Southwest Gas 10-Q 7 Aug 19 2019 Q2 Quarterly report Financial data

Cover

Cover - shares	6 Months Ended
	Jun. 30, 2019	Jul. 31, 2019
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2019
Document Transition Report	false
Entity File Number	001-37976
Entity Registrant Name	Southwest Gas Holdings, Inc.
Entity Incorporation, State or Country Code	CA
Entity Tax Identification Number	81-3881866
Entity Address, Address Line One	5241 Spring Mountain Road
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		54,324,289
Entity Central Index Key	0001692115
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Amendment Flag	false
Southwest Gas Corporation
Entity Information [Line Items]
Document Type	10-Q
Document Period End Date	Jun. 30, 2019
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	5241 Spring Mountain Road
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
Trading Symbol	SWX
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Central Index Key	0000092416
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Amendment Flag	false

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Utility plant:
Gas plant	$ 7,404,724	$ 7,134,239
Less: accumulated depreciation	(2,282,770)	(2,234,029)
Construction work in progress	248,270	193,028
Net utility plant	5,370,224	5,093,238
Other property and investments	761,652	623,551
Current assets:
Cash and cash equivalents	38,446	85,361
Accounts receivable, net of allowances	417,639	413,926
Accrued utility revenue	34,900	77,200
Income taxes receivable	15,190	14,653
Deferred purchased gas costs	58,240	4,928
Prepaid and other current assets	187,666	243,701
Total current assets	752,081	839,769
Noncurrent assets:
Goodwill	345,872	359,045
Deferred income taxes	1,021	1,264
Deferred charges and other assets	436,035	440,862
Total noncurrent assets	782,928	801,171
Total assets	7,666,885	7,357,729
Capitalization:
Common stock, $1 par (authorized - 60,000,000 shares; issued and outstanding - 54,321,217 and 53,026,848 shares)	55,951	54,656
Additional paid-in capital	1,409,923	1,305,769
Accumulated other comprehensive income (loss), net	(48,581)	(52,668)
Retained earnings	1,002,070	944,285
Total Southwest Gas Holdings, Inc. equity	2,419,363	2,252,042
Noncontrolling interest	0	(452)
Total equity	2,419,363	2,251,590
Redeemable noncontrolling interest	83,182	81,831
Long-term debt, less current maturities	2,373,003	2,107,258
Total capitalization	4,875,548	4,440,679
Current liabilities:
Current maturities of long-term debt	36,807	33,060
Short-term debt	0	152,000
Accounts payable	196,915	248,993
Customer deposits	69,165	67,940
Income taxes payable	0	1,083
Accrued general taxes	42,778	43,560
Accrued interest	21,431	21,369
Deferred purchased gas costs	89,734	79,762
Other current liabilities	269,674	290,878
Total current liabilities	726,504	938,645
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	563,478	529,201
Accumulated removal costs	389,000	383,000
Other deferred credits and other long-term liabilities	1,112,355	1,066,204
Total deferred income taxes and other credits	2,064,833	1,978,405
Total capitalization and liabilities	7,666,885	7,357,729
Southwest Gas Corporation
Utility plant:
Gas plant	7,404,724	7,134,239
Less: accumulated depreciation	(2,282,770)	(2,234,029)
Construction work in progress	248,270	193,028
Net utility plant	5,370,224	5,093,238
Other property and investments	127,211	116,146
Current assets:
Cash and cash equivalents	32,443	31,962
Accounts receivable, net of allowances	93,430	140,057
Accrued utility revenue	34,900	77,200
Income taxes receivable	12,713	13,444
Deferred purchased gas costs	58,240	4,928
Prepaid and other current assets	167,906	229,562
Total current assets	399,632	497,153
Noncurrent assets:
Goodwill	10,095	10,095
Deferred charges and other assets	417,231	424,952
Total noncurrent assets	427,326	435,047
Total assets	6,324,393	6,141,584
Capitalization:
Common stock, $1 par (authorized - 60,000,000 shares; issued and outstanding - 54,321,217 and 53,026,848 shares)	49,112	49,112
Additional paid-in capital	1,169,549	1,065,242
Accumulated other comprehensive income (loss), net	(46,540)	(49,049)
Retained earnings	776,101	717,155
Total Southwest Gas Holdings, Inc. equity	1,948,222	1,782,460
Total equity	1,948,222
Long-term debt, less current maturities	2,011,559	1,818,669
Total capitalization	3,959,781	3,601,129
Current liabilities:
Short-term debt	0	152,000
Accounts payable	106,773	184,982
Customer deposits	69,165	67,940
Accrued general taxes	42,778	43,560
Accrued interest	21,258	20,243
Deferred purchased gas costs	89,734	79,762
Payable to parent	580	472
Other current liabilities	110,851	94,136
Total current liabilities	441,139	643,095
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	519,432	490,458
Accumulated removal costs	389,000	383,000
Other deferred credits and other long-term liabilities	1,015,041	1,023,902
Total deferred income taxes and other credits	1,923,473	1,897,360
Total capitalization and liabilities	$ 6,324,393	$ 6,141,584

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Common stock, par (in USD per share)	$ 1	$ 1
Common stock, authorized (in shares)	60,000,000	60,000,000
Common stock, issued (in shares)		53,026,848
Common stock, outstanding (in shares)	54,321,217	53,026,848

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating revenues:
Gas operating revenues	$ 258,711	$ 275,679	$ 779,388	$ 769,992	$ 1,367,124	$ 1,349,536
Utility infrastructure services revenues	454,300	395,204	767,162	655,221	1,634,226	1,409,263
Total operating revenues	713,011	670,883	1,546,550	1,425,213	3,001,350	2,758,799
Operating expenses:
Net cost of gas sold	65,182	83,466	257,786	269,198	407,976	407,943
Operations and maintenance	105,293	105,435	211,538	207,786	410,145	398,051
Depreciation and amortization	70,342	61,307	147,881	123,785	273,308	244,176
Taxes other than income taxes	15,126	14,666	31,332	29,923	61,307	58,590
Utility infrastructure services expenses	402,199	352,671	702,664	611,623	1,478,730	1,296,629
Total operating expenses	658,142	617,545	1,351,201	1,242,315	2,631,466	2,405,389
Operating income	54,869	53,338	195,349	182,898	369,884	353,410
Other income and (expenses):
Net interest deductions	(26,831)	(23,652)	(53,228)	(46,283)	(103,616)	(86,978)
Other income (deductions)	1,146	(2,706)	7,985	(7,040)	(2,401)	(9,270)
Total other income and (expenses)	(25,685)	(26,358)	(45,243)	(53,323)	(106,017)	(96,248)
Income before income taxes	29,184	26,980	150,106	129,575	263,867	257,162
Income tax expense (benefit)	6,352	5,429	31,890	29,730	63,844	50,501
Net income	22,832	21,551	118,216	99,845	200,023	206,661
Net income (loss) attributable to noncontrolling interests	776	0	1,351	(797)	1,523	(650)
Net income attributable to Southwest Gas Holdings, Inc.	$ 22,056	$ 21,551	$ 116,865	$ 100,642	$ 198,500	$ 207,311
Basic earnings per share (in USD per share)	$ 0.41	$ 0.44	$ 2.18	$ 2.07	$ 3.82	$ 4.29
Diluted earnings per share (in USD per share)	$ 0.41	$ 0.44	$ 2.18	$ 2.07	$ 3.82	$ 4.28
Average number of common shares (in shares)	53,935	48,826	53,654	48,622	51,914	48,338
Average shares (assuming dilution) (in shares)	54,003	48,880	53,716	48,671	51,977	48,387
Southwest Gas Corporation
Operating revenues:
Gas operating revenues	$ 258,711	$ 275,679	$ 779,388	$ 769,992	$ 1,367,124	$ 1,349,536
Operating expenses:
Net cost of gas sold	65,182	83,466	257,786	269,198	407,976	407,943
Operations and maintenance	104,991	105,208	210,533	207,398	407,948	397,251
Depreciation and amortization	49,343	47,664	106,955	97,625	201,146	192,098
Taxes other than income taxes	15,126	14,666	31,332	29,923	61,307	58,590
Total operating expenses	234,642	251,004	606,606	604,144	1,078,377	1,055,882
Operating income	24,069	24,675	172,782	165,848	288,747	293,654
Other income and (expenses):
Net interest deductions	(23,345)	(20,149)	(46,444)	(39,404)	(88,780)	(74,936)
Other income (deductions)	1,592	(2,094)	7,538	(6,697)	(3,005)	(9,036)
Total other income and (expenses)	(21,753)	(22,243)	(38,906)	(46,101)	(91,785)	(83,972)
Income before income taxes	2,316	2,432	133,876	119,747	196,962	209,682
Income tax expense (benefit)	(1,053)	(190)	27,118	26,776	44,333	46,353
Net income	3,369	2,622	106,758	92,971	152,629	163,329
Net income attributable to Southwest Gas Holdings, Inc.	$ 3,369	$ 2,622	$ 106,758	$ 92,971	$ 152,629	$ 163,329

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net income	$ 22,832	$ 21,551	$ 118,216	$ 99,845	$ 200,023	$ 206,661
Defined benefit pension plans:
Net actuarial gain (loss)	0	0	0	0	(15,524)	(32,701)
Amortization of prior service cost	242	254	483	508	990	922
Amortization of net actuarial loss	4,441	6,387	8,883	12,774	21,658	20,662
Regulatory adjustment	(4,064)	(5,744)	(8,128)	(11,490)	(2,895)	8,212
Net defined benefit pension plans	619	897	1,238	1,792	4,229	(2,905)
Forward-starting interest rate swaps (“FSIRS”):
Amounts reclassified into net income	636	636	1,271	1,271	2,541	2,308
Net forward-starting interest rate swaps	636	636	1,271	1,271	2,541	2,308
Foreign currency translation adjustments	787	(690)	1,578	(1,601)	169	(679)
Total other comprehensive income (loss), net of tax	2,042	843	4,087	1,462	6,939	(1,276)
Comprehensive income	24,874	22,394	122,303	101,307	206,962	205,385
Comprehensive income (loss) attributable to noncontrolling interests	776	0	1,351	(797)	1,523	(668)
Comprehensive income attributable to Southwest Gas Holdings, Inc.	24,098	22,394	120,952	102,104	205,439	206,053
Southwest Gas Corporation
Net income	3,369	2,622	106,758	92,971	152,629	163,329
Defined benefit pension plans:
Net actuarial gain (loss)	0	0	0	0	(15,524)	(32,701)
Amortization of prior service cost	242	254	483	508	990	922
Amortization of net actuarial loss	4,441	6,387	8,883	12,774	21,658	20,662
Regulatory adjustment	(4,064)	(5,744)	(8,128)	(11,490)	(2,895)	8,212
Net defined benefit pension plans	619	897	1,238	1,792	4,229	(2,905)
Forward-starting interest rate swaps (“FSIRS”):
Amounts reclassified into net income	636	636	1,271	1,271	2,541	2,308
Net forward-starting interest rate swaps	636	636	1,271	1,271	2,541	2,308
Total other comprehensive income (loss), net of tax	1,255	1,533	2,509	3,063	6,770	(597)
Comprehensive income	$ 4,624	$ 4,155	$ 109,267	$ 96,034	$ 159,399	$ 162,732

