) (58) (656) Centuri - total other income (deductions) (102) (242) (67) (651) Corporate and administrative — 8 — 92 Consolidated Southwest Gas Holdings, Inc. - total other income (deductions) $ 448 $ (20,770) $ 14,429 $ (17,524) Included in the table above is the change in cash surrender values of COLI policies (including net death benefits recognized). Current tax regulations provide for tax-free treatment of life insurance (death benefit) proceeds. Therefore, changes in the cash surrender values of COLI policies, as they progress towards the ultimate death benefits, are also recorded without tax consequences. Refer to Other Property and Investments above and also to Note 2 – Components of Net Periodic Benefit Cost . 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. 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 $38 million during the first quarter of 2021 . Adjustment to the redemption value also impacts retained earnings, as reflected in the Company’s Condensed Consolidated Statement of Equity, but does not impact net income. The following depicts the change to the balance of the redeemable noncontrolling interest: (Thousands of dollars): Redeemable Noncontrolling Interest Balance, December 31, 2020 $ 165,716 Net income attributable to redeemable noncontrolling interest 1,552 Redemption value adjustment 38,018 Balance, March 31, 2021 $ 205,286 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. A reconciliation of the denominator used in Basic and Diluted EPS calculations is shown in the following table: Three Months Ended Twelve Months Ended (In thousands) 2021 2020 2021 2020 Weighted average basic shares 57,600 55,310 56,564 54,726 Effect of dilutive securities: Management Incentive Plan shares — — — 9 Restricted stock units (1) 79 53 85 57 Weighted average diluted shares 57,679 55,363 56,649 54,792 (1) The number of securities included 75,000 and 50,000 performance shares during the three months ending March 31, 2021 and 2020, and 76,000 and 48,000 performance shares during the twelve months ending March 31, 2021 and 2020, respectively, the total of which was derived by assuming that target performance will be achi eved during the relevant performance period. Recent Accounting Standards Updates. Accounting pronouncements adopted in 2021: In December 2019, the Financial Accounting Standards Board (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 Company and Southwest adopted the update in the first quarter of 2021, the impact of which was not material to the condensed consolidated financial statements of the Company or Southwest. 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 Topic 848) to replace a reference rate affected by such reform. The update applies only to contracts and hedging relationships that reference the London Interbank Offered Rate (“LIBOR”) or another 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 5 – 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 what impacts, if any, this update might have on the Company’s consolidated financial statements and disclosures.

Components of Net Periodic Bene

Components of Net Periodic Benefit Cost	3 Months Ended
	Mar. 31, 2021
Retirement Benefits [Abstract]
Components of Net Periodic Benefit Cost	Note 2 – Components of Net Periodic Benefit Cost Southwest has a noncontributory qualified retirement plan with defined benefits covering substantially all employees and a separate unfunded supplemental retirement plan (“SERP”) which is limited to officers. Southwest also provides postretirement benefits other than pensions (“PBOP”) to its qualified retirees for health care, dental, and life insurance. The service cost component of net periodic benefit costs included in the table below is a component of an overhead loading process associated with the cost of labor. The overhead process ultimately results in allocation of service cost 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 other components of net periodic benefit cost are reflected in Other income (deductions) on the Condensed Consolidated Statements of Income of each entity. Qualified Retirement Plan March 31, Three Months Twelve Months 2021 2020 2021 2020 (Thousands of dollars) Service cost $ 10,290 $ 8,574 $ 36,015 $ 27,972 Interest cost 10,108 11,388 44,275 48,142 Expected return on plan assets (18,088) (16,324) (67,060) (61,507) Amortization of net actuarial loss 10,489 9,007 37,507 25,774 Net periodic benefit cost $ 12,799 $ 12,645 $ 50,737 $ 40,381 SERP March 31, Three Months Twelve Months 2021 2020 2021 2020 (Thousands of dollars) Service cost $ 131 $ 98 $ 422 $ 298 Interest cost 358 401 1,561 1,721 Amortization of net actuarial loss 660 451 2,014 1,216 Net periodic benefit cost $ 1,149 $ 950 $ 3,997 $ 3,235 PBOP March 31, Three Months Twelve Months 2021 2020 2021 2020 (Thousands of dollars) Service cost $ 423 $ 396 $ 1,608 $ 1,353 Interest cost 548 645 2,485 2,929 Expected return on plan assets (810) (852) (3,366) (3,219) Amortization of prior service costs 240 289 1,106 1,243 Net periodic benefit cost $ 401 $ 478 $ 1,833 $ 2,306

Revenue

Revenue	3 Months Ended
	Mar. 31, 2021
Revenue from Contract with Customer [Abstract]
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 : Gas operating revenues on the Condensed 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: Three Months Ended Twelve Months Ended March 31, (Thousands of dollars) 2021 2020 2021 2020 Residential $ 403,143 $ 378,555 $ 983,108 $ 934,115 Small commercial 81,398 82,463 220,476 241,970 Large commercial 12,673 12,667 44,639 47,640 Industrial/other 13,770 6,702 33,310 22,298 Transportation 24,536 24,406 88,345 91,884 Revenue from contracts with customers 535,520 504,793 1,369,878 1,337,907 Alternative revenue program revenues (deferrals) (16,373) (3,765) (468) 5,668 Other revenues (1) 2,785 1,799 280 7,514 Total Gas operating revenues $ 521,932 $ 502,827 $ 1,369,690 $ 1,351,089 (1) Amounts include late fees and other miscellaneous revenues, and may also include the impact of certain regulatory mechanisms, such as cost-of-service components in customer rates expected to be returned to customers in future periods. Late fees and certain other fees were reduced, for the three- and twelve-month periods ended March 31, 2021, due to a moratorium on late fees and disconnection for nonpayment during the COVID-19 pandemic. Utility Infrastructure Services Segment : The following tables display Centuri’s revenue, reflected as Utility infrastructure services revenues on the Condensed Consolidated Statements of Income of the Company, representing revenue from contracts with customers disaggregated by service and contract types: Three Months Ended Twelve Months Ended March 31, (Thousands of dollars) 2021 2020 2021 2020 Service Types: Gas infrastructure services $ 221,837 $ 217,709 $ 1,265,288 $ 1,258,790 Electric power infrastructure services 93,961 72,320 433,467 267,736 Other 48,177 43,464 280,015 245,083 Total Utility infrastructure services revenues $ 363,975 $ 333,493 $ 1,978,770 $ 1,771,609 Three Months Ended Twelve Months Ended March 31, (Thousands of dollars) 2021 2020 2021 2020 Contract Types: Master services agreement $ 293,680 $ 263,545 $ 1,520,144 $ 1,411,267 Bid contract 70,295 69,948 458,626 360,342 Total Utility infrastructure services revenues $ 363,975 $ 333,493 $ 1,978,770 $ 1,771,609 Unit price contracts $ 234,449 $ 243,136 $ 1,347,953 $ 1,387,706 Fixed price contracts 34,594 27,545 164,750 101,931 Time and materials contracts 94,932 62,812 466,067 281,972 Total Utility infrastructure services revenues $ 363,975 $ 333,493 $ 1,978,770 $ 1,771,609 The following table provides information about contracts receivable and revenue earned on contracts in progress in excess of billings (contract asset), which are both included within Accounts receivable, net of allowances, as well as amounts billed in excess of revenue earned on contracts (contract liability), which are included in Other current liabilities as of March 31, 2021 and December 31, 2020 on the Company’s Condensed Consolidated Balance Sheets: (Thousands of dollars) March 31, 2021 December 31, 2020 Contracts receivable, net $ 197,357 $ 278,316 Revenue earned on contracts in progress in excess of billings 107,336 96,996 Amounts billed in excess of revenue earned on contracts 5,985 4,507 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 for billing at the reporting date. These contract assets are transferred to contracts receivable when the rights become unconditional. 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, 2020 to March 31, 2021 is due to revenue recognized of $4.5 million th at was included in this item as of January 1, 2021, 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 uses the practical expedient applicable to such contracts and does not consider/compute an interest component based on the time value of money. Further, because of the short duration of these contracts, Centuri 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 March 31, 2021, Centuri had 18 c ontracts 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 March 31, 2021 was $48.7 million . Centuri expects to recognize the remaining performance obligation s over approximately the next two years; ho wever, 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 are provided by the customer. Utility infrastructure services contracts receivable consists of the following: (Thousands of dollars) March 31, 2021 December 31, 2020 Billed on completed contracts and contracts in progress $ 193,042 $ 273,778 Other receivables 5,789 6,692 Contracts receivable, gross 198,831 280,470 Allowance for doubtful accounts (1,474) (2,154) Contracts receivable, net $ 197,357 $ 278,316

Common Stock

Common Stock	3 Months Ended
	Mar. 31, 2021
Equity [Abstract]
Common Stock	Note 4 – 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 stock awards to be issued in shares of Southwest Gas Holdings, Inc. On May 8, 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 8, 2019, between the Company and BNY Mellon Capital Markets, LLC (the “Equity Shelf Program”). The following table provides the activity under the Equity Shelf Program for the three-month and life-to-date periods ended March 31, 2021: Three Months Ended Life-To-Date Ended March 31, 2021 Gross proceeds $ 46,448,484 $ 299,999,974 Less: agent commissions (464,485) (3,000,000) Net proceeds $ 45,983,999 $ 296,999,974 Number of shares sold 705,957 4,102,414 Weighted average price per share $ 65.80 $ 73.13 During the quarter ended March 31, 2021, the Company sold essentially all of the remaining common stock available for sale under the program. Net proceeds from the sale of shares of common stock under the Equity Shelf Program 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. On April 8, 2021, the Company entered into a Sales Agency Agreement between the Company and BNY Mellon Capital Markets, LLC and J.P. Morgan Securities LLC for the offer and sale of up to $500 million of common stock from time to time in a new at-the-market offering program. The shares will be issued pursuant to the Company’s automatic shelf registration statement on Form S-3 (File No. 333-251074), which became effective upon filing with the SEC on December 2, 2020. During the three months ended March 31, 2021, the Company issued approximately 47,000 shares of common stock through the Restricted Stock/Unit Plan and Omnibus Incentive Plan. Additionally, during the three months ended March 31, 2021, the Company issued 49,500 shares of common stock through the Dividend Reinvestment and Stock Purchase Plan, raising approximately $3.1 million.