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$ 118,216	$ 99,845	$ 200,023	$ 206,661
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	147,881	123,785	273,308	244,176
Deferred income taxes	31,831	33,318	49,554	48,871
Changes in current assets and liabilities:
Accounts receivable, net of allowances	(2,612)	(12,704)	(5,770)	(57,196)
Accrued utility revenue	42,300	44,000	(700)	(600)
Deferred purchased gas costs	(43,341)	34,105	5,128	11,693
Accounts payable	(48,155)	(44,465)	8,088	25,943
Accrued taxes	(2,272)	(17,350)	3,123	(9,138)
Other current assets and liabilities	54,988	(16,806)	17,721	(45,399)
Gains on sale	(831)	(250)	(2,284)	(3,019)
Changes in undistributed stock compensation	4,702	3,300	7,513	6,457
Equity AFUDC	(1,967)	(586)	(5,008)	(1,773)
Changes in other assets and deferred charges	(15,870)	(5,122)	(16,486)	(15,870)
Changes in other liabilities and deferred credits	(6,454)	5,952	26,040	7,979
Net cash provided by operating activities	278,416	247,022	560,250	418,785
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(471,578)	(339,011)	(898,481)	(700,426)
Acquisition of businesses, net of cash acquired	(19,533)	(4,209)	(266,697)	(98,413)
Changes in customer advances	9,483	8,158	14,788	9,911
Miscellaneous inflows	1,159	2,564	2,936	12,304
Net cash used in investing activities	(480,469)	(332,498)	(1,147,454)	(776,624)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	102,743	69,139	388,006	110,390
Dividends paid	(56,738)	(48,985)	(107,993)	(96,166)
Issuance of long-term debt, net	421,594	455,398	531,368	781,882
Retirement of long-term debt	(53,604)	(100,776)	(190,586)	(379,704)
Change in credit facility and commercial paper	(104,000)	(102,000)	(2,000)	(44,000)
Change in short-term debt	(152,000)	(192,000)	(22,500)	20,000
Principal payments on finance lease obligations	(107)	(316)	(439)	(723)
Redemption of Centuri shares from noncontrolling parties	0	0	0	(23,000)
Withholding remittance - share-based compensation	(1,856)	(2,854)	(2,112)	(2,910)
Other	(966)	(898)	(2,812)	(2,936)
Net cash provided by financing activities	155,066	76,708	590,932	362,833
Effects of currency translation on cash and cash equivalents	72	(124)	(12)	34
Change in cash and cash equivalents	(46,915)	(8,892)	3,716	5,028
Cash and cash equivalents at beginning of period	85,361	43,622	34,730	29,702
Cash and cash equivalents at end of period	38,446	34,730	38,446	34,730
Supplemental information:
Interest paid, net of amounts capitalized	49,996	40,082	96,476	76,843
Income taxes paid (received)	1,832	16,507	(13,454)	19,137
Southwest Gas Corporation
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	106,758	92,971	152,629	163,329
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	106,955	97,625	201,146	192,098
Deferred income taxes	28,183	27,371	43,811	47,666
Changes in current assets and liabilities:
Accounts receivable, net of allowances	46,626	36,104	(9,787)	(9,282)
Accrued utility revenue	42,300	44,000	(700)	(600)
Deferred purchased gas costs	(43,341)	34,105	5,128	11,693
Accounts payable	(74,787)	(52,095)	716	8,157
Accrued taxes	(52)	(12,776)	(6,008)	430
Other current assets and liabilities	74,589	(24,366)	7,511	(62,357)
Changes in undistributed stock compensation	3,760	3,220	5,895	5,577
Equity AFUDC	(1,967)	(586)	(5,008)	(1,773)
Changes in other assets and deferred charges	(20,336)	(5,490)	(21,895)	(16,607)
Changes in other liabilities and deferred credits	(6,836)	5,477	25,356	7,323
Net cash provided by operating activities	261,852	245,560	398,794	345,654
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(365,251)	(285,999)	(762,121)	(622,362)
Changes in customer advances	9,483	8,158	14,788	9,911
Miscellaneous inflows	(49)	778	(813)	2,165
Net cash used in investing activities	(355,817)	(277,063)	(748,146)	(610,286)
CASH FLOW FROM FINANCING ACTIVITIES:
Contributions from parent	102,092	56,596	159,045	97,955
Dividends paid	(46,300)	(43,000)	(90,300)	(84,601)
Issuance of long-term debt, net	297,222	297,495	297,222	297,495
Change in credit facility and commercial paper	(104,000)	(102,000)	(2,000)	(44,000)
Change in short-term debt	(152,000)	(191,000)	0	0
Withholding remittance - share-based compensation	(1,856)	(2,855)	(2,111)	(2,911)
Other	(712)	(779)	(961)	(850)
Net cash provided by financing activities	94,446	14,457	360,895	263,088
Change in cash and cash equivalents	481	(17,046)	11,543	(1,544)
Cash and cash equivalents at beginning of period	31,962	37,946	20,900	22,444
Cash and cash equivalents at end of period	32,443	20,900	32,443	20,900
Supplemental information:
Interest paid, net of amounts capitalized	42,853	33,452	83,206	66,037
Income taxes paid (received)	$ (22)	$ 10,886	$ (16,764)	$ 3,013

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 income (loss)	Accumulated other comprehensive income (loss)Southwest Gas Corporation	Retained Earnings	Retained EarningsSouthwest Gas Corporation	Southwest Gas Holdings, Inc.	Non-controlling Interest
Beginning balance (in shares) at Dec. 31, 2017			48,090,000	47,482,000
Beginning balance at Dec. 31, 2017			$ 49,720	$ 49,112	$ 955,332	$ 948,767	$ (47,682)	$ (47,073)	$ 857,398	$ 659,193		$ (2,365)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			1,036,000
Common stock issuances			$ 1,036		68,886
Net income (loss)									100,642	92,971		(797)
Change in ownership of noncontrolling interest					(2,710)							2,710
Foreign currency exchange translation adjustment							(1,601)
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax							1,792	1,792
FSIRS amounts reclassified to net income, net of tax	$ 1,271	$ 1,271					1,271	1,271
Reclassification of excess deferred taxes							(9,300)	(9,300)	9,300	9,300
Share-based compensation						702				(338)
Dividends declared									(51,065)	(44,000)
Contributions from Southwest Gas Holdings, Inc.						56,596
Ending balance (in shares) at Jun. 30, 2018			49,126,000
Ending balance at Jun. 30, 2018	$ 1,932,567	1,718,993	$ 50,756		1,021,508	1,006,065	(55,520)	(53,310)	916,275	717,126	$ 1,933,019	(452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 1.04
Beginning balance (in shares) at Mar. 31, 2018			48,337,000	47,482,000
Beginning balance at Mar. 31, 2018			$ 49,967	$ 49,112	965,480	948,199	(56,363)	(54,843)	920,454	736,676		(3,162)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			789,000
Common stock issuances			$ 789		56,028
Net income (loss)									21,551	2,622		0
Change in ownership of noncontrolling interest					0							2,710
Foreign currency exchange translation adjustment							(690)
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax							897	897
FSIRS amounts reclassified to net income, net of tax	$ 636	636					636	636
Reclassification of excess deferred taxes							0	0	0	0
Share-based compensation						1,270				(172)
Dividends declared									(25,730)	(22,000)
Contributions from Southwest Gas Holdings, Inc.						56,596
Ending balance (in shares) at Jun. 30, 2018			49,126,000
Ending balance at Jun. 30, 2018	$ 1,932,567	1,718,993	$ 50,756		1,021,508	1,006,065	(55,520)	(53,310)	916,275	717,126	1,933,019	(452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 0.52
FSIRS amounts reclassified to net income, net of tax	$ 2,541	2,541
Ending balance (in shares) at Jun. 30, 2019	54,321,217		54,321,000
Ending balance at Jun. 30, 2019	$ 2,419,363	1,948,222	$ 55,951		1,409,923	1,169,549	(48,581)	(46,540)	1,002,070	776,101	2,419,363	0
Beginning balance (in shares) at Dec. 31, 2018	53,026,848		53,026,000	47,482,000
Beginning balance at Dec. 31, 2018	$ 2,251,590		$ 54,656	$ 49,112	1,305,769	1,065,242	(52,668)	(49,049)	944,285	717,155		(452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			1,295,000
Common stock issuances			$ 1,295		104,606
Net income (loss)									116,865	106,758		0
Change in ownership of noncontrolling interest					(452)							452
Foreign currency exchange translation adjustment							1,578
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax							1,238	1,238
FSIRS amounts reclassified to net income, net of tax	$ 1,271	1,271					1,271	1,271
Reclassification of excess deferred taxes							0	0	0	0
Share-based compensation						2,215				(312)
Dividends declared									(59,080)	(47,500)
Contributions from Southwest Gas Holdings, Inc.						102,092
Ending balance (in shares) at Jun. 30, 2019	54,321,217		54,321,000
Ending balance at Jun. 30, 2019	$ 2,419,363	1,948,222	$ 55,951		1,409,923	1,169,549	(48,581)	(46,540)	1,002,070	776,101	2,419,363	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 1.09
Beginning balance (in shares) at Mar. 31, 2019			53,391,000	47,482,000
Beginning balance at Mar. 31, 2019			$ 55,021	$ 49,112	1,332,793	1,089,002	(50,623)	(47,795)	1,009,809	797,584		(452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			930,000
Common stock issuances			$ 930		77,582
Net income (loss)									22,056	3,369		0
Change in ownership of noncontrolling interest					(452)							452
Foreign currency exchange translation adjustment							787
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax							619	619
FSIRS amounts reclassified to net income, net of tax	$ 636	636					636	636
Reclassification of excess deferred taxes							0	0	0	0
Share-based compensation						1,297				(152)
Dividends declared									(29,795)	(24,700)
Contributions from Southwest Gas Holdings, Inc.						79,250
Ending balance (in shares) at Jun. 30, 2019	54,321,217		54,321,000
Ending balance at Jun. 30, 2019	$ 2,419,363	$ 1,948,222	$ 55,951		$ 1,409,923	$ 1,169,549	$ (48,581)	$ (46,540)	$ 1,002,070	$ 776,101	$ 2,419,363	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 0.545