Debt

Debt	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Debt	Note 5 – Debt Long-Term Debt Long-term debt is recognized in the Company’s and Southwest’s Condensed Consolidated Balance Sheets generally at the carrying value of the obligations outstanding. Details surrounding the fair value, as described in Note 1 – Background, Organization, and Summary of Significant Accounting Policies, and individual carrying values of instruments are provided in the table that follows. March 31, 2021 December 31, 2020 Carrying Fair Carrying Fair (Thousands of dollars) Southwest Gas Corporation: Debentures: Notes, 6.1%, due 2041 $ 125,000 $ 160,651 $ 125,000 $ 174,858 Notes, 3.875%, due 2022 250,000 256,693 250,000 258,825 Notes, 4.875%, due 2043 250,000 288,500 250,000 317,190 Notes, 3.8%, due 2046 300,000 306,261 300,000 347,046 Notes, 3.7%, due 2028 300,000 324,069 300,000 344,553 Notes, 4.15%, due 2049 300,000 324,183 300,000 370,278 Notes, 2.2%, due 2030 450,000 438,912 450,000 474,552 8% Series, due 2026 75,000 95,894 75,000 99,723 Medium-term notes, 7.78% series, due 2022 25,000 26,201 25,000 26,663 Medium-term notes, 7.92% series, due 2027 25,000 32,138 25,000 33,802 Medium-term notes, 6.76% series, due 2027 7,500 9,127 7,500 9,613 Unamortized discount and debt issuance costs (17,501) (17,822) 2,089,999 2,089,678 Revolving credit facility and commercial paper 150,000 150,000 150,000 150,000 Industrial development revenue bonds: Variable-rate 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 (1,411) (1,472) 198,589 198,528 Less: current maturities (25,000) — Long-term debt, less current maturities - Southwest Gas Corporation $ 2,413,588 $ 2,438,206 Centuri: Centuri term loan facility $ 222,668 $ 225,269 $ 226,648 $ 230,824 Unamortized debt issuance costs (750) (820) 221,918 225,828 Centuri secured revolving credit facility 26,163 26,180 26,626 26,645 Centuri other debt obligations 77,235 78,378 81,973 84,246 Less: current maturities (42,334) (40,433) Long-term debt, less current maturities - Centuri $ 282,982 $ 293,994 Consolidated Southwest Gas Holdings, Inc.: Southwest Gas Corporation long-term debt $ 2,438,588 $ 2,438,206 Centuri long-term debt 325,316 334,427 Less: current maturities (67,334) (40,433) Long-term debt, less current maturities - Southwest Gas Holdings, Inc. $ 2,696,570 $ 2,732,200 The fair values of Southwest's 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. 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, and as such are categorized as Level 2 in the hierarchy. Centuri's secured revolving credit and term loan facility and other debt obligations (not actively traded) are categorized as Level 3; fair values were based on a conventional discounted cash flow methodology utilizing current market pricing yield curves. Southwest has a $400 million credit facility that is scheduled to expire in April 2025. Southwest designates $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 March 31, 2021, 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. At March 31, 2021, $150 million was outstanding on the long-term portion (including $50 million under the commercial paper program, discussed below) of the facility and $17 million of borrowings were outstanding on the short-term portion of this credit facility discussed below. Southwest has a $50 million commercial paper program. Issuances under the commercial paper program are supported by Southwest’s revolving credit facility and, therefore, do not represent additional borrowing capacity under the credit facility. Borrowings under the commercial paper program are designated a s long-term debt. Interest rates for the program are calculated at the then current commercial paper rate. At March 31, 2021, as noted above, $50 million of borrowings were outstanding under the commercial paper program. 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. The $590 million facility 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 March 31, 2021 totaled $1.3 billion . At March 31, 2021 , $249 million in bor rowings were outstanding under Centuri’s combined secured revolving credit and term loan facility. Short-Term Debt Southwest Gas Holdings, Inc. has a $100 million credit facility that is scheduled to expire in April 2025 and is primarily used for short-term financing needs. There was $43 million outstanding under this credit facility as of March 31, 2021. Under Southwest’s $400 million credit facility, Southwest had $17 million in short-term borrowings outstanding at March 31, 2021. In March 2021, Southwest issued a $250 million Term Loan that matures March 22, 2022. The proceeds were used to fund the increased cost of natural gas supply during the month of February 2021, caused by extreme weather conditions in the central U.S. (see Deferred Purchased Gas Costs in Note 1 – Background, Organization, and Summary of Significant Accounting Policies ). Interest rates for the term loan 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 long-term debt rating. The applicable margin ranges from 0.550% to 1.000% for loans bearing interest with reference to LIBOR and 0.000% for loans bearing interest with reference to an alternate base rate. The effective interest rate was 0.77% at March 31, 2021. The 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. LIBOR LIBOR is scheduled to be discontinued as a benchmark or reference rate after 2021. In order to mitigate the impact of a discontinuance on the Company’s and Southwest’s financial condition and results of operations, management will 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.

Other Comprehensive Income and

Other Comprehensive Income and Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2021
Equity [Abstract]
Other Comprehensive Income and Accumulated Other Comprehensive Income	Note 6 – Other Comprehensive Income and Accumulated Other Comprehensive Income The following information presents the Company’s Other comprehensive income (loss), both before and after-tax impacts, within the Condensed Consolidated Statements of Comprehensive Income, which also impact Accumulated other comprehensive income (“AOCI”) in the Condensed Consolidated Balance Sheets and the Condensed Consolidated Statements of Equity. Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) Three Months Ended Three Months Ended (Thousands of dollars) Before- Tax Net-of- Before- Tax Net-of- Defined benefit pension plans: Amortization of prior service cost $ 240 $ (58) $ 182 $ 289 $ (69) $ 220 Amortization of net actuarial (gain)/loss 11,149 (2,675) 8,474 9,458 (2,270) 7,188 Regulatory adjustment (9,575) 2,298 (7,277) (8,395) 2,015 (6,380) Pension plans other comprehensive income (loss) 1,814 (435) 1,379 1,352 (324) 1,028 FSIRS (designated hedging activities): Amounts reclassified into net income 544 (131) 413 837 (201) 636 FSIRS other comprehensive income (loss) 544 (131) 413 837 (201) 636 Total other comprehensive income (loss) - Southwest Gas Corporation 2,358 (566) 1,792 2,189 (525) 1,664 Foreign currency translation adjustments: Translation adjustments 823 — 823 (4,005) — (4,005) Foreign currency other comprehensive income (loss) 823 — 823 (4,005) — (4,005) Total other comprehensive income (loss) - Southwest Gas Holdings, Inc. $ 3,181 $ (566) $ 2,615 $ (1,816) $ (525) $ (2,341) Twelve Months Ended Twelve Months Ended (Thousands of dollars) Before- Tax Net-of- Before- Tax Net-of- Defined benefit pension plans: Net actuarial gain/(loss) $ (57,539) $ 13,809 $ (43,730) $ (71,087) $ 17,061 $ (54,026) Amortization of prior service cost 1,106 (266) 840 1,243 (298) 945 Amortization of net actuarial (gain)/loss 39,521 (9,484) 30,037 26,990 (6,477) 20,513 Prior service cost — — — (1,878) 452 (1,426) Regulatory adjustment 6,255 (1,502) 4,753 33,896 (8,136) 25,760 Pension plans other comprehensive income (loss) (10,657) 2,557 (8,100) (10,836) 2,602 (8,234) FSIRS (designated hedging activities): Amounts reclassified into net income 2,954 (710) 2,244 3,345 (803) 2,542 FSIRS other comprehensive income (loss) 2,954 (710) 2,244 3,345 (803) 2,542 Total other comprehensive income (loss) - Southwest Gas Corporation (7,703) 1,847 (5,856) (7,491) 1,799 (5,692) Foreign currency translation adjustments: Translation adjustments 6,541 — 6,541 (2,758) — (2,758) Foreign currency other comprehensive income (loss) 6,541 — 6,541 (2,758) — (2,758) Total other comprehensive income (loss) - Southwest Gas Holdings, Inc. $ (1,162) $ 1,847 $ 685 $ (10,249) $ 1,799 $ (8,450) (1) Tax amounts are calculated using a 24% rate. The Company has elected to indefinitely reinvest the earnings of Centuri’s Canadian subsidiaries in Canada, thus precluding 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 reported in Other comprehensive income (loss). Approximately $1.7 million of realized losses (net of tax) related to the remaining balance of Southwest’s previously settled forward-starting interest rate swap (“FSIRS”), included in AOCI at March 31, 2021, will be reclassified into interest expense within the next 12 months as the related interest payments on long-term debt occur. The following table represents a rollforward of AOCI, presented on the Company’s Condensed Consolidated Balance Sheets and its Condensed Consolidated Statements of Equity: Defined Benefit Plans FSIRS Foreign Currency Items (Thousands of dollars) Before-Tax Tax After-Tax Before-Tax Tax After-Tax Before-Tax Tax After-Tax AOCI Beginning Balance AOCI December 31, 2020 $ (77,720) $ 18,653 $ (59,067) $ (2,719) $ 651 $ (2,068) $ 132 $ — $ 132 $ (61,003) Translation adjustments — — — — — — 823 — 823 823 Other comprehensive income (loss) before reclassifications — — — — — — 823 — 823 823 FSIRS amount reclassified from AOCI (1) — — — 544 (131) 413 — — — 413 Amortization of prior service cost (2) 240 (58) 182 — — — — — — 182 Amortization of net actuarial loss (2) 11,149 (2,675) 8,474 — — — — — — 8,474 Regulatory adjustment (3) (9,575) 2,298 (7,277) — — — — — — (7,277) Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. 1,814 (435) 1,379 544 (131) 413 823 — 823 2,615 Ending Balance AOCI March 31, 2021 $ (75,906) $ 18,218 $ (57,688) $ (2,175) $ 520 $ (1,655) $ 955 $ — $ 955 $ (58,388) (1) The FSIRS reclassification amount is included in Net interest deductions on the Company’s Condensed Consolidated Statements of Income. (2) These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost 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 Condensed Consolidated Balance Sheets). (4) Tax amounts are calculated using a 24% rate. The following table represents a rollforward of AOCI, presented on Southwest’s Condensed Consolidated Balance Sheets: Defined Benefit Plans FSIRS (Thousands of dollars) Before-Tax Tax After-Tax Before-Tax Tax After-Tax AOCI Beginning Balance AOCI December 31, 2020 $ (77,720) $ 18,653 $ (59,067) $ (2,719) $ 651 $ (2,068) $ (61,135) FSIRS amount reclassified from AOCI (5) — — — 544 (131) 413 413 Amortization of prior service cost (6) 240 (58) 182 — — — 182 Amortization of net actuarial loss (6) 11,149 (2,675) 8,474 — — — 8,474 Regulatory adjustment (7) (9,575) 2,298 (7,277) — — — (7,277) Net current period other comprehensive income attributable to Southwest Gas Corporation 1,814 (435) 1,379 544 (131) 413 1,792 Ending Balance AOCI March 31, 2021 $ (75,906) $ 18,218 $ (57,688) $ (2,175) $ 520 $ (1,655) $ (59,343) (5) The FSIRS reclassification amount is included in Net interest deductions on Southwest’s Condensed Consolidated Statements of Income. (6) These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost for additional details). (7) 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 Southwest’s Condensed Consolidated Balance Sheets). (8) 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: (Thousands of dollars) March 31, 2021 December 31, 2020 Net actuarial loss $ (491,634) $ (502,783) Prior service cost (2,247) (2,487) Less: amount recognized in regulatory assets 417,975 427,550 Recognized in AOCI $ (75,906) $ (77,720)