Background, Organization, and S

Background, Organization, and Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
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). At the annual meeting of shareholders of Southwest Gas Holdings, Inc., held on May 2, 2019, shareholders voted to approve changing the state of incorporation for Southwest Gas Holdings, Inc. from California to Delaware. The reincorporation remains subject to certain regulatory approvals, which are currently pending. 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 from installation, replacement, repair, and maintenance of energy distribution systems, and developing industrial construction solutions. 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. In November 2017, Centuri acquired Neuco, thereby expanding its core services in the northeast region of the U.S. Additionally, in November 2018, Centuri expanded its operations in the southeast region of the U.S. through the acquisition of an 80% interest in Linetec. See Note 12 – Business Acquisitions for more information. Basis of Presentation. The condensed consolidated financial statements for 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”). The year-end condensed balance sheet data was derived from audited financial statements. 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, or in the primary businesses comprising those segments as a result of the foregoing acquisitions of Neuco and Linetec. 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. It is suggested that these condensed consolidated financial statements be read in conjunction with the consolidated financial statements and the notes thereto included in the 2018 Annual Report to Shareholders, which is incorporated by reference into the 2018 Form 10-K. Centuri, through its subsidiary, NPL, had historically held a 65% interest in a venture to market natural gas engine-driven heating, ventilating, and air conditioning technology and products. During the second quarter of 2018 , an additional $1 million of capital was contributed by NPL, thereby increasing NPL’s ownership interest to 95% . During the second quarter of 2019 , the remaining 5% ownership interest was assumed by NPL. The carrying amount of the noncontrolling interest has been adjusted with a corresponding charge to Additional paid-in capital on the Company’s Consolidated Balance Sheet. Fair Value Measurements . Certain assets and liabilities are reported at fair value, which is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. U.S. GAAP states that a fair value measurement should be based on the assumptions that market participants would use in pricing the asset or liability and establishes a fair value hierarchy that ranks the inputs used to measure fair value by their reliability. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements) and the lowest priority to fair values derived from unobservable inputs (Level 3 measurements). Financial assets and liabilities are categorized in their entirety based on the lowest level of input that is significant to the fair value measurement. The three levels of the fair value hierarchy are as follows: Level 1 – quoted prices (unadjusted) in active markets for identical assets or liabilities that a company has the ability to access at the measurement date. Level 2 – inputs other than quoted prices included within Level 1 that are observable for similar assets or liabilities, either directly or indirectly. Level 3 – unobservable inputs for the asset or liability. Unobservable inputs are used to measure fair value to the extent that observable inputs are not available, thereby allowing for situations in which there is little, if any, market activity for the asset or liability at the measurement date. The Company primarily used quoted market prices and other observable market pricing information in valuing cash and cash equivalents, derivatives, long-term debt outstanding, and assets of the qualified pension plan and 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): June 30, 2019 December 31, 2018 Southwest Gas Corporation: Net cash surrender value of COLI policies $ 125,491 $ 114,405 Other property 1,720 1,741 Total Southwest Gas Corporation 127,211 116,146 Centuri property, equipment, and intangibles 952,120 792,191 Centuri accumulated depreciation/amortization (332,683 ) (298,939 ) Other property 15,004 14,153 Total Southwest Gas Holdings, Inc. $ 761,652 $ 623,551 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. Cash and Cash Equivalents. For purposes of reporting consolidated cash flows, cash and cash equivalents include cash on hand and financial instruments with original maturities of three months or less. Such investments are carried at cost, which approximates market value. Cash and cash equivalents for Southwest and the Company also include money market fund investments totaling approximately $19.6 million and $23.8 million , respectively, at June 30, 2019 , and $18 million and $59.9 million , respectively, at December 31, 2018 , which fall within Level 2 of the fair value hierarchy, due to the asset valuation methods used by money market funds. Typical non-cash investing activities for Southwest include customer advances applied as contributions toward utility construction activity and capital expenditures that were not paid as of quarter end that are included in accounts payable. Amounts related to such activities were immaterial for the periods presented herein. Non-cash investing activities for the twelve months ended June 30, 2019 included $31.3 million of purchase consideration related to the Linetec acquisition by Centuri, in the form of liabilities incurred that remained unpaid as of June 30, 2019 ; such amounts are included in Other current liabilities on the Condensed Consolidated Balance Sheets of the Company. Also, see Recent Accounting Standards Updates and Note 4 – Leases for information related to right-of-use assets obtained in exchange for lease liabilities, which are non-cash investing and financing activities. Intercompany Transactions . Centuri recognizes revenues generated from contracts with Southwest (see Note 10 – Segment Information ). Centuri’s accounts receivable for these services are presented in the table below (thousands of dollars): June 30, 2019 December 31, 2018 Centuri accounts receivable for services provided to Southwest $ 19,260 $ 18,830 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. Income Taxes. In 2017, the Tax Cuts and Jobs Act (the “TCJA”) was enacted. The TCJA had significant impacts on the taxation of business entities, including specific provisions related to regulated public utilities. The more significant changes that impacted the Company and Southwest include the reduction in the corporate federal income tax rate from 35% to 21%, and limiting the utilization of net operating losses (“NOLs”) to 80% of taxable income, with the ability to indefinitely carryforward unutilized NOLs to reduce future taxable income. Prepaid and Other Current Assets. Prepaid and other current assets includes gas pipe materials and operating supplies of $62 million at June 30, 2019 and $56 million at December 31, 2018 (carried at weighted average cost), in addition to $52 million at June 30, 2019 and $74 million at December 31, 2018 related to a regulatory asset associated with the Arizona decoupling mechanism (an alternative revenue program). 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. No impairment was deemed to have occurred in the first six months of 2019 . (Thousands of dollars) Natural Gas Operations Utility Infrastructure Services Total Company December 31, 2018 $ 10,095 $ 348,950 $ 359,045 Measurement-period adjustments - Linetec acquisition (a) — (17,511 ) (17,511 ) Foreign currency translation adjustment — 4,338 4,338 June 30, 2019 $ 10,095 $ 335,777 $ 345,872 (a) See Note 12 – Business Acquisitions for details regarding Linetec measurement-period adjustments. Other Current Liabilities . Other current liabilities for Southwest include $24.7 million and $23.5 million of dividends declared by Southwest Gas Corporation, but not yet paid to Southwest Gas Holdings, Inc. at June 30, 2019 and December 31, 2018 , respectively. In addition, the balances in both periods include amounts payable under regulatory mechanisms in the next twelve months and miscellaneous other accrued liabilities. Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) in the Condensed Consolidated Statements of Income (thousands of dollars): Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2019 2018 2019 2018 2019 2018 Southwest Gas Corporation - natural gas operations segment: Change in COLI policies $ 3,400 $ 2,000 $ 11,000 $ 1,300 $ 6,500 $ 6,900 Interest income 1,822 1,377 3,419 2,795 6,644 4,401 Equity AFUDC 1,007 357 1,967 586 5,008 1,773 Other components of net periodic benefit cost (3,765 ) (5,264 ) (7,530 ) (10,529 ) (18,060 ) (20,241 ) Miscellaneous income and (expense) (872 ) (564 ) (1,318 ) (849 ) (3,097 ) (1,869 ) Southwest Gas Corporation - total other income (deductions) 1,592 (2,094 ) 7,538 (6,697 ) (3,005 ) (9,036 ) Utility infrastructure services segment: Interest income — 1 — 2 86 4 Foreign transaction gain (loss) 21 202 552 349 (19 ) (207 ) Miscellaneous income and (expense) (498 ) (835 ) (154 ) (720 ) 462 (69 ) Centuri - total other income (deductions) (477 ) (632 ) 398 (369 ) 529 (272 ) Corporate and administrative 31 20 49 26 75 38 Consolidated Southwest Gas Holdings, Inc. - total other income (deductions) $ 1,146 $ (2,706 ) $ 7,985 $ (7,040 ) $ (2,401 ) $ (9,270 ) 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 also to Note 2 – Components of Net Periodic Benefit Cost . Recent Accounting Standards Updates. Accounting pronouncements adopted in 2019: In February 2016, the Financial Accounting Standards Board (“FASB”) issued the update “Leases (Topic 842).” Under the update, lessees were required to recognize a lease liability for the obligation to make lease payments, measured on a discounted basis; and a right-of-use asset for the right to use, or control the use of, a specified asset for the lease term. The Company and Southwest adopted Topic 842 in the first quarter of 2019 through an optional transition method, which was elected, permitting the application of the provisions of the standard at the adoption date, rather than to earlier comparative periods. As a result, the Company and Southwest have not recast prior periods to reflect the adoption of this standard. See Note 4 – Leases . Accounting pronouncements that will be effective after 2019: In June 2016, the FASB issued Accounting Standards Update (“ASU”) 2016-13 update “Financial Instruments — Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” The update requires the measurement of all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. The inputs currently used to estimate credit losses will still be used; however, they may be adapted to reflect the full amount of expected losses. The update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. All entities may adopt the amendments in this update earlier as of fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Management is evaluating what impact, if any, this update might have on the Company’s and Southwest’s consolidated financial statements and disclosures. In January 2017, the FASB issued ASU 2017-04 “Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment.” Currently, unless meeting the criteria for qualitative assessment only, an entity is required to perform a two-step test to determine the amount, if any, of goodwill impairment. In Step 1, an entity compares the fair value of a reporting unit with its carrying amount, including goodwill. If the carrying amount of the reporting unit exceeds its fair value, the entity performs Step 2 and compares the implied fair value of goodwill with the carrying amount of that goodwill for that reporting unit, requiring a hypothetical purchase price allocation to measure the amount of a goodwill impairment. An impairment charge equal to the amount by which the carrying amount of goodwill for the reporting unit exceeds the implied fair value of that goodwill is recorded, limited to the amount of goodwill allocated to that reporting unit. Under the update, an entity will apply a one-step quantitative test and record the amount of goodwill impairment as the excess of a reporting unit's carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit. The new guidance does not amend the optional qualitative assessment. The amount of any goodwill impairment calculated under the update could vary from the calculation under existing guidance, largely due to the consideration to be given to unrecognized differences between the fair value and carrying values of the other assets and liabilities in the reporting unit under the new guidance. The amendments should be applied on a prospective basis. The update is effective for fiscal and interim periods beginning after December 15, 2019. Early adoption has been permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. Management is evaluating the impacts this update might have on the Company’s and Southwest’s consolidated financial statements and disclosures. In August 2018, the FASB issued ASU 2018-15 “Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract.” The update generally aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement (that is a service contract) with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. Once capitalized, the update requires the entity to expense the amount capitalized over the term of the hosting arrangement, including reasonably certain renewal periods. The update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption of the amendments in this update is permitted for interim and related annual fiscal periods after December 15, 2018. Management is evaluating the impacts this update might have on the Company’s and Southwest’s consolidated financial statements and disclosures. In August 2018, the FASB issued ASU 2018-14 “Compensation—Retirement Benefits—Defined Benefit Plans—General (Subtopic 715-20): Disclosure Framework—Changes to the Disclosure Requirements for Defined Benefit Plans.” This update removes disclosures that are no longer considered cost-beneficial, clarifies the specific requirements of disclosures, and adds disclosure requirements identified as relevant. The update applies to all employers that sponsor defined benefit pension or other postretirement plans. The update is effective for fiscal years ending after December 15, 2020. Upon adoption, the Company and Southwest will modify their disclosures to conform to the requirements of the update. In August 2018, the FASB issued ASU 2018-13 “Fair Value Measurement: Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement.” The update is intended to improve the effectiveness of fair value measurement disclosures and removes the following disclosure requirements: the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy; the policy for timing of transfers between levels; and the valuation processes for Level 3 fair value measurements. The update also modifies or clarifies for investments in certain entities that calculate net asset value, a requirement to disclose the timing of liquidation of an investee’s assets and the date when restrictions from redemption might lapse (in cases when the timing has been communicated or announced publicly). It also clarifies communication requirements about measurement uncertainty as of the reporting date. For certain unobservable inputs, an entity may disclose other quantitative information in lieu of the weighted average if it would be a more reasonable and rational method to reflect the distribution of inputs to the measurements. The update is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Management is evaluating the impacts this update might have on its disclosures.

Components of Net Periodic Bene

Components of Net Periodic Benefit Cost	6 Months Ended
	Jun. 30, 2019
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 are components of an overhead loading process associated with the cost of labor. The overhead process ultimately results in allocation of that portion of overall net periodic benefit costs to the same accounts to which productive labor is charged. As a result, service costs become components of various accounts, primarily operations and maintenance expense, net utility plant, and deferred charges and other assets for both the Company and Southwest. The 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 Period Ended June 30, Three Months Six Months Twelve Months 2019 2018 2019 2018 2019 2018 (Thousands of dollars) Service cost $ 6,466 $ 7,139 $ 12,932 $ 14,278 $ 27,209 $ 25,974 Interest cost 12,251 11,044 24,503 22,087 46,590 45,129 Expected return on plan assets (15,061 ) (14,688 ) (30,122 ) (29,377 ) (59,500 ) (56,975 ) Amortization of net actuarial loss 5,589 8,028 11,178 16,057 27,236 28,059 Net periodic benefit cost $ 9,245 $ 11,523 $ 18,491 $ 23,045 $ 41,535 $ 42,187 SERP Period Ended June 30, Three Months Six Months Twelve Months 2019 2018 2019 2018 2019 2018 (Thousands of dollars) Service cost $ 67 $ 61 $ 133 $ 122 $ 256 $ 276 Interest cost 440 414 880 829 1,709 1,770 Amortization of net actuarial loss 255 376 510 751 1,261 1,472 Net periodic benefit cost $ 762 $ 851 $ 1,523 $ 1,702 $ 3,226 $ 3,518 PBOP Period Ended June 30, Three Months Six Months Twelve Months 2019 2018 2019 2018 2019 2018 (Thousands of dollars) Service cost $ 319 $ 369 $ 638 $ 737 $ 1,374 $ 1,471 Interest cost 762 687 1,524 1,374 2,898 2,990 Expected return on plan assets (789 ) (930 ) (1,578 ) (1,860 ) (3,436 ) (3,539 ) Amortization of prior service costs 318 334 635 668 1,302 1,335 Net periodic benefit cost $ 610 $ 460 $ 1,219 $ 919 $ 2,138 $ 2,257

Revenue

Revenue	6 Months Ended
	Jun. 30, 2019
Revenue from Contract with Customer [Abstract]
Revenue	Note 3 – Revenue The following information about the Company’s revenues is presented by segment. Southwest encompasses one segment – natural gas operations. 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 Six Months Ended Twelve Months Ended June 30, June 30, June 30, (Thousands of dollars) 2019 2018 2019 2018 2019 2018 Residential $ 164,873 $ 166,702 $ 582,101 $ 511,313 $ 958,008 $ 867,338 Small commercial 50,184 55,653 139,794 143,596 251,281 254,966 Large commercial 11,123 13,134 25,085 28,574 49,703 54,577 Industrial/other 5,413 5,491 11,891 12,001 23,379 23,533 Transportation 21,815 20,719 46,717 44,773 88,934 88,842 Revenue from contracts with customers 253,408 261,699 805,588 740,257 1,371,305 1,289,256 Alternative revenue program revenues (deferrals) 3,392 10,393 (31,153 ) 37,602 (22,776 ) 67,046 Other revenues (a) 1,911 3,587 4,953 (7,867 ) 18,595 (6,766 ) Total Gas operating revenues $ 258,711 $ 275,679 $ 779,388 $ 769,992 $ 1,367,124 $ 1,349,536 (a) Comprised of various other revenue impacts, including $(1.1) million , $(2.9) million , and $(3.9) million for the three, six, and twelve months ending June 30, 2019 , respectively; and, $1.6 million during the three months, and $(12.5) million in both the six and twelve months ending June 30, 2018 related to tax reform savings reserves/adjustments. 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: (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2019 2018 2019 2018 2019 2018 Service Types: Gas infrastructure services $ 323,817 $ 300,346 $ 521,709 $ 493,894 $ 1,205,197 $ 1,072,082 Electric power infrastructure services 61,366 3,994 113,667 9,396 136,900 19,350 Other 69,117 90,864 131,786 151,931 292,129 317,831 Total Utility infrastructure services revenues $ 454,300 $ 395,204 $ 767,162 $ 655,221 $ 1,634,226 $ 1,409,263 (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2019 2018 2019 2018 2019 2018 Contract Types: Master services agreement $ 375,860 $ 290,075 $ 611,515 $ 484,539 $ 1,229,388 $ 1,014,517 Bid contract 78,440 105,129 155,647 170,682 404,838 394,746 Total Utility infrastructure services revenues $ 454,300 $ 395,204 $ 767,162 $ 655,221 $ 1,634,226 $ 1,409,263 Unit priced contracts $ 355,487 $ 329,236 $ 591,172 $ 526,557 $ 1,323,034 $ 1,102,335 Fixed priced contracts 4,426 27,025 42,965 52,566 107,697 138,658 Time and materials contracts 94,387 38,943 133,025 76,098 203,495 168,270 Total Utility infrastructure services revenues $ 454,300 $ 395,204 $ 767,162 $ 655,221 $ 1,634,226 $ 1,409,263 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 June 30, 2019 and December 31, 2018 on the Company’s Condensed Consolidated Balance Sheets: (Thousands of dollars) June 30, 2019 December 31, 2018 Contracts receivable, net $ 205,571 $ 186,249 Revenue earned on contracts in progress in excess of billings 118,529 87,520 Amounts billed in excess of revenue earned on contracts 6,786 4,211 The revenue earned on contracts in progress in excess of billings (contract asset) primarily relates to Centuri’s rights to consideration for work completed but not billed and/or approved at the reporting date. These contract assets are transferred to contracts receivable when the rights become unconditional. The amounts billed in excess of revenue earned (contract liability) primarily relates 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, 2018 to June 30, 2019 is due to revenue recognized of $4.2 million that was included in this item as of January 1, 2019, 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 June 30, 2019 , Centuri has eighteen contracts that had 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 June 30, 2019 is $64.8 million . Centuri expects to recognize the remaining performance obligations over the next three years ; however, the timing of that recognition is largely within the control of the customer, including when the necessary equipment and materials required to complete the work are provided by the customer. Utility infrastructure services contracts receivable consists of the following: (Thousands of dollars) June 30, 2019 December 31, 2018 Billed on completed contracts and contracts in progress $ 207,057 $ 184,100 Other receivables 1,863 2,588 Contracts receivable, gross 208,920 186,688 Allowance for doubtful accounts (3,349 ) (439 ) Contracts receivable, net $ 205,571 $ 186,249