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2021
Segment Reporting [Abstract]
Segment Information	Note 7 – Segment Information 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: (Thousands of dollars) March 31, 2021 December 31, 2020 Centuri accounts receivable for services provided to Southwest $ 11,344 $ 13,956 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 Condensed 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 is as follows: (Thousands of dollars) Natural Gas Utility Infrastructure Other Total Three Months Ended March 31, 2021 Revenues from external customers $ 521,932 $ 339,772 $ — $ 861,704 Intersegment revenues — 24,203 — 24,203 Total $ 521,932 $ 363,975 $ — $ 885,907 Segment net income (loss) $ 118,715 $ (859) $ (563) $ 117,293 Three Months Ended March 31, 2020 Revenues from external customers $ 502,827 $ 300,291 $ — $ 803,118 Intersegment revenues — 33,202 — 33,202 Total $ 502,827 $ 333,493 $ — $ 836,320 Segment net income (loss) $ 83,599 $ (10,204) $ (853) $ 72,542 (Thousands of dollars) Natural Gas Utility Infrastructure Other Total Twelve Months Ended March 31, 2021 Revenues from external customers $ 1,369,690 $ 1,852,910 $ — $ 3,222,600 Intersegment revenues — 125,860 — 125,860 Total $ 1,369,690 $ 1,978,770 $ — $ 3,348,460 Segment net income (loss) $ 194,234 $ 84,207 $ (1,366) $ 277,075 Twelve Months Ended March 31, 2020 Revenues from external customers $ 1,351,089 $ 1,618,354 $ — $ 2,969,443 Intersegment revenues — 153,255 — 153,255 Total $ 1,351,089 $ 1,771,609 $ — $ 3,122,698 Segment net income (loss) $ 143,381 $ 50,231 $ (1,943) $ 191,669

Background, Organization, and_2

Background, Organization, and Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations. Southwest Gas Holdings, Inc. is a holding company, owning all of the shares of common stock of Southwest Gas Corporation (“Southwest” or the “natural gas operations” segment) 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.
Basis of Presentation	Basis of Presentation. The condensed consolidated financial statements of Southwest Gas Holdings, Inc. and subsidiaries (the “Company”) and Southwest included herein have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) have been condensed or omitted pursuant to such rules and regulations. No substantive change has occurred with regard to the Company’s business segments on the whole. The preparation of the condensed consolidated financial statements 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. In the opinion of management, all adjustments, consisting of normal recurring items and estimates necessary for a fair depiction of results for the interim periods, have been made. In association with the novel Coronavirus (“COVID-19”) pandemic environment, utility operations, and to a large extent, utility infrastructure services, have been deemed “essential services.” Management has considered the impact of the pandemic and adjusted certain estimates, where relevant, used in the preparation of the condensed consolidated financial statements. These condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the 2020 Annual Report to Stockholders, which is incorporated by reference into the 2020 Form 10-K.
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.
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 of Southwest and the Company include money market fund investments totaling approximately $40,000 for each entity at March 31, 2021 and December 31, 2020, respectively, 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 include customer advances applied as contributions toward utility construction activity, and capital expenditures that were not paid as of period-end reporting dates, but rather included in accounts payable. Typical activities that represent aspects of both non-cash investing and non-cash financing activities relate to right-of-use assets obtained in exchange for lease liabilities (including, at times, lease terminations and modifications). Amounts related to these collective activities were immaterial for the periods presented herein.
Intercompany Transactions	Intercompany Transactions . Centuri recognizes revenues generated from contracts with Southwest (see Note 7 – Segment Information ). The accounts receivable balance, revenues, and associated profits are included in the condensed consolidated financial statements of the Company and Southwest and were not eliminated during consolidation in accordance with accounting treatment for rate-regulated entities.
Accounts Receivable, net of allowances	Accounts Receivable, net of 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. Due to the ongoing COVID-19 pandemic, Southwest continued the moratorium initiated in March 2020 on disconnection of natural gas service for non-payment, but recommenced assessing late fees in Nevada and Arizona in April 2021, with late fees in California expected to recommence in the latter half of 2021. Southwest is actively working with customers experiencing financial hardship by means of flexible payment options. Management continues to monitor expected credit losses in light of the evolving impact of COVID-19. The allowance as of March 31, 2021 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 on disconnections 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 has not been significantly impacted, nor does it anticipate it will experience significant difficulty in collecting amounts due, as a result of the current environment surrounding COVID-19 given the nature of its customers.
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 supply issues due to wellhead freeze-offs, power outages, or other adverse operating conditions upstream of Southwest’s distribution systems. These conditions caused daily natural gas prices to reach unprecedented levels. During this time, Southwest secured natural gas supplies, albeit at substantially higher prices, maintaining service to its customers. The incremental cost for these supplies was approximately $250 million, funded using a 364-day $250 million Bank Term Loan executed in March 2021 (see Note 5 – Debt) . The incremental gas costs are expected to be collected from customers through the purchased gas adjustment (“PGA”) mechanisms. Following the extreme weather event, an interstate transmission pipeline company billed Southwest, in addition to customary transmission costs, $65 million for pipeline imbalance charges, allegedly incurred during the period of the pipeline’s critical operation condition. However, Southwest has formally disputed these imbalance charges and believes that no amounts are due to the pipeline. Consequently, Southwest has not recognized this charge. Pipeline transmission costs, including periodic imbalance charges, are components of the cost of gas recovered from customers through the PGA and similar mechanisms.
Prepaid and other current assets	Prepaid and other current assets. Prepaid and other current assets includes gas pipe materials and operating supplies of $47 million at March 31, 2021 and $50 million at December 31, 2020 (carried at weighted average cost).
Goodwill	Goodwill. Goodwill is assessed as of October 1 st
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 amounts payable under regulatory mechanisms, customary accrued expenses for employee compensation and benefits, declared but unpaid dividends, and miscellaneous other accrued liabilities. Other current liabilities for the Company includes $34.5 million and $32.6 million of dividends declared, as well as $37 million and $25.2 million of accrued property taxes, as of March 31, 2021 and December 31, 2020, respectively.
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.
Recent Accounting Standards Updates	Recent Accounting Standards Updates. Accounting pronouncements adopted in 2021: In December 2019, the Financial Accounting Standards Board (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 Company and Southwest adopted the update in the first quarter of 2021, the impact of which was not material to the condensed consolidated financial statements of the Company or Southwest. 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 Topic 848) to replace a reference rate affected by such reform. The update applies only to contracts and hedging relationships that reference the London Interbank Offered Rate (“LIBOR”) or another 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 5 – 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 what impacts, if any, this update might have on the Company’s consolidated financial statements and disclosures.