Leases

Leases	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Leases	Note 4 – Leases The Company and Southwest adopted FASB Topic 842 as of January 1, 2019. In association with the adoption, the Company recorded adjustments to its Condensed Consolidated Balance Sheet to record right-of-use (“ROU”) assets and lease liabilities of $58.4 million and $60.8 million , respectively. Included in those amounts, Southwest recorded $1.9 million related to both its ROU assets and lease liabilities. Neither the Company nor Southwest experienced a material impact to the Condensed Consolidated Statements of Income from the adoption and no cumulative-effect adjustment to the opening balance of retained earnings was recognized. Management elected to adopt the standard under the optional transition method (refer to Recent Accounting Standards Updates in Note 1 – Background, Organization, and Summary of Significant Accounting Policies ), and elected the following Topic 842 practical expedients and accounting policy elections: • To use the “package”, which is a set of three practical expedients that must be elected as a package and applied consistently to all of Southwest’s and Centuri’s leases. These include: not reassessing whether any expired or existing contracts are or contain leases; not reassessing the lease classification for expired or existing leases (that is, existing operating and capital leases in accordance with current lease guidance will in each case be classified as operating and finance leases, respectively, under the updated guidance); and not reassessing initial direct costs for any existing leases. • To utilize the practical expedient to exclude all easements in place prior to January 1, 2019 from treatment under Topic 842. However, Southwest will evaluate new easements entered into after the effective date of the standard to determine if the arrangements should be accounted for as leases. • To make an accounting policy election by asset class to include both the lease and non-lease components (as defined in the guidance) as a single component. • To make an accounting policy election to not apply Topic 842 to short-term leases, as permitted. • To not elect to use hindsight in determining the lease term and in assessing impairment of ROU assets. • To utilize a portfolio approach to effectively account for the operating lease ROU assets and liabilities with regard to certain equipment leases at Centuri. Southwest and Centuri determine if an arrangement is a lease at inception. ROU assets represent the right to use an underlying asset for the lease term; lease liabilities represent obligations to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at the commencement date based on the present value of lease payments over the lease term. As most of Southwest’s and Centuri’s leases do not provide an implicit interest rate, an incremental borrowing rate based on information available at commencement is used in determining the present value of lease payments; an implicit rate, if readily determinable, is used. Lease terms utilized in the computations may include options to extend or terminate the lease when it is reasonably certain that the option will be exercised. Southwest’s leases are comprised primarily of operating leases of buildings, land, and equipment. Southwest has no finance leases and no significant short-term leases. Southwest’s leases have a remaining term of 1 to 10 years , some of which include options to extend the lease up to 3 years . Southwest is currently not a lessor in any significant lease arrangements. Southwest’s ROU assets are included in Gas plant, and its lease liabilities are included in, depending upon maturity, Other current liabilities or Other deferred credits and other long-term liabilities on the Company’s and Southwest’s Condensed Consolidated Balance Sheet as of June 30, 2019 . Centuri has operating and finance leases for corporate and field offices, construction equipment, and transportation vehicles. Centuri is currently not a lessor in any significant lease arrangements. Centuri’s leases have remaining lease terms of 1 to 14 years . Some of these include options to extend the leases, generally for optional terms of up to 5 years , and some include options to terminate the leases within 1 year . Centuri’s equipment leases may include variable payment terms in addition to the fixed lease payments if machinery is used in excess of the standard work periods. These variable payments are not probable of occurring under the current operating environment and have not been included in consideration of lease payments. Due to the seasonality of Centuri’s business, expense for short-term leases will fluctuate throughout the year with higher expense incurred during the warmer months. Short-term leases of Centuri are not accounted for under the provisions of Topic 842, as permitted. Centuri’s ROU assets are included in Other property and investments, and its lease liabilities for operating and finance leases are included, depending upon maturity, in Other current liabilities or Other deferred credits and other long-term liabilities on the Company’s Condensed Consolidated Balance Sheet as of June 30, 2019 . The components of lease expense were as follows: (Thousands of dollars) Three Months Ended Six Months Ended June 30, 2019 June 30, 2019 Southwest: Operating lease cost $ 409 $ 815 Centuri: Operating lease cost $ 3,238 $ 6,043 Finance lease cost: Amortization of ROU assets $ 34 $ 69 Interest on lease liabilities 7 14 Total finance lease cost 41 83 Short-term lease cost 3,357 5,932 Total lease cost $ 7,045 $ 12,873 Supplemental cash flow information related to leases for the six months ended June 30, 2019 was as follows: (Thousands of dollars) Southwest Centuri Consolidated Total Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 671 $ 5,293 $ 5,964 Operating cash flows from finance leases — 14 14 Financing cash flows from finance leases — 107 107 ROU assets obtained in exchange for lease obligations: Operating leases $ 901 $ 8,968 $ 9,869 Finance leases — 339 339 Supplemental information related to leases, including location in the Condensed Consolidated Balance Sheets, is as follows: (Thousands of dollars) June 30, 2019 Southwest: Operating leases: Net Utility Plant $ 2,072 Other current liabilities $ 897 Other deferred credits and other long-term liabilities 1,200 Total operating lease liabilities $ 2,097 Weighted average remaining lease term (in years) 3.81 Weighted average discount rate 3.36 % Centuri: Operating leases: Other property and investments $ 60,301 Other current liabilities 7,965 Other deferred credits and other long-term liabilities 55,185 Total operating lease liabilities $ 63,150 Finance leases: Other property and investments $ 854 Other current liabilities 314 Other deferred credits and other long-term liabilities 394 Total finance lease liabilities $ 708 Weighted average remaining lease term (in years) Operating leases 9.07 Finance leases 1.84 Weighted average discount rate Operating leases 4.08 % Finance leases 5.90 % The following are schedules of maturities of lease liabilities as of June 30, 2019 : (Thousands of dollars) Operating leases Southwest: 2020 $ 1,018 2021 534 2022 213 2023 102 2024 78 Thereafter 287 Total lease payments 2,232 Less imputed interest 135 Total $ 2,097 (Thousands of dollars) Operating leases Finance leases Centuri: 2020 $ 10,714 $ 349 2021 9,551 157 2022 8,736 168 2023 7,548 72 2024 5,992 35 Thereafter 34,239 — Total lease payments 76,780 781 Less imputed interest 13,630 73 Total $ 63,150 $ 708 As the Company and Southwest adopted Topic 842 using the optional transition method referred to in Note 1 – Background, Organization, and Summary of Significant Accounting Policies , the recent annual disclosure of rental and lease payments as of December 31, 2018 in accordance with Topic 840 is presented in the table below (thousands of dollars): 2018 2017 Southwest Gas Corporation $ 4,556 $ 4,926 Centuri 59,491 62,310 Consolidated rental payments/lease expense $ 64,047 $ 67,236 The following is a schedule of future minimum lease payments for operating leases (with initial or remaining terms in excess of one year) as of December 31, 2018 (thousands of dollars): Southwest Centuri Consolidated Total 2019 $ 898 $ 10,053 $ 10,951 2020 363 7,656 8,019 2021 299 5,760 6,059 2022 163 5,163 5,326 2023 79 3,681 3,760 Thereafter 177 10,511 10,688 Total minimum lease payments $ 1,979 $ 42,824 $ 44,803 As of December 31, 2018 Centuri leased certain construction equipment under capital leases arrangements which were not significant.
Leases	Note 4 – Leases The Company and Southwest adopted FASB Topic 842 as of January 1, 2019. In association with the adoption, the Company recorded adjustments to its Condensed Consolidated Balance Sheet to record right-of-use (“ROU”) assets and lease liabilities of $58.4 million and $60.8 million , respectively. Included in those amounts, Southwest recorded $1.9 million related to both its ROU assets and lease liabilities. Neither the Company nor Southwest experienced a material impact to the Condensed Consolidated Statements of Income from the adoption and no cumulative-effect adjustment to the opening balance of retained earnings was recognized. Management elected to adopt the standard under the optional transition method (refer to Recent Accounting Standards Updates in Note 1 – Background, Organization, and Summary of Significant Accounting Policies ), and elected the following Topic 842 practical expedients and accounting policy elections: • To use the “package”, which is a set of three practical expedients that must be elected as a package and applied consistently to all of Southwest’s and Centuri’s leases. These include: not reassessing whether any expired or existing contracts are or contain leases; not reassessing the lease classification for expired or existing leases (that is, existing operating and capital leases in accordance with current lease guidance will in each case be classified as operating and finance leases, respectively, under the updated guidance); and not reassessing initial direct costs for any existing leases. • To utilize the practical expedient to exclude all easements in place prior to January 1, 2019 from treatment under Topic 842. However, Southwest will evaluate new easements entered into after the effective date of the standard to determine if the arrangements should be accounted for as leases. • To make an accounting policy election by asset class to include both the lease and non-lease components (as defined in the guidance) as a single component. • To make an accounting policy election to not apply Topic 842 to short-term leases, as permitted. • To not elect to use hindsight in determining the lease term and in assessing impairment of ROU assets. • To utilize a portfolio approach to effectively account for the operating lease ROU assets and liabilities with regard to certain equipment leases at Centuri. Southwest and Centuri determine if an arrangement is a lease at inception. ROU assets represent the right to use an underlying asset for the lease term; lease liabilities represent obligations to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at the commencement date based on the present value of lease payments over the lease term. As most of Southwest’s and Centuri’s leases do not provide an implicit interest rate, an incremental borrowing rate based on information available at commencement is used in determining the present value of lease payments; an implicit rate, if readily determinable, is used. Lease terms utilized in the computations may include options to extend or terminate the lease when it is reasonably certain that the option will be exercised. Southwest’s leases are comprised primarily of operating leases of buildings, land, and equipment. Southwest has no finance leases and no significant short-term leases. Southwest’s leases have a remaining term of 1 to 10 years , some of which include options to extend the lease up to 3 years . Southwest is currently not a lessor in any significant lease arrangements. Southwest’s ROU assets are included in Gas plant, and its lease liabilities are included in, depending upon maturity, Other current liabilities or Other deferred credits and other long-term liabilities on the Company’s and Southwest’s Condensed Consolidated Balance Sheet as of June 30, 2019 . Centuri has operating and finance leases for corporate and field offices, construction equipment, and transportation vehicles. Centuri is currently not a lessor in any significant lease arrangements. Centuri’s leases have remaining lease terms of 1 to 14 years . Some of these include options to extend the leases, generally for optional terms of up to 5 years , and some include options to terminate the leases within 1 year . Centuri’s equipment leases may include variable payment terms in addition to the fixed lease payments if machinery is used in excess of the standard work periods. These variable payments are not probable of occurring under the current operating environment and have not been included in consideration of lease payments. Due to the seasonality of Centuri’s business, expense for short-term leases will fluctuate throughout the year with higher expense incurred during the warmer months. Short-term leases of Centuri are not accounted for under the provisions of Topic 842, as permitted. Centuri’s ROU assets are included in Other property and investments, and its lease liabilities for operating and finance leases are included, depending upon maturity, in Other current liabilities or Other deferred credits and other long-term liabilities on the Company’s Condensed Consolidated Balance Sheet as of June 30, 2019 . The components of lease expense were as follows: (Thousands of dollars) Three Months Ended Six Months Ended June 30, 2019 June 30, 2019 Southwest: Operating lease cost $ 409 $ 815 Centuri: Operating lease cost $ 3,238 $ 6,043 Finance lease cost: Amortization of ROU assets $ 34 $ 69 Interest on lease liabilities 7 14 Total finance lease cost 41 83 Short-term lease cost 3,357 5,932 Total lease cost $ 7,045 $ 12,873 Supplemental cash flow information related to leases for the six months ended June 30, 2019 was as follows: (Thousands of dollars) Southwest Centuri Consolidated Total Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 671 $ 5,293 $ 5,964 Operating cash flows from finance leases — 14 14 Financing cash flows from finance leases — 107 107 ROU assets obtained in exchange for lease obligations: Operating leases $ 901 $ 8,968 $ 9,869 Finance leases — 339 339 Supplemental information related to leases, including location in the Condensed Consolidated Balance Sheets, is as follows: (Thousands of dollars) June 30, 2019 Southwest: Operating leases: Net Utility Plant $ 2,072 Other current liabilities $ 897 Other deferred credits and other long-term liabilities 1,200 Total operating lease liabilities $ 2,097 Weighted average remaining lease term (in years) 3.81 Weighted average discount rate 3.36 % Centuri: Operating leases: Other property and investments $ 60,301 Other current liabilities 7,965 Other deferred credits and other long-term liabilities 55,185 Total operating lease liabilities $ 63,150 Finance leases: Other property and investments $ 854 Other current liabilities 314 Other deferred credits and other long-term liabilities 394 Total finance lease liabilities $ 708 Weighted average remaining lease term (in years) Operating leases 9.07 Finance leases 1.84 Weighted average discount rate Operating leases 4.08 % Finance leases 5.90 % The following are schedules of maturities of lease liabilities as of June 30, 2019 : (Thousands of dollars) Operating leases Southwest: 2020 $ 1,018 2021 534 2022 213 2023 102 2024 78 Thereafter 287 Total lease payments 2,232 Less imputed interest 135 Total $ 2,097 (Thousands of dollars) Operating leases Finance leases Centuri: 2020 $ 10,714 $ 349 2021 9,551 157 2022 8,736 168 2023 7,548 72 2024 5,992 35 Thereafter 34,239 — Total lease payments 76,780 781 Less imputed interest 13,630 73 Total $ 63,150 $ 708 As the Company and Southwest adopted Topic 842 using the optional transition method referred to in Note 1 – Background, Organization, and Summary of Significant Accounting Policies , the recent annual disclosure of rental and lease payments as of December 31, 2018 in accordance with Topic 840 is presented in the table below (thousands of dollars): 2018 2017 Southwest Gas Corporation $ 4,556 $ 4,926 Centuri 59,491 62,310 Consolidated rental payments/lease expense $ 64,047 $ 67,236 The following is a schedule of future minimum lease payments for operating leases (with initial or remaining terms in excess of one year) as of December 31, 2018 (thousands of dollars): Southwest Centuri Consolidated Total 2019 $ 898 $ 10,053 $ 10,951 2020 363 7,656 8,019 2021 299 5,760 6,059 2022 163 5,163 5,326 2023 79 3,681 3,760 Thereafter 177 10,511 10,688 Total minimum lease payments $ 1,979 $ 42,824 $ 44,803 As of December 31, 2018 Centuri leased certain construction equipment under capital leases arrangements which were not significant.

Derivatives

Derivatives	6 Months Ended
	Jun. 30, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives	Note 5 – Derivatives In managing its natural gas supply portfolios, Southwest has historically entered into fixed- and variable-price contracts, which qualify as derivatives. Additionally, Southwest utilizes fixed-for-floating swap contracts (“Swaps”) to supplement its fixed-price contracts. The fixed-price contracts, firm commitments to purchase a fixed amount of gas in the future at a fixed price, qualify for the normal purchases and normal sales exception that is allowed for contracts that are probable of delivery in the normal course of business, and are exempt from fair value reporting. The variable-price contracts qualify as derivative instruments; however, because the contract prices are the prevailing prices at the future transaction dates, the contracts have no determinable fair value. The Swap contract prices are determined at the beginning of each month to reflect that month’s published first of month index price and are recorded at fair value. Southwest does not utilize derivative financial instruments for speculative purposes, nor does it have trading operations. The fixed-price contracts and Swaps are utilized by Southwest under its volatility mitigation programs to effectively fix the price on a portion (up to 25% in the Arizona and California jurisdictions) of its natural gas supply portfolios. The maturities of the Swaps highly correlate to forecasted purchases of natural gas, with the longest maturity date of the Swaps being October 2020. Under such contracts, Southwest pays the counterparty a fixed rate and receives from the counterparty a floating rate per MMBtu (“dekatherm”) of natural gas. Only the net differential is actually paid or received. The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms): June 30, 2019 December 31, 2018 Contract notional amounts 17,309 13,387 The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps: Three Months Ended Six Months Ended Twelve Months Ended (Thousands of dollars) June 30, 2019 June 30, 2019 June 30, 2019 Paid to counterparties $ 4,265 $ 6,147 $ 8,997 Received from counterparties $ — $ 1,047 $ 1,647 Pursuant to regulatory deferral accounting treatment for rate-regulated entities, unrealized gains and losses in fair value of the Swaps are recorded as a regulatory asset and/or liability. When the Swaps mature, any prior positions held are reversed and the settled position is recorded as an increase or decrease of purchased gas under the related purchase gas adjustment (“PGA”) mechanism in determining the deferred PGA balances. Neither changes in fair value nor settled amounts of Swaps have a direct effect on earnings or other comprehensive income, since following settlement, amounts are reflected in Net cost of gas sold at the same time they are included in Gas operating revenues through updates to the PGA component of rates. Previously, Southwest entered into forward-starting interest rate swaps (“FSIRS”), the settled positions for which are immaterial and continue to be amortized from Accumulated other comprehensive income (loss) into interest expense. The estimated fair value of Southwest’s Swaps were determined at June 30, 2019 and December 31, 2018 using futures settlement prices for the delivery of natural gas at Henry Hub adjusted by the price of future settlement bases, which reflect the difference between the price of natural gas at a given delivery basin and the Henry Hub pricing points. These Level 2 inputs are observable in the marketplace throughout the full term of the Swaps and have been credit-risk adjusted with no significant impact to the overall fair value measurement. The following table sets forth the fair value of the Swaps and their location in the Condensed Consolidated Balance Sheets for both the Company and Southwest (thousands of dollars). It also sets forth the location of regulatory assets or liabilities offsetting, dollar-for-dollar, the fair value of the Swaps (pursuant to Southwest’s rate-regulation): Fair values of derivatives not designated as hedging instruments: June 30, 2019 Swap Position Instrument Balance Sheet Location Asset Derivatives Liability Derivatives Net Total Offsetting Balance Sheet Location (Regulatory Asset/(Liability)) Swaps Deferred charges and other assets $ 37 $ (6 ) $ 31 Other deferred credits Swaps Other current liabilities 669 (6,309 ) (5,640 ) Prepaid and other current assets Swaps Other deferred credits — (1,314 ) (1,314 ) Deferred charges and other assets Total $ 706 $ (7,629 ) $ (6,923 ) December 31, 2018 Swap Position Instrument Balance Sheet Location Asset Derivatives Liability Derivatives Net Total Offsetting Balance Sheet Location (Regulatory Asset/(Liability)) Swaps Prepaid and other current assets $ 243 $ (99 ) $ 144 Other current liabilities Swaps Other current liabilities 1,595 (3,347 ) (1,752 ) Prepaid and other current assets Swaps Other deferred credits 141 (251 ) (110 ) Deferred charges and other assets Total $ 1,979 $ (3,697 ) $ (1,718 ) Master netting arrangements exist with each counterparty that provide for the net settlement (in the settlement month) of all contracts through a single payment. As applicable, management has elected to reflect the net amounts in its balance sheets. There was no outstanding collateral associated with the Swaps during either period shown in the above table.

Common Stock

Common Stock	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Common Stock	Note 6 – 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 is based on stock awards of Southwest Gas Corporation 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 quarter and life-to-date ended June 30, 2019 : Gross proceeds $ 74,999,771 Less: agent commissions (749,998 ) Net proceeds $ 74,249,773 Number of shares sold 873,361 Weighted average price per share $ 85.87 As of June 30, 2019 , the Company had up to $225,000,229 of common stock available for sale under the program. Net proceeds from the sales 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. Net proceeds during the six months ended June 30, 2019 were contributed to, and reflected in the records of, Southwest (as a capital contribution from Southwest Gas Holdings, Inc.). During the quarter ended March 31, 2019, the Company sold approximately 278,000 shares of common stock under a previously effective equity shelf program. Those issuances reflected the remaining shares available under that previous program. During the six months ended June 30, 2019 , the Company issued approximately 76,000 shares of common stock through the Restricted Stock/Unit Plan and Management Incentive Plan. Also during the six months ended June 30, 2019 , the Company issued 67,000 shares of common stock through the Dividend Reinvestment and Stock Purchase Plan (“DRSPP”), raising approximately $5.5 million . On May 2, 2019, at the Company’s annual meeting of shareholders, the Company’s shareholders approved an amendment to the Company’s Articles of Incorporation to increase the number of shares of common stock available for issuance from 60,000,000 to 120,000,000 . However, the Company has not filed such amendment to its Articles of Incorporation as it intends to cause such increase in availability of shares of common stock to take effect in connection with the Delaware reincorporation.