Background, Organization, and_3

Background, Organization, and Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Other Property and Investments	Other Property and Investments. Other property and investments on the Condensed Consolidated Balance Sheets includes: (Thousands of dollars) March 31, 2021 December 31, 2020 Southwest Gas Corporation: Net cash surrender value of COLI policies $ 143,618 $ 140,874 Other property 2,720 2,737 Total Southwest Gas Corporation 146,338 143,611 Centuri property, equipment, and intangibles 1,106,855 1,089,414 Centuri accumulated provision for depreciation and amortization (439,126) (422,741) Other property and investments 28,605 23,961 Total Southwest Gas Holdings, Inc. $ 842,672 $ 834,245
Schedule of Goodwill	Goodwill on Southwest’s and the Company’s Condensed Consolidated Balance Sheets includes: (Thousands of dollars) Natural Gas Utility Infrastructure Total Company December 31, 2020 $ 10,095 $ 335,089 $ 345,184 Foreign currency translation adjustment — 1,369 1,369 March 31, 2021 $ 10,095 $ 336,458 $ 346,553
Schedule of Significant Items Included in Other Income (Deductions)	Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) in the Condensed Consolidated Statements of Income: Three Months Ended March 31, Twelve Months Ended (Thousands of dollars) 2021 2020 2021 2020 Southwest Gas Corporation - natural gas operations segment: Change in COLI policies $ 2,700 $ (15,500) $ 27,400 $ (5,700) Interest income 716 1,388 3,343 6,147 Equity AFUDC 981 1,061 4,644 4,262 Other components of net periodic benefit cost (3,505) (5,005) (18,522) (16,299) Miscellaneous income and (expense) (342) (2,480) (2,369) (5,375) Southwest Gas Corporation - total other income (deductions) 550 (20,536) 14,496 (16,965) Utility infrastructure services segment: Foreign transaction gain (loss) (3) (10) (9) 5 Miscellaneous income and (expense) (99) (232) (58) (656) Centuri - total other income (deductions) (102) (242) (67) (651) Corporate and administrative — 8 — 92 Consolidated Southwest Gas Holdings, Inc. - total other income (deductions) $ 448 $ (20,770) $ 14,429 $ (17,524)
Summary of Redeemable Noncontrolling Interest	The following depicts the change to the balance of the redeemable noncontrolling interest: (Thousands of dollars): Redeemable Noncontrolling Interest Balance, December 31, 2020 $ 165,716 Net income attributable to redeemable noncontrolling interest 1,552 Redemption value adjustment 38,018 Balance, March 31, 2021 $ 205,286
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: Three Months Ended Twelve Months Ended (In thousands) 2021 2020 2021 2020 Weighted average basic shares 57,600 55,310 56,564 54,726 Effect of dilutive securities: Management Incentive Plan shares — — — 9 Restricted stock units (1) 79 53 85 57 Weighted average diluted shares 57,679 55,363 56,649 54,792 (1) The number of securities included 75,000 and 50,000 performance shares during the three months ending March 31, 2021 and 2020, and 76,000 and 48,000 performance shares during the twelve months ending March 31, 2021 and 2020, respectively, the total of which was derived by assuming that target performance will be achi eved during the relevant performance period.

Components of Net Periodic Be_2

Components of Net Periodic Benefit Cost (Tables)	3 Months Ended
	Mar. 31, 2021
Retirement Benefits [Abstract]
Schedule of Net Periodic Benefit Costs	The service cost component of net periodic benefit costs included in the table below is a component of an overhead loading process associated with the cost of labor. The overhead process ultimately results in allocation of service cost 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 other components of net periodic benefit cost are reflected in Other income (deductions) on the Condensed Consolidated Statements of Income of each entity. Qualified Retirement Plan March 31, Three Months Twelve Months 2021 2020 2021 2020 (Thousands of dollars) Service cost $ 10,290 $ 8,574 $ 36,015 $ 27,972 Interest cost 10,108 11,388 44,275 48,142 Expected return on plan assets (18,088) (16,324) (67,060) (61,507) Amortization of net actuarial loss 10,489 9,007 37,507 25,774 Net periodic benefit cost $ 12,799 $ 12,645 $ 50,737 $ 40,381 SERP March 31, Three Months Twelve Months 2021 2020 2021 2020 (Thousands of dollars) Service cost $ 131 $ 98 $ 422 $ 298 Interest cost 358 401 1,561 1,721 Amortization of net actuarial loss 660 451 2,014 1,216 Net periodic benefit cost $ 1,149 $ 950 $ 3,997 $ 3,235 PBOP March 31, Three Months Twelve Months 2021 2020 2021 2020 (Thousands of dollars) Service cost $ 423 $ 396 $ 1,608 $ 1,353 Interest cost 548 645 2,485 2,929 Expected return on plan assets (810) (852) (3,366) (3,219) Amortization of prior service costs 240 289 1,106 1,243 Net periodic benefit cost $ 401 $ 478 $ 1,833 $ 2,306

Revenue (Tables)

Revenue (Tables)	3 Months Ended
	Mar. 31, 2021
Revenue from Contract with Customer [Abstract]
Summary of Revenue Disaggregated by Service Type and Contract Type	Gas operating revenues on the Condensed 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: Three Months Ended Twelve Months Ended March 31, (Thousands of dollars) 2021 2020 2021 2020 Residential $ 403,143 $ 378,555 $ 983,108 $ 934,115 Small commercial 81,398 82,463 220,476 241,970 Large commercial 12,673 12,667 44,639 47,640 Industrial/other 13,770 6,702 33,310 22,298 Transportation 24,536 24,406 88,345 91,884 Revenue from contracts with customers 535,520 504,793 1,369,878 1,337,907 Alternative revenue program revenues (deferrals) (16,373) (3,765) (468) 5,668 Other revenues (1) 2,785 1,799 280 7,514 Total Gas operating revenues $ 521,932 $ 502,827 $ 1,369,690 $ 1,351,089 (1) Amounts include late fees and other miscellaneous revenues, and may also include the impact of certain regulatory mechanisms, such as cost-of-service components in customer rates expected to be returned to customers in future periods. Late fees and certain other fees were reduced, for the three- and twelve-month periods ended March 31, 2021, due to a moratorium on late fees and disconnection for nonpayment during the COVID-19 pandemic. The following tables display Centuri’s revenue, reflected as Utility infrastructure services revenues on the Condensed Consolidated Statements of Income of the Company, representing revenue from contracts with customers disaggregated by service and contract types: Three Months Ended Twelve Months Ended March 31, (Thousands of dollars) 2021 2020 2021 2020 Service Types: Gas infrastructure services $ 221,837 $ 217,709 $ 1,265,288 $ 1,258,790 Electric power infrastructure services 93,961 72,320 433,467 267,736 Other 48,177 43,464 280,015 245,083 Total Utility infrastructure services revenues $ 363,975 $ 333,493 $ 1,978,770 $ 1,771,609 Three Months Ended Twelve Months Ended March 31, (Thousands of dollars) 2021 2020 2021 2020 Contract Types: Master services agreement $ 293,680 $ 263,545 $ 1,520,144 $ 1,411,267 Bid contract 70,295 69,948 458,626 360,342 Total Utility infrastructure services revenues $ 363,975 $ 333,493 $ 1,978,770 $ 1,771,609 Unit price contracts $ 234,449 $ 243,136 $ 1,347,953 $ 1,387,706 Fixed price contracts 34,594 27,545 164,750 101,931 Time and materials contracts 94,932 62,812 466,067 281,972 Total Utility infrastructure services revenues $ 363,975 $ 333,493 $ 1,978,770 $ 1,771,609
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	The following table provides information about contracts receivable and revenue earned on contracts in progress in excess of billings (contract asset), which are both included within Accounts receivable, net of allowances, as well as amounts billed in excess of revenue earned on contracts (contract liability), which are included in Other current liabilities as of March 31, 2021 and December 31, 2020 on the Company’s Condensed Consolidated Balance Sheets: (Thousands of dollars) March 31, 2021 December 31, 2020 Contracts receivable, net $ 197,357 $ 278,316 Revenue earned on contracts in progress in excess of billings 107,336 96,996 Amounts billed in excess of revenue earned on contracts 5,985 4,507
Schedule of Utility Infrastructure Services Contracts Receivable	Utility infrastructure services contracts receivable consists of the following: (Thousands of dollars) March 31, 2021 December 31, 2020 Billed on completed contracts and contracts in progress $ 193,042 $ 273,778 Other receivables 5,789 6,692 Contracts receivable, gross 198,831 280,470 Allowance for doubtful accounts (1,474) (2,154) Contracts receivable, net $ 197,357 $ 278,316

Common Stock (Tables)

Common Stock (Tables)	3 Months Ended
	Mar. 31, 2021
Equity [Abstract]
Schedule of Common Stock Activity	The following table provides the activity under the Equity Shelf Program for the three-month and life-to-date periods ended March 31, 2021: Three Months Ended Life-To-Date Ended March 31, 2021 Gross proceeds $ 46,448,484 $ 299,999,974 Less: agent commissions (464,485) (3,000,000) Net proceeds $ 45,983,999 $ 296,999,974 Number of shares sold 705,957 4,102,414 Weighted average price per share $ 65.80 $ 73.13

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt	March 31, 2021 December 31, 2020 Carrying Fair Carrying Fair (Thousands of dollars) Southwest Gas Corporation: Debentures: Notes, 6.1%, due 2041 $ 125,000 $ 160,651 $ 125,000 $ 174,858 Notes, 3.875%, due 2022 250,000 256,693 250,000 258,825 Notes, 4.875%, due 2043 250,000 288,500 250,000 317,190 Notes, 3.8%, due 2046 300,000 306,261 300,000 347,046 Notes, 3.7%, due 2028 300,000 324,069 300,000 344,553 Notes, 4.15%, due 2049 300,000 324,183 300,000 370,278 Notes, 2.2%, due 2030 450,000 438,912 450,000 474,552 8% Series, due 2026 75,000 95,894 75,000 99,723 Medium-term notes, 7.78% series, due 2022 25,000 26,201 25,000 26,663 Medium-term notes, 7.92% series, due 2027 25,000 32,138 25,000 33,802 Medium-term notes, 6.76% series, due 2027 7,500 9,127 7,500 9,613 Unamortized discount and debt issuance costs (17,501) (17,822) 2,089,999 2,089,678 Revolving credit facility and commercial paper 150,000 150,000 150,000 150,000 Industrial development revenue bonds: Variable-rate 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 (1,411) (1,472) 198,589 198,528 Less: current maturities (25,000) — Long-term debt, less current maturities - Southwest Gas Corporation $ 2,413,588 $ 2,438,206 Centuri: Centuri term loan facility $ 222,668 $ 225,269 $ 226,648 $ 230,824 Unamortized debt issuance costs (750) (820) 221,918 225,828 Centuri secured revolving credit facility 26,163 26,180 26,626 26,645 Centuri other debt obligations 77,235 78,378 81,973 84,246 Less: current maturities (42,334) (40,433) Long-term debt, less current maturities - Centuri $ 282,982 $ 293,994 Consolidated Southwest Gas Holdings, Inc.: Southwest Gas Corporation long-term debt $ 2,438,588 $ 2,438,206 Centuri long-term debt 325,316 334,427 Less: current maturities (67,334) (40,433) Long-term debt, less current maturities - Southwest Gas Holdings, Inc. $ 2,696,570 $ 2,732,200

Other Comprehensive Income an_2

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2021
Equity [Abstract]
Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss)	Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) Three Months Ended Three Months Ended (Thousands of dollars) Before- Tax Net-of- Before- Tax Net-of- Defined benefit pension plans: Amortization of prior service cost $ 240 $ (58) $ 182 $ 289 $ (69) $ 220 Amortization of net actuarial (gain)/loss 11,149 (2,675) 8,474 9,458 (2,270) 7,188 Regulatory adjustment (9,575) 2,298 (7,277) (8,395) 2,015 (6,380) Pension plans other comprehensive income (loss) 1,814 (435) 1,379 1,352 (324) 1,028 FSIRS (designated hedging activities): Amounts reclassified into net income 544 (131) 413 837 (201) 636 FSIRS other comprehensive income (loss) 544 (131) 413 837 (201) 636 Total other comprehensive income (loss) - Southwest Gas Corporation 2,358 (566) 1,792 2,189 (525) 1,664 Foreign currency translation adjustments: Translation adjustments 823 — 823 (4,005) — (4,005) Foreign currency other comprehensive income (loss) 823 — 823 (4,005) — (4,005) Total other comprehensive income (loss) - Southwest Gas Holdings, Inc. $ 3,181 $ (566) $ 2,615 $ (1,816) $ (525) $ (2,341) Twelve Months Ended Twelve Months Ended (Thousands of dollars) Before- Tax Net-of- Before- Tax Net-of- Defined benefit pension plans: Net actuarial gain/(loss) $ (57,539) $ 13,809 $ (43,730) $ (71,087) $ 17,061 $ (54,026) Amortization of prior service cost 1,106 (266) 840 1,243 (298) 945 Amortization of net actuarial (gain)/loss 39,521 (9,484) 30,037 26,990 (6,477) 20,513 Prior service cost — — — (1,878) 452 (1,426) Regulatory adjustment 6,255 (1,502) 4,753 33,896 (8,136) 25,760 Pension plans other comprehensive income (loss) (10,657) 2,557 (8,100) (10,836) 2,602 (8,234) FSIRS (designated hedging activities): Amounts reclassified into net income 2,954 (710) 2,244 3,345 (803) 2,542 FSIRS other comprehensive income (loss) 2,954 (710) 2,244 3,345 (803) 2,542 Total other comprehensive income (loss) - Southwest Gas Corporation (7,703) 1,847 (5,856) (7,491) 1,799 (5,692) Foreign currency translation adjustments: Translation adjustments 6,541 — 6,541 (2,758) — (2,758) Foreign currency other comprehensive income (loss) 6,541 — 6,541 (2,758) — (2,758) Total other comprehensive income (loss) - Southwest Gas Holdings, Inc. $ (1,162) $ 1,847 $ 685 $ (10,249) $ 1,799 $ (8,450) (1) Tax amounts are calculated using a 24% rate. The Company has elected to indefinitely reinvest the earnings of Centuri’s Canadian subsidiaries in Canada, thus precluding 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 reported in Other comprehensive income (loss).
Rollforward of Accumulated Other Comprehensive Income	The following table represents a rollforward of AOCI, presented on the Company’s Condensed Consolidated Balance Sheets and its Condensed Consolidated Statements of Equity: Defined Benefit Plans FSIRS Foreign Currency Items (Thousands of dollars) Before-Tax Tax After-Tax Before-Tax Tax After-Tax Before-Tax Tax After-Tax AOCI Beginning Balance AOCI December 31, 2020 $ (77,720) $ 18,653 $ (59,067) $ (2,719) $ 651 $ (2,068) $ 132 $ — $ 132 $ (61,003) Translation adjustments — — — — — — 823 — 823 823 Other comprehensive income (loss) before reclassifications — — — — — — 823 — 823 823 FSIRS amount reclassified from AOCI (1) — — — 544 (131) 413 — — — 413 Amortization of prior service cost (2) 240 (58) 182 — — — — — — 182 Amortization of net actuarial loss (2) 11,149 (2,675) 8,474 — — — — — — 8,474 Regulatory adjustment (3) (9,575) 2,298 (7,277) — — — — — — (7,277) Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. 1,814 (435) 1,379 544 (131) 413 823 — 823 2,615 Ending Balance AOCI March 31, 2021 $ (75,906) $ 18,218 $ (57,688) $ (2,175) $ 520 $ (1,655) $ 955 $ — $ 955 $ (58,388) (1) The FSIRS reclassification amount is included in Net interest deductions on the Company’s Condensed Consolidated Statements of Income. (2) These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost 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 Condensed Consolidated Balance Sheets). (4) Tax amounts are calculated using a 24% rate. The following table represents a rollforward of AOCI, presented on Southwest’s Condensed Consolidated Balance Sheets: Defined Benefit Plans FSIRS (Thousands of dollars) Before-Tax Tax After-Tax Before-Tax Tax After-Tax AOCI Beginning Balance AOCI December 31, 2020 $ (77,720) $ 18,653 $ (59,067) $ (2,719) $ 651 $ (2,068) $ (61,135) FSIRS amount reclassified from AOCI (5) — — — 544 (131) 413 413 Amortization of prior service cost (6) 240 (58) 182 — — — 182 Amortization of net actuarial loss (6) 11,149 (2,675) 8,474 — — — 8,474 Regulatory adjustment (7) (9,575) 2,298 (7,277) — — — (7,277) Net current period other comprehensive income attributable to Southwest Gas Corporation 1,814 (435) 1,379 544 (131) 413 1,792 Ending Balance AOCI March 31, 2021 $ (75,906) $ 18,218 $ (57,688) $ (2,175) $ 520 $ (1,655) $ (59,343) (5) The FSIRS reclassification amount is included in Net interest deductions on Southwest’s Condensed Consolidated Statements of Income. (6) These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost for additional details). (7) 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 Southwest’s Condensed Consolidated Balance Sheets). (8) Tax amounts are calculated using a 24% rate.
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: (Thousands of dollars) March 31, 2021 December 31, 2020 Net actuarial loss $ (491,634) $ (502,783) Prior service cost (2,247) (2,487) Less: amount recognized in regulatory assets 417,975 427,550 Recognized in AOCI $ (75,906) $ (77,720)

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2021
Segment Reporting [Abstract]
Accounts Receivable for Services	Accounts receivable for these services, which are not eliminated during consolidation, are presented in the table below: (Thousands of dollars) March 31, 2021 December 31, 2020 Centuri accounts receivable for services provided to Southwest $ 11,344 $ 13,956
Schedule of Segment Reporting Information	The financial information pertaining to the natural gas operations and utility infrastructure services segments is as follows: (Thousands of dollars) Natural Gas Utility Infrastructure Other Total Three Months Ended March 31, 2021 Revenues from external customers $ 521,932 $ 339,772 $ — $ 861,704 Intersegment revenues — 24,203 — 24,203 Total $ 521,932 $ 363,975 $ — $ 885,907 Segment net income (loss) $ 118,715 $ (859) $ (563) $ 117,293 Three Months Ended March 31, 2020 Revenues from external customers $ 502,827 $ 300,291 $ — $ 803,118 Intersegment revenues — 33,202 — 33,202 Total $ 502,827 $ 333,493 $ — $ 836,320 Segment net income (loss) $ 83,599 $ (10,204) $ (853) $ 72,542 (Thousands of dollars) Natural Gas Utility Infrastructure Other Total Twelve Months Ended March 31, 2021 Revenues from external customers $ 1,369,690 $ 1,852,910 $ — $ 3,222,600 Intersegment revenues — 125,860 — 125,860 Total $ 1,369,690 $ 1,978,770 $ — $ 3,348,460 Segment net income (loss) $ 194,234 $ 84,207 $ (1,366) $ 277,075 Twelve Months Ended March 31, 2020 Revenues from external customers $ 1,351,089 $ 1,618,354 $ — $ 2,969,443 Intersegment revenues — 153,255 — 153,255 Total $ 1,351,089 $ 1,771,609 $ — $ 3,122,698 Segment net income (loss) $ 143,381 $ 50,231 $ (1,943) $ 191,669