Long-Term Debt

Long-Term Debt	6 Months Ended
	Jun. 30, 2019
Debt Disclosure [Abstract]
Long-Term Debt	Note 7 – Long-Term Debt Long-term debt is recognized in the Company’s and Southwest’s balance sheets generally at the carrying value of the obligations outstanding. However, details surrounding the fair value and individual carrying values of instruments are discussed below or provided in the table that follows. Southwest believes its revolving credit facility (including commercial paper) and the variable-rate Industrial Development Revenue Bonds (“IDRBs”) approximate their carrying values. The revolving credit facility and IDRBs are categorized as Level 1 due to Southwest’s ability to access similar debt arrangements at measurement dates with comparable terms, including variable/market rates. Additionally, the revolving credit facility is generally repaid quickly and the IDRBs have interest rates that reset frequently. The fair values of Southwest’s debentures (which includes senior and medium-term notes) were determined utilizing a market-based valuation approach, where fair values are determined based on evaluated pricing data, such as broker quotes and yields for similar securities adjusted for observable differences. Significant inputs used in the valuation generally include benchmark yield curves, credit ratings, and issuer spreads. The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable. The fair values of debentures are categorized as Level 2 (observable market inputs based on market prices of similar securities). The Centuri secured revolving credit and term loan facility and Centuri’s other debt obligations (not actively traded) are categorized as Level 3, based on significant unobservable inputs to their fair values. Because Centuri’s debt is not publicly traded, fair values for the secured revolving credit and term loan facility and other debt obligations were based on a conventional discounted cash flow methodology and utilized current market pricing yield curves, across Centuri’s debt maturity spectrum, of other industrial bonds with an assumed credit rating comparable to the Company’s. June 30, 2019 December 31, 2018 Carrying Amount Market Value Carrying Amount Market Value (Thousands of dollars) Southwest Gas Corporation: Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 127,030 $ 125,000 $ 126,213 Notes, 6.1%, due 2041 125,000 158,916 125,000 150,728 Notes, 3.875%, due 2022 250,000 257,128 250,000 254,195 Notes, 4.875%, due 2043 250,000 287,903 250,000 268,985 Notes, 3.8%, due 2046 300,000 296,868 300,000 267,030 Notes, 3.7%, due 2028 300,000 316,170 300,000 298,926 Notes, 4.15%, due 2049 300,000 315,426 — — 8% Series, due 2026 75,000 96,803 75,000 93,827 Medium-term notes, 7.78% series, due 2022 25,000 27,774 25,000 27,497 Medium-term notes, 7.92% series, due 2027 25,000 31,660 25,000 30,016 Medium-term notes, 6.76% series, due 2027 7,500 9,064 7,500 8,651 Unamortized discount and debt issuance costs (15,102 ) (11,807 ) 1,767,398 1,470,693 Revolving credit facility and commercial paper 46,000 46,000 150,000 150,000 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 (1,839 ) (2,024 ) 198,161 197,976 Less: current maturities — — Long-term debt, less current maturities - Southwest Gas Corporation $ 2,011,559 $ 1,818,669 Centuri: Centuri term loan facility $ 253,120 $ 259,002 $ 255,959 $ 260,135 Unamortized debt issuance costs (1,270 ) (1,414 ) 251,850 254,545 Centuri secured revolving credit facility 88,174 88,213 — — Centuri other debt obligations 58,227 59,440 67,104 67,053 Less: current maturities (36,807 ) (33,060 ) Long-term debt, less current maturities - Centuri $ 361,444 $ 288,589 Consolidated Southwest Gas Holdings, Inc.: Southwest Gas Corporation long-term debt $ 2,011,559 $ 1,818,669 Centuri long-term debt 398,251 321,649 Less: current maturities (36,807 ) (33,060 ) Long-term debt, less current maturities - Southwest Gas Holdings, Inc. $ 2,373,003 $ 2,107,258 Southwest has a $400 million credit facility that is scheduled to expire in March 2022 . Southwest designates $150 million of capacity related to the facility as long-term debt and has designated the remaining $250 million for working capital purposes. Interest rates for the credit facility are calculated at either the London Interbank Offered Rate (“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 June 30, 2019 , 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 June 30, 2019 , $46 million was outstanding on the long-term portion (including $0 under the commercial paper program, discussed below) and no borrowings were outstanding on the short-term portion of this credit facility (see Note 8 – Short-Term Debt ). Southwest has a $50 million commercial paper program. Any issuance under the commercial paper program is supported by Southwest’s current revolving credit facility and, therefore, does not represent additional borrowing capacity under the credit facility. Borrowings under the commercial paper program are designated as long-term debt. Interest rates for the program are calculated at the then current commercial paper rate. At June 30, 2019 , as noted above, no borrowings were outstanding under the commercial paper program. In May 2019, Southwest issued $300 million in 4.15% Senior Notes at a discount of 0.051% . The notes will mature in June 2049 . The net proceeds were used to repay a portion of amounts then outstanding under its credit facility and commercial paper program. In November 2018, Centuri, in association with the acquisition of Linetec, amended and restated its senior secured revolving credit and term loan facility, increasing the capacity from $450 million to $590 million ; the amended facility is scheduled to expire in November 2023 . This facility includes a revolving credit facility and a term loan facility. The line of credit portion of the facility is $325 million ; amounts borrowed and repaid under the revolving credit facility are available to be re-borrowed. The term loan facility portion has a limit of approximately $265 million . The $590 million revolving credit and term loan 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 June 30, 2019 totaled $1.3 billion . At June 30, 2019 , $341 million in borrowings were outstanding under the Centuri facility. It is currently anticipated that LIBOR may be discontinued as a benchmark or reference rate after 2021. As of June 30, 2019 , no borrowings were outstanding for the holding company under its credit facility (see Note 8 – Short-Term Debt ), and therefore, there was no related indebtedness with reference to LIBOR. However, all of Southwest’s outstanding borrowings of $46 million noted above under its credit facility and approximately $215 million of Centuri’s indebtedness under its facility have interest rates with reference to LIBOR and credit facility maturity dates that extend beyond 2021. The outstanding amounts reflect approximately 2% of total Southwest long-term debt and 11% of long-term debt (including current maturities) for the Company overall. The Company intends to continue to monitor developments with respect to alternative rates and take appropriate steps, that will include Southwest and Centuri working with lenders to determine the appropriate alternative reference rate for variable rate indebtedness, in order to mitigate the impact of the discontinuation on financial condition and results of operations. However, at this time the Company and Southwest can provide no assurances as to the impact a LIBOR discontinuation will have on their financial condition or results of operations. Any alternative rate may be less predictable or less attractive than LIBOR.

Short-Term Debt

Short-Term Debt	6 Months Ended
	Jun. 30, 2019
Debt Disclosure [Abstract]
Short-Term Debt	Note 8 – Short-Term Debt Southwest Gas Holdings, Inc. has a $100 million credit facility that is scheduled to expire in March 2022 . There were no borrowings outstanding under this credit facility at June 30, 2019 . As discussed in Note 7 – Long-Term Debt , Southwest has a $ 400 million credit facility that is scheduled to expire in March 2022 , of which $ 250 million has been designated by management for working capital purposes. Southwest had no short-term borrowings outstanding at June 30, 2019 under this facility.

Other Comprehensive Income and

Other Comprehensive Income and Accumulated Other Comprehensive Income	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Other Comprehensive Income and Accumulated Other Comprehensive Income	Note 9 – Other Comprehensive Income and Accumulated Other Comprehensive Income The following information provides insight into amounts impacting 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. See Note 5 – Derivatives for additional information on the FSIRS. Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Thousands of dollars) Three Months Ended June 30, 2019 Three Months Ended June 30, 2018 Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Defined benefit pension plans: Amortization of prior service cost $ 318 $ (76 ) $ 242 $ 334 $ (80 ) $ 254 Amortization of net actuarial (gain)/loss 5,844 (1,403 ) 4,441 8,404 (2,017 ) 6,387 Regulatory adjustment (5,348 ) 1,284 (4,064 ) (7,559 ) 1,815 (5,744 ) Pension plans other comprehensive income 814 (195 ) 619 1,179 (282 ) 897 FSIRS (designated hedging activities): Amounts reclassified into net income 836 (200 ) 636 836 (200 ) 636 FSIRS other comprehensive income 836 (200 ) 636 836 (200 ) 636 Total other comprehensive income - Southwest Gas Corporation 1,650 (395 ) 1,255 2,015 (482 ) 1,533 Foreign currency translation adjustments: Translation adjustments 787 — 787 (690 ) — (690 ) Foreign currency other comprehensive income (loss) 787 — 787 (690 ) — (690 ) Total other comprehensive income - Southwest Gas Holdings, Inc. $ 2,437 $ (395 ) $ 2,042 $ 1,325 $ (482 ) $ 843 Six Months Ended June 30, 2019 Six Months Ended June 30, 2018 Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Defined benefit pension plans: Amortization of prior service cost $ 635 $ (152 ) $ 483 $ 668 $ (160 ) $ 508 Amortization of net actuarial (gain)/loss 11,688 (2,805 ) 8,883 16,808 (4,034 ) 12,774 Regulatory adjustment (10,695 ) 2,567 (8,128 ) (15,119 ) 3,629 (11,490 ) Pension plans other comprehensive income 1,628 (390 ) 1,238 2,357 (565 ) 1,792 FSIRS (designated hedging activities): Amounts reclassified into net income 1,672 (401 ) 1,271 1,673 (402 ) 1,271 FSIRS other comprehensive income 1,672 (401 ) 1,271 1,673 (402 ) 1,271 Total other comprehensive income - Southwest Gas Corporation 3,300 (791 ) 2,509 4,030 (967 ) 3,063 Foreign currency translation adjustments: Translation adjustments 1,578 — 1,578 (1,601 ) — (1,601 ) Foreign currency other comprehensive income (loss) 1,578 — 1,578 (1,601 ) — (1,601 ) Total other comprehensive income - Southwest Gas Holdings, Inc. $ 4,878 $ (791 ) $ 4,087 $ 2,429 $ (967 ) $ 1,462 Twelve Months Ended June 30, 2019 Twelve Months Ended June 30, 2018 Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Defined benefit pension plans: Net actuarial gain/(loss) $ (20,426 ) $ 4,902 $ (15,524 ) $ (43,027 ) $ 10,326 $ (32,701 ) Amortization of prior service cost 1,302 (312 ) 990 1,335 (413 ) 922 Amortization of net actuarial (gain)/loss 28,497 (6,839 ) 21,658 29,531 (8,869 ) 20,662 Regulatory adjustment (3,809 ) 914 (2,895 ) 8,691 (479 ) 8,212 Pension plans other comprehensive income (loss) 5,564 (1,335 ) 4,229 (3,470 ) 565 (2,905 ) FSIRS (designated hedging activities): Amounts reclassified into net income 3,344 (803 ) 2,541 3,345 (1,037 ) 2,308 FSIRS other comprehensive income 3,344 (803 ) 2,541 3,345 (1,037 ) 2,308 Total other comprehensive income (loss) - Southwest Gas Corporation 8,908 (2,138 ) 6,770 (125 ) (472 ) (597 ) Foreign currency translation adjustments: Translation adjustments 169 — 169 (679 ) — (679 ) Foreign currency other comprehensive income (loss) 169 — 169 (679 ) — (679 ) Total other comprehensive income (loss) - Southwest Gas Holdings, Inc. $ 9,077 $ (2,138 ) $ 6,939 $ (804 ) $ (472 ) $ (1,276 ) (1) Tax amounts are calculated using a 24% rate following the December 22, 2017 enactment date of U.S. tax reform. For periods prior to the enactment date (and included in specific line items of the tables for the twelve months ended June 30, 2018 ), tax amounts were calculated using a 38% rate. The tax effect of before-tax amounts remaining in the balance of Accumulated other comprehensive income (loss) as of June 30, 2019 is effectively computed using a 24% tax rate overall. With regard to foreign currency translation adjustments, the Company has elected to indefinitely reinvest the earnings of Centuri’s Canadian subsidiaries in Canada, thus preventing deferred taxes on such earnings. As a result of this assertion, and no repatriation of earnings anticipated, the Company is not recognizing a tax effect or presenting a tax expense or benefit for currency translation adjustments reported in Other comprehensive income (loss). Approximately $2.5 million of realized losses (net of tax) related to the FSIRS, reported in Accumulated other comprehensive income (loss) at June 30, 2019 , 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 (thousands of dollars): Defined Benefit Plans FSIRS Foreign Currency Items Before-Tax Tax (Expense) Benefit (4,5) After-Tax (5) Before-Tax Tax (Expense) Benefit (4,5) After-Tax (5) Before-Tax Tax (Expense) Benefit After-Tax AOCI Beginning Balance AOCI December 31, 2018 $ (55,227 ) $ 13,254 $ (41,973 ) $ (9,310 ) $ 2,234 $ (7,076 ) $ (3,619 ) $ — $ (3,619 ) $ (52,668 ) Translation adjustments — — — — — — 1,578 — 1,578 1,578 Other comprehensive income (loss) before reclassifications — — — — — — 1,578 — 1,578 1,578 FSIRS amounts reclassified from AOCI (1) — — — 1,672 (401 ) 1,271 — — — 1,271 Amortization of prior service cost (2) 635 (152 ) 483 — — — — — — 483 Amortization of net actuarial loss (2) 11,688 (2,805 ) 8,883 — — — — — — 8,883 Regulatory adjustment (3) (10,695 ) 2,567 (8,128 ) — — — — — — (8,128 ) Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. 1,628 (390 ) 1,238 1,672 (401 ) 1,271 1,578 — 1,578 4,087 Ending Balance AOCI June 30, 2019 $ (53,599 ) $ 12,864 $ (40,735 ) $ (7,638 ) $ 1,833 $ (5,805 ) $ (2,041 ) $ — $ (2,041 ) $ (48,581 ) (1) The FSIRS reclassification amounts are 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. (5) The beginning balances depict amounts attributable to the individual components of AOCI (Defined Benefit Plans and FSIRS) following the adoption of ASU No. 2018-02, with no impact to the total balance of AOCI resulting from the depiction. The following table represents a rollforward of AOCI, presented on Southwest’s Condensed Consolidated Balance Sheets (thousands of dollars): Defined Benefit Plans FSIRS Before-Tax Tax (Expense) Benefit (9,10) After-Tax (10) Before-Tax Tax (Expense) Benefit (9,10) After-Tax (10) AOCI Beginning Balance AOCI December 31, 2018 $ (55,227 ) $ 13,254 $ (41,973 ) $ (9,310 ) $ 2,234 $ (7,076 ) $ (49,049 ) FSIRS amounts reclassified from AOCI (6) — — — 1,672 (401 ) 1,271 1,271 Amortization of prior service cost (7) 635 (152 ) 483 — — — 483 Amortization of net actuarial loss (7) 11,688 (2,805 ) 8,883 — — — 8,883 Regulatory adjustment (8) (10,695 ) 2,567 (8,128 ) — — — (8,128 ) Net current period other comprehensive income attributable to Southwest Gas Corporation 1,628 (390 ) 1,238 1,672 (401 ) 1,271 2,509 Ending Balance AOCI June 30, 2019 $ (53,599 ) $ 12,864 $ (40,735 ) $ (7,638 ) $ 1,833 $ (5,805 ) $ (46,540 ) (6) The FSIRS reclassification amounts are included in Net interest deductions on Southwest’s Condensed Consolidated Statements of Income. (7) 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). (8) The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in Deferred charges and other assets on Southwest’s Condensed Consolidated Balance Sheets). (9) Tax amounts are calculated using a 24% rate. (10) The beginning balances depict amounts attributable to the individual components of AOCI (Defined Benefit Plans and FSIRS) following the adoption of ASU No. 2018-02, with no impact to the total balance of AOCI resulting from the depiction. 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): June 30, 2019 December 31, 2018 Net actuarial (loss) gain $ (423,676 ) $ (435,364 ) Prior service cost (2,398 ) (3,033 ) Less: amount recognized in regulatory assets 372,475 383,170 Recognized in AOCI $ (53,599 ) $ (55,227 )