Background, Organization, and_4

Background, Organization, and Summary of Significant Accounting Policies - Narratives (Details)	1 Months Ended		2 Months Ended	3 Months Ended
	Mar. 31, 2021USD ($)state	Feb. 28, 2021USD ($)	Mar. 31, 2021USD ($)state	Mar. 31, 2021USD ($)state	Dec. 31, 2020USD ($)	Nov. 30, 2018
Significant Accounting Policies [Line Items]
Number of states in which entity operates | state	3		3	3
Accounts receivable, extending period				1 month
Deferred purchased gas costs, recovered or refunded period				1 year
Deferred purchased gas costs, estimated incremental costs			$ 250,000,000
Deferred purchased gas costs, pipeline imbalance charges		$ 65,000,000
Goodwill impairment				$ 0
Dividends declared but not yet paid	$ 34,500,000		34,500,000	34,500,000	$ 32,600,000
Accrued property taxes	37,000,000		37,000,000	37,000,000	25,200,000
Redemption value adjustments				38,018,000
Linetec
Significant Accounting Policies [Line Items]
Ownership percentage by noncontrolling owners						20.00%
Gas Pipe Materials and Supplies | Prepaid and other current assets
Significant Accounting Policies [Line Items]
Gas pipe materials and operating supplies	47,000,000		47,000,000	47,000,000	50,000,000
Pooled Funds and Mutual Funds
Significant Accounting Policies [Line Items]
Money market fund investments	$ 40,000		40,000	40,000	40,000
Southwest Gas Corporation | $250 Million Term Loan
Significant Accounting Policies [Line Items]
Debt instrument, term	364 days
Debt instrument, face amount	$ 250,000,000		250,000,000	250,000,000
Southwest Gas Corporation | Pooled Funds and Mutual Funds
Significant Accounting Policies [Line Items]
Money market fund investments	$ 40,000		$ 40,000	$ 40,000	$ 40,000

Background, Organization, and_5

Background, Organization, and Summary of Significant Accounting Policies - Schedule of Other Property and Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Other property and investments	$ 28,605	$ 23,961
Total	842,672	834,245
Southwest Gas Corporation
Property, Plant and Equipment [Line Items]
Net cash surrender value of COLI policies	143,618	140,874
Other property and investments	2,720	2,737
Total	146,338	143,611
Centuri
Property, Plant and Equipment [Line Items]
Centuri property, equipment, and intangibles	1,106,855	1,089,414
Centuri accumulated provision for depreciation and amortization	$ (439,126)	$ (422,741)

Background, Organization, and_6

Background, Organization, and Summary of Significant Accounting Policies - Schedule of Goodwill (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 345,184
Foreign currency translation adjustment	1,369
Goodwill, ending balance	346,553
Natural Gas Operations
Goodwill [Roll Forward]
Goodwill, beginning balance	10,095
Foreign currency translation adjustment	0
Goodwill, ending balance	10,095
Utility Infrastructure Services
Goodwill [Roll Forward]
Goodwill, beginning balance	335,089
Foreign currency translation adjustment	1,369
Goodwill, ending balance	$ 336,458

Background, Organization, and_7

Background, Organization, and Summary of Significant Accounting Policies - Other Income (Deductions) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Other income (deductions)
Total other income (deductions)	$ 448	$ (20,770)	$ 14,429	$ (17,524)
Southwest Gas Corporation
Other income (deductions)
Total other income (deductions)	550	(20,536)	14,496	(16,965)
Revenues from external customers | Natural Gas Operations | Southwest Gas Corporation
Other income (deductions)
Change in COLI policies	2,700	(15,500)	27,400	(5,700)
Interest income	716	1,388	3,343	6,147
Equity AFUDC	981	1,061	4,644	4,262
Other components of net periodic benefit cost	(3,505)	(5,005)	(18,522)	(16,299)
Miscellaneous income and (expense)	(342)	(2,480)	(2,369)	(5,375)
Total other income (deductions)	550	(20,536)	14,496	(16,965)
Revenues from external customers | Utility Infrastructure Services Segment | Centuri
Other income (deductions)
Miscellaneous income and (expense)	(99)	(232)	(58)	(656)
Foreign transaction gain (loss)	(3)	(10)	(9)	5
Total other income (deductions)	(102)	(242)	(67)	(651)
Corporate and administrative
Other income (deductions)
Total other income (deductions)	$ 0	$ 8	$ 0	$ 92

Background, Organization, and_8

Background, Organization, and Summary of Significant Accounting Policies - Summary of Redeemable Noncontrolling Interest (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Movement In Redeemable Noncontrolling Interest [Roll Forward]
Redeemable noncontrolling interest, beginning balance	$ 165,716
Net income attributable to redeemable noncontrolling interest	1,552
Redemption value adjustment	38,018
Redeemable noncontrolling interest, ending balance	$ 205,286

Background, Organization, and_9

Background, Organization, and Summary of Significant Accounting Policies - Schedule of Earnings Per Share (Details) - shares shares in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Weighted average basic shares (in shares)	57,600	55,310	56,564	54,726
Effect of dilutive securities:
Management Incentive Plan shares (in shares)	0	0	0	9
Restricted stock units (in shares)	79	53	85	57
Weighted average diluted shares (in shares)	57,679	55,363	56,649	54,792
Number of performance share units granted (in shares)	75	50	76	48

Components of Net Periodic Be_3

Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 10,290	$ 8,574	$ 36,015	$ 27,972
Interest cost	10,108	11,388	44,275	48,142
Expected return on plan assets	(18,088)	(16,324)	(67,060)	(61,507)
Amortization of net actuarial loss	10,489	9,007	37,507	25,774
Net periodic benefit cost	12,799	12,645	50,737	40,381
SERP
Defined Benefit Plan Disclosure [Line Items]
Service cost	131	98	422	298
Interest cost	358	401	1,561	1,721
Amortization of net actuarial loss	660	451	2,014	1,216
Net periodic benefit cost	1,149	950	3,997	3,235
PBOP
Defined Benefit Plan Disclosure [Line Items]
Service cost	423	396	1,608	1,353
Interest cost	548	645	2,485	2,929
Expected return on plan assets	(810)	(852)	(3,366)	(3,219)
Amortization of prior service costs	240	289	1,106	1,243
Net periodic benefit cost	$ 401	$ 478	$ 1,833	$ 2,306

Revenue - Schedule of Gas Opera

Revenue - Schedule of Gas Operating Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 535,520	$ 504,793	$ 1,369,878	$ 1,337,907
Alternative revenue program revenues (deferrals)	(16,373)	(3,765)	(468)	5,668
Other revenues (1)	2,785	1,799	280	7,514
Total Gas operating revenues	521,932	502,827	1,369,690	1,351,089
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	403,143	378,555	983,108	934,115
Small commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	81,398	82,463	220,476	241,970
Large commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	12,673	12,667	44,639	47,640
Industrial/other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	13,770	6,702	33,310	22,298
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 24,536	$ 24,406	$ 88,345	$ 91,884

Revenue - Summary of Revenue Di

Revenue - Summary of Revenue Disaggregated by Service Type, and Contract Type (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 535,520	$ 504,793	$ 1,369,878	$ 1,337,907
Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	363,975	333,493	1,978,770	1,771,609
Master services agreement | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	293,680	263,545	1,520,144	1,411,267
Bid contract | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	70,295	69,948	458,626	360,342
Unit price contracts | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	234,449	243,136	1,347,953	1,387,706
Fixed price contracts | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	34,594	27,545	164,750	101,931
Time and materials contracts | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	94,932	62,812	466,067	281,972
Gas infrastructure services | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	221,837	217,709	1,265,288	1,258,790
Electric power infrastructure services | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	93,961	72,320	433,467	267,736
Other | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 48,177	$ 43,464	$ 280,015	$ 245,083

Revenue - Summary of Informatio

Revenue - Summary of Information about Receivables (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Contracts receivable, net	$ 197,357	$ 278,316
Revenue earned on contracts in progress in excess of billings	107,336	96,996
Amounts billed in excess of revenue earned on contracts	$ 5,985	$ 4,507

Revenue - Narratives (Details)

Revenue - Narratives (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)Contract
Segment Reporting Information [Line Items]
Change in contract liability, revenue recognized | $	$ 4.5
Centuri
Segment Reporting Information [Line Items]
Number of contracts with original duration more than one year | Contract	18

Revenue - Revenue Performance O

Revenue - Revenue Performance Obligation (Details) - Centuri - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-04-01 $ in Millions	Mar. 31, 2021USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to unsatisfied performance obligations of contracts	$ 48.7
Transaction price allocated to unsatisfied performance obligations of contracts, period	2 years

Revenue - Schedule of Utility I

Revenue - Schedule of Utility Infrastructure Services Contracts Receivable (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Contracts receivable, gross	$ 198,831	$ 280,470
Allowance for doubtful accounts	(1,474)	(2,154)
Contracts receivable, net	197,357	278,316
Billed On Completed Contracts And Contracts In Progress
Disaggregation of Revenue [Line Items]
Contracts receivable, gross	193,042	273,778
Other Receivables
Disaggregation of Revenue [Line Items]
Contracts receivable, gross	$ 5,789	$ 6,692

Common Stock (Details)

Common Stock (Details) - USD ($)	3 Months Ended		12 Months Ended		23 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Apr. 08, 2021	May 08, 2019
Class of Stock [Line Items]
Net proceeds	$ 48,990,000	$ 3,148,000	$ 185,087,000	$ 135,215,000
Restricted Stock/Unit Plan, and Management Incentive Plan
Class of Stock [Line Items]
Common stock issued (in shares)	47,000
Dividend Reinvestment and Stock Purchase Plan
Class of Stock [Line Items]
Common stock issued (in shares)	49,500
Common stock issued, amount	$ 3,100,000
Equity Shelf Program
Class of Stock [Line Items]
Sale of stock, amount of common stock offered for sale (up to)							$ 300,000,000
Gross proceeds	46,448,484				$ 299,999,974
Less: agent commissions	(464,485)				(3,000,000)
Net proceeds	$ 45,983,999				$ 296,999,974
Number of shares sold (in shares)	705,957				4,102,414
Weighted average price per share	$ 65.80				$ 73.13
Equity Shelf Program | Subsequent Event
Class of Stock [Line Items]
Sale of stock, amount of common stock offered for sale (up to)						$ 500,000,000

Debt - Schedule of Carrying Amo

Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Less: current maturities	$ (67,334)	$ (40,433)
Long-term debt, less current maturities	2,696,570	2,732,200
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(67,334)	(40,433)
Long-term debt, less current maturities	2,696,570	2,732,200
Southwest Gas Corporation
Debt Instrument [Line Items]
Less: current maturities	(25,000)	0
Long-term debt, less current maturities	2,413,588	2,438,206
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(25,000)	0
Long-term debt, less current maturities	$ 2,413,588	2,438,206
Debentures | Notes, 6.1%, due 2041 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument interest rate	6.10%
Debentures | Notes, 3.875%, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument interest rate	3.875%
Debentures | Notes, 4.875%, due 2043 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument interest rate	4.875%
Debentures | Notes, 3.8%, due 2046 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument interest rate	3.80%
Debentures | Notes, 3.7%, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument interest rate	3.70%
Debentures | Notes, 4.15%, due 2049 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument interest rate	4.15%
Debentures | Notes, 2.2%, due 2030 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument interest rate	2.20%
Debentures | 8% Series, due 2026 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument interest rate	8.00%
Debentures | Medium-term notes, 7.78% series, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument interest rate	7.78%
Debentures | Medium-term notes, 7.92% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument interest rate	7.92%
Debentures | Medium-term notes, 6.76% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt instrument interest rate	6.76%
Carrying Amount
Debt Instrument [Line Items]
Less: current maturities	$ (67,334)	(40,433)
Long-term debt, less current maturities	2,696,570	2,732,200
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(67,334)	(40,433)
Long-term debt, less current maturities	2,696,570	2,732,200
Carrying Amount | Southwest Gas Corporation
Debt Instrument [Line Items]
Less: current maturities	(25,000)	0
Long-term debt, less current maturities	2,413,588	2,438,206
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(25,000)	0
Long-term debt, less current maturities	2,413,588	2,438,206
Long-term debt	2,438,588	2,438,206
Carrying Amount | Centuri
Debt Instrument [Line Items]
Less: current maturities	(42,334)	(40,433)
Long-term debt, less current maturities	282,982	293,994
Unamortized debt issuance costs	(750)	(820)
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(42,334)	(40,433)
Long-term debt, less current maturities	282,982	293,994
Long-term debt	325,316	334,427
Centuri secured revolving credit facility	221,918	225,828
Carrying Amount | Centuri secured revolving credit facility | Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	26,163	26,626
Carrying Amount | Centuri term loan facility | Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	222,668	226,648
Carrying Amount | Centuri other debt obligations | Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	77,235	81,973
Carrying Amount | Debentures | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	2,089,999	2,089,678
Unamortized discount and debt issuance costs	(17,501)	(17,822)
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	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 | Debentures | Revolving credit facility and commercial paper | Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit facility	150,000	150,000
Carrying Amount | Industrial development revenue bonds | Southwest Gas Corporation
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(1,411)	(1,472)
Unsecured debt	198,589	198,528
Carrying Amount | Industrial development revenue bonds | Tax-exempt Series A, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount | Industrial development revenue bonds | 2003 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount | Industrial development revenue bonds | 2008 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount | Industrial development revenue bonds | 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 secured revolving credit facility	26,180	26,645
Fair Value | Centuri term loan facility | Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	225,269	230,824
Fair Value | Centuri other debt obligations | Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	78,378	84,246
Fair Value | Debentures | Notes, 6.1%, due 2041 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	160,651	174,858
Fair Value | Debentures | Notes, 3.875%, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	256,693	258,825
Fair Value | Debentures | Notes, 4.875%, due 2043 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	288,500	317,190
Fair Value | Debentures | Notes, 3.8%, due 2046 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	306,261	347,046
Fair Value | Debentures | Notes, 3.7%, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	324,069	344,553
Fair Value | Debentures | Notes, 4.15%, due 2049 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	324,183	370,278
Fair Value | Debentures | Notes, 2.2%, due 2030 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	438,912	474,552
Fair Value | Debentures | 8% Series, due 2026 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	95,894	99,723
Fair Value | Debentures | Medium-term notes, 7.78% series, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	26,201	26,663
Fair Value | Debentures | Medium-term notes, 7.92% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	32,138	33,802
Fair Value | Debentures | Medium-term notes, 6.76% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	9,127	9,613
Fair Value | Debentures | 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 | Industrial development revenue bonds | Tax-exempt Series A, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Fair Value | Industrial development revenue bonds | 2003 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Fair Value | Industrial development revenue bonds | 2008 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Fair Value | Industrial development revenue bonds | 2009 Series A, due 2039 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 50,000	$ 50,000

Debt - Narratives (Details)

Debt - Narratives (Details) - USD ($)	1 Months Ended	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Current maturities of long-term debt	$ 67,334,000	$ 67,334,000	$ 40,433,000
Short term borrowings outstanding	310,000,000	310,000,000	107,000,000
Southwest Gas Corporation
Debt Instrument [Line Items]
Current maturities of long-term debt	25,000,000	25,000,000	0
Short term borrowings outstanding	267,000,000	267,000,000	$ 57,000,000
Southwest Gas Corporation | $250 Million Term Loan
Debt Instrument [Line Items]
Debt instrument, face amount	$ 250,000,000	$ 250,000,000
Effective interest rate (in percent)	0.77%	0.77%
Debt covenant, ratio of funded debt to total capitalization	0.70	0.70
Southwest Gas Corporation | Southwest Gas Credit Facility
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
Current maturities of long-term debt	17,000,000	17,000,000
Southwest Gas Corporation | Commercial Paper Program
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
Southwest Gas Corporation | LIBOR | Minimum | $250 Million Term Loan
Debt Instrument [Line Items]
Applicable margin	0.55%
Southwest Gas Corporation | LIBOR | Maximum | $250 Million Term Loan
Debt Instrument [Line Items]
Applicable margin	1.00%
Southwest Gas Corporation | LIBOR | Southwest Gas Credit Facility
Debt Instrument [Line Items]
Applicable margin		1.00%
Southwest Gas Corporation | Alternative base rate | $250 Million Term Loan
Debt Instrument [Line Items]
Applicable margin		0.00%
Southwest Gas Corporation | Alternative base rate | Southwest Gas Credit Facility
Debt Instrument [Line Items]
Applicable margin		0.00%
Centuri | Secured Revolving Credit Facility and Term Loan
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	$ 590,000,000	$ 590,000,000
Borrowings outstanding under facility	249,000,000	249,000,000
Debt secured by assets	1,300,000,000	1,300,000,000
Centuri | Line of Credit
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	325,000,000	325,000,000
Centuri | Centuri term loan facility
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	265,000,000	265,000,000
$400 Million 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
Borrowings outstanding under facility	$ 43,000,000	$ 43,000,000

Other Comprehensive Income an_3

Other Comprehensive Income and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to OCI (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, net of tax	$ 823
Other comprehensive income (loss), before tax	3,181	$ (1,816)	$ (1,162)	$ (10,249)
Other comprehensive income (loss), tax	(566)	(525)	1,847	1,799
Total other comprehensive income (loss), net of tax	$ 2,615	(2,341)	$ 685	(8,450)
Effective income tax rate	24.00%		24.00%
Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	$ 2,358	2,189	$ (7,703)	(7,491)
Other comprehensive income (loss), tax	(566)	(525)	1,847	1,799
Total other comprehensive income (loss), net of tax	1,792	1,664	(5,856)	(5,692)
Amortization of prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	240	289	1,106	1,243
Reclassification from AOCI, tax	(58)	(69)	(266)	(298)
Reclassification from AOCI, net of tax	182	220	840	945
Other comprehensive income (loss), before tax	240
Other comprehensive income (loss), tax	(58)
Total other comprehensive income (loss), net of tax	182
Amortization of prior service cost | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	240
Other comprehensive income (loss), tax	(58)
Total other comprehensive income (loss), net of tax	182
Amortization of net actuarial (gain)/loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax			(57,539)	(71,087)
Other comprehensive income (loss) before reclassifications, tax			13,809	17,061
Other comprehensive income (loss) before reclassifications, net of tax			(43,730)	(54,026)
Reclassification from AOCI, before Tax	11,149	9,458	39,521	26,990
Reclassification from AOCI, tax	(2,675)	(2,270)	(9,484)	(6,477)
Reclassification from AOCI, net of tax	8,474	7,188	30,037	20,513
Other comprehensive income (loss), before tax	11,149
Other comprehensive income (loss), tax	(2,675)
Total other comprehensive income (loss), net of tax	8,474
Amortization of net actuarial (gain)/loss | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	11,149
Other comprehensive income (loss), tax	(2,675)
Total other comprehensive income (loss), net of tax	8,474
Regulatory adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	(9,575)	(8,395)	6,255	33,896
Reclassification from AOCI, tax	2,298	2,015	(1,502)	(8,136)
Reclassification from AOCI, net of tax	(7,277)	(6,380)	4,753	25,760
Other comprehensive income (loss), before tax	(9,575)
Other comprehensive income (loss), tax	2,298
Total other comprehensive income (loss), net of tax	(7,277)
Regulatory adjustment | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	(9,575)
Other comprehensive income (loss), tax	2,298
Total other comprehensive income (loss), net of tax	(7,277)
Prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax			0	(1,878)
Other comprehensive income (loss) before reclassifications, tax			0	452
Other comprehensive income (loss) before reclassifications, net of tax			0	(1,426)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	1,814	1,352	(10,657)	(10,836)
Other comprehensive income (loss), tax	(435)	(324)	2,557	2,602
Total other comprehensive income (loss), net of tax	1,379	1,028	(8,100)	(8,234)
Defined Benefit Plans | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	1,814
Other comprehensive income (loss), tax	(435)
Total other comprehensive income (loss), net of tax	1,379
FSIRS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	544	837	2,954	3,345
Reclassification from AOCI, tax	(131)	(201)	(710)	(803)
Reclassification from AOCI, net of tax	413	636	2,244	2,542
Other comprehensive income (loss), before tax	544	837	2,954	3,345
Other comprehensive income (loss), tax	(131)	(201)	(710)	(803)
Total other comprehensive income (loss), net of tax	413	636	2,244	2,542
FSIRS | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	544
Reclassification from AOCI, tax	(131)
Reclassification from AOCI, net of tax	413
Other comprehensive income (loss), before tax	544
Other comprehensive income (loss), tax	(131)
Total other comprehensive income (loss), net of tax	413
Foreign Currency Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	823	(4,005)	6,541	(2,758)
Other comprehensive income (loss) before reclassifications, tax	0	0	0	0
Other comprehensive income (loss) before reclassifications, net of tax	$ 823	$ (4,005)	$ 6,541	$ (2,758)

Other Comprehensive Income an_4

Other Comprehensive Income and Accumulated Other Comprehensive Income - Additional Information (Details) $ in Millions	Mar. 31, 2021USD ($)
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$ 1.7

Other Comprehensive Income an_5

Other Comprehensive Income and Accumulated Other Comprehensive Income - AOCI Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent	$ 2,674,953
Other comprehensive income (loss) before reclassifications, net of tax	823
Other comprehensive income (loss), before tax	3,181	$ (1,816)	$ (1,162)	$ (10,249)
Other comprehensive income (loss), tax	(566)	(525)	1,847	1,799
Total other comprehensive income (loss), net of tax	2,615	(2,341)	685	(8,450)
Ending balance, attributable to parent	$ 2,773,722		$ 2,773,722
Effective income tax rate	24.00%		24.00%
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	$ (77,720)
Beginning balance, tax	18,653
Beginning balance	(59,067)
Other comprehensive income (loss), before tax	1,814	1,352	$ (10,657)	(10,836)
Other comprehensive income (loss), tax	(435)	(324)	2,557	2,602
Total other comprehensive income (loss), net of tax	1,379	1,028	(8,100)	(8,234)
Ending balance, before tax	(75,906)		(75,906)
Ending balance, tax	18,218		18,218
Ending balance	(57,688)		(57,688)
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	240	289	1,106	1,243
Reclassification from AOCI, tax	(58)	(69)	(266)	(298)
Reclassification from AOCI, net of tax	182	220	840	945
Other comprehensive income (loss), before tax	240
Other comprehensive income (loss), tax	(58)
Total other comprehensive income (loss), net of tax	182
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)
Other comprehensive income (loss) before reclassifications, net of tax			(43,730)	(54,026)
Reclassification from AOCI, before Tax	11,149	9,458	39,521	26,990
Reclassification from AOCI, tax	(2,675)	(2,270)	(9,484)	(6,477)
Reclassification from AOCI, net of tax	8,474	7,188	30,037	20,513
Other comprehensive income (loss), before tax	11,149
Other comprehensive income (loss), tax	(2,675)
Total other comprehensive income (loss), net of tax	8,474
Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	(9,575)	(8,395)	6,255	33,896
Reclassification from AOCI, tax	2,298	2,015	(1,502)	(8,136)
Reclassification from AOCI, net of tax	(7,277)	(6,380)	4,753	25,760
Other comprehensive income (loss), before tax	(9,575)
Other comprehensive income (loss), tax	2,298
Total other comprehensive income (loss), net of tax	(7,277)
FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	(2,719)
Beginning balance, tax	651
Beginning balance	(2,068)
Reclassification from AOCI, before Tax	544	837	2,954	3,345
Reclassification from AOCI, tax	(131)	(201)	(710)	(803)
Reclassification from AOCI, net of tax	413	636	2,244	2,542
Other comprehensive income (loss), before tax	544	837	2,954	3,345
Other comprehensive income (loss), tax	(131)	(201)	(710)	(803)
Total other comprehensive income (loss), net of tax	413	636	2,244	2,542
Ending balance, before tax	(2,175)		(2,175)
Ending balance, tax	520		520
Ending balance	(1,655)		(1,655)
Foreign Currency Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	132
Beginning balance	132
Other comprehensive income (loss) before reclassifications, before tax	823
Other comprehensive income (loss) before reclassifications, net of tax	823
Ending balance, before tax	955		955
Ending balance	955		955
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	2,615
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(61,003)	(56,732)	(59,073)
Ending balance	(58,388)	(59,073)	(58,388)	(59,073)
Southwest Gas Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			2,114,276
Beginning balance, attributable to parent	2,233,468
Other comprehensive income (loss), before tax	2,358	2,189	(7,703)	(7,491)
Other comprehensive income (loss), tax	(566)	(525)	1,847	1,799
Total other comprehensive income (loss), net of tax	1,792	1,664	(5,856)	(5,692)
Ending balance, attributable to parent	2,374,124		2,374,124
Ending balance	2,374,124	2,114,276	2,374,124	2,114,276
Southwest Gas Corporation | Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax	(77,720)
Beginning balance, attributable to parent, tax	18,653
Beginning balance, attributable to parent	(59,067)
Other comprehensive income (loss), before tax	1,814
Other comprehensive income (loss), tax	(435)
Total other comprehensive income (loss), net of tax	1,379
Ending balance, attributable to parent, before tax	(75,906)		(75,906)
Ending balance, attributable to parent, tax	18,218		18,218
Ending balance, attributable to parent	(57,688)		(57,688)
Southwest Gas Corporation | Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	240
Other comprehensive income (loss), tax	(58)
Total other comprehensive income (loss), net of tax	182
Southwest Gas Corporation | Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	11,149
Other comprehensive income (loss), tax	(2,675)
Total other comprehensive income (loss), net of tax	8,474
Southwest Gas Corporation | Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	(9,575)
Other comprehensive income (loss), tax	2,298
Total other comprehensive income (loss), net of tax	(7,277)
Southwest Gas Corporation | FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax	(2,719)
Beginning balance, attributable to parent, tax	651
Beginning balance, attributable to parent	(2,068)
Reclassification from AOCI, before Tax	544
Reclassification from AOCI, tax	(131)
Reclassification from AOCI, net of tax	413
Other comprehensive income (loss), before tax	544
Other comprehensive income (loss), tax	(131)
Total other comprehensive income (loss), net of tax	413
Ending balance, attributable to parent, before tax	(2,175)		(2,175)
Ending balance, attributable to parent, tax	520		520
Ending balance, attributable to parent	(1,655)		(1,655)
Southwest Gas Corporation | AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(61,135)	(55,151)	(53,487)
Beginning balance, attributable to parent	(61,135)
Ending balance, attributable to parent	(59,343)		(59,343)
Ending balance	$ (59,343)	$ (53,487)	$ (59,343)	$ (53,487)

Other Comprehensive Income an_6

Other Comprehensive Income and Accumulated Other Comprehensive Income - Amounts Recognized Before Tax, Defined Benefit Plans (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Equity [Abstract]
Net actuarial loss	$ (491,634)	$ (502,783)
Prior service cost	(2,247)	(2,487)
Less: amount recognized in regulatory assets	417,975	427,550
Recognized in AOCI	$ (75,906)	$ (77,720)

Segment Information - Accounts

Segment Information - Accounts Receivable for Services (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Centuri
Segment Reporting Information [Line Items]
Centuri accounts receivable for services provided to Southwest	$ 11,344	$ 13,956

Segment Information - Narrative

Segment Information - Narratives (Details)	3 Months Ended
	Mar. 31, 2021Segment
Segment Reporting [Abstract]
Number of reportable segments	2

Segment Information - Schedule

Segment Information - Schedule of Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Revenues	$ 885,907	$ 836,320	$ 3,348,460	$ 3,122,698
Segment net income (loss)	117,293	72,542	277,075	191,669
Revenues from external customers
Segment Reporting Information [Line Items]
Revenues	861,704	803,118	3,222,600	2,969,443
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	24,203	33,202	125,860	153,255
Segment net income (loss)	(563)	(853)	(1,366)	(1,943)
Natural Gas Operations
Segment Reporting Information [Line Items]
Revenues	521,932	502,827	1,369,690	1,351,089
Segment net income (loss)	118,715	83,599	194,234	143,381
Natural Gas Operations | Revenues from external customers
Segment Reporting Information [Line Items]
Revenues	521,932	502,827	1,369,690	1,351,089
Natural Gas Operations | Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Utility Infrastructure Services
Segment Reporting Information [Line Items]
Revenues	363,975	333,493	1,978,770	1,771,609
Segment net income (loss)	(859)	(10,204)	84,207	50,231
Utility Infrastructure Services | Revenues from external customers
Segment Reporting Information [Line Items]
Revenues	339,772	300,291	1,852,910	1,618,354
Utility Infrastructure Services | Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	$ 24,203	$ 33,202	$ 125,860	$ 153,255