Segment Information

Segment Information	6 Months Ended
	Jun. 30, 2019
Segment Reporting [Abstract]
Segment Information	Note 10 – Segment Information 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 following tables present revenues from external customers, intersegment revenues, and segment net income for the two reportable segments (thousands of dollars): Natural Gas Operations Utility Infrastructure Services Other Total Three Months Ended June 30, 2019 Revenues from external customers $ 258,711 $ 405,218 $ — $ 663,929 Intersegment revenues — 49,082 — 49,082 Total $ 258,711 $ 454,300 $ — $ 713,011 Segment net income (loss) $ 3,369 $ 18,918 $ (231 ) $ 22,056 Three Months Ended June 30, 2018 Revenues from external customers $ 275,679 $ 362,132 $ — $ 637,811 Intersegment revenues — 33,072 — 33,072 Total $ 275,679 $ 395,204 $ — $ 670,883 Segment net income (loss) $ 2,622 $ 19,236 $ (307 ) $ 21,551 Natural Gas Operations Utility Infrastructure Services Other Total Six Months Ended June 30, 2019 Revenues from external customers $ 779,388 $ 679,407 $ — $ 1,458,795 Intersegment revenues — 87,755 — 87,755 Total $ 779,388 $ 767,162 $ — $ 1,546,550 Segment net income (loss) $ 106,758 $ 10,887 $ (780 ) $ 116,865 Six Months Ended June 30, 2018 Revenues from external customers $ 769,992 $ 594,991 $ — $ 1,364,983 Intersegment revenues — 60,230 — 60,230 Total $ 769,992 $ 655,221 $ — $ 1,425,213 Segment net income (loss) $ 92,971 $ 8,235 $ (564 ) $ 100,642 Natural Gas Operations Utility Infrastructure Services Other Total Twelve Months Ended June 30, 2019 Revenues from external customers $ 1,367,124 $ 1,470,787 $ — $ 2,837,911 Intersegment revenues — 163,439 — 163,439 Total $ 1,367,124 $ 1,634,226 $ — $ 3,001,350 Segment net income (loss) $ 152,629 $ 47,629 $ (1,758 ) $ 198,500 Twelve Months Ended June 30, 2018 Revenues from external customers $ 1,349,536 $ 1,296,093 $ — $ 2,645,629 Intersegment revenues — 113,170 — 113,170 Total $ 1,349,536 $ 1,409,263 $ — $ 2,758,799 Segment net income (loss) $ 163,329 $ 45,213 $ (1,231 ) $ 207,311

Redeemable Noncontrolling Inter

Redeemable Noncontrolling Interests	6 Months Ended
	Jun. 30, 2019
Noncontrolling Interest [Abstract]
Redeemable Noncontrolling Interests	Note 11 – Redeemable Noncontrolling Interest In connection with the acquisition of Linetec in November 2018, the previous owner retained a 20% equity interest in Linetec, the reduction of which is subject to certain rights based on the passage of time or upon the occurrence of certain triggering events. Effective January 2022, the Company has the right, but not the obligation, to purchase at fair value (subject to a floor) a portion of the interest held by the noncontrolling party, and in incremental amounts each year thereafter. The shares subject to the election accumulate (if earlier elections are not made) such that 100% of the interest retained by the noncontrolling party is subject to the election beginning in 2024. If the Company does not exercise its rights at each or any of the specified intervals, the noncontrolling party has the ability, but not the obligation, to exit their investment retained by requiring Centuri to purchase a similar portion of their interest up to the maximum cumulative amounts specified at each interval discussed above. The Company has determined that this noncontrolling interest is a redeemable noncontrolling interest and, in accordance with SEC guidance, is classified as mezzanine equity (temporary equity) in the Company’s Condensed Consolidated Balance Sheets. Significant changes in the value of the redeemable noncontrolling interest, above a floor established at the acquisition date, are recognized as they occur, and the carrying value is adjusted as necessary at each reporting date. The fair value is estimated using a market approach that utilizes certain financial metrics from guideline public companies of similar industry and operating characteristics. However, the carrying value of the redeemable noncontrolling interest was greater than its fair value as of June 30, 2019 , and no previous upward redemption value adjustments were made following the acquisition date. SEC guidance indicates that a redemption value adjustment would not be made under these circumstances. The following depicts the change to the balance of the redeemable noncontrolling interest: (Thousands of dollars): Redeemable Noncontrolling Interest Balance, December 31, 2018 $ 81,831 Net income attributable to redeemable noncontrolling interest 1,351 Balance, June 30, 2019 $ 83,182

Business Acquisitions

Business Acquisitions	6 Months Ended
	Jun. 30, 2019
Business Combinations [Abstract]
Business Acquisitions	Note 12 – Business Acquisitions In November 2018, the Company, through its subsidiaries, led principally by Centuri, completed the acquisition of an 80% interest in a privately held infrastructure services business, Linetec, with the remaining 20% retained by the seller. See the Company’s 2018 Form 10-K for additional information about this acquisition. Assets acquired and liabilities assumed in the transaction were recorded, generally, at their acquisition date fair values. Transaction costs associated with the acquisition were expensed as incurred. The Company’s allocation of the purchase price was based on an evaluation of the appropriate fair values and represented management’s best estimate based on available data (including market data, data regarding customers of the acquired business, terms of acquisition-related agreements, analysis of historical and projected results, and other types of data). The analysis included consideration of types of intangibles that were acquired, including customer relationships, trade names, and customer contracts. Certain payments were estimated as of the acquisition date and will be adjusted when paid or as estimates change based on available data; the final purchase accounting has not yet been completed. Further refinement is expected to occur, including potential changes to income taxes and intangibles, as well as additional consideration payments held back. Subsequent to the acquisition date and through June 30, 2019 , Centuri recorded a net reduction to the overall purchase price of $20.1 million related to the combined effects of a mutual tax election under Internal Revenue Code Section 338(h)(10), working capital adjustments, and other refinements to the valuation, which impacted the remaining unremitted consideration. In addition, approximately $19.5 million of previously unremitted consideration was paid during the three months ending June 30, 2019 . As of that date, remaining unpaid consideration was $31.3 million . The preliminary estimated fair values of assets acquired and liabilities assumed as of November 30, 2018, are as follows (millions of dollars): Acquisition Date Measurement Period Adjustments Revised Acquisition Date Cash and cash equivalents $ 3.9 $ — $ 3.9 Accounts receivable 32.8 (0.5 ) 32.3 Revenue earned on contracts in progress in excess of billings 21.6 (0.2 ) 21.4 Prepaid expenses and other current assets 1.1 0.2 1.3 Property and equipment 89.4 (0.5 ) 88.9 Intangible assets 89.3 — 89.3 Goodwill 188.5 (17.5 ) 171.0 Total assets acquired 426.6 (18.5 ) 408.1 Accounts payable 8.0 — 8.0 Accrued liabilities 6.9 1.6 8.5 Deferred compensation and related accrued taxes 3.4 — 3.4 Redeemable noncontrolling interest 81.7 — 81.7 Total liabilities assumed and noncontrolling interest 100.0 1.6 101.6 Net assets acquired $ 326.6 $ (20.1 ) $ 306.5 The Company incurred and expensed acquisition costs of $ 6.9 million for the twelve months ended June 30, 2019 which are included in Utility infrastructure services expenses on the Company’s Condensed Consolidated Statement of Income. No acquisition-related costs were incurred during the three and six months ended June 30, 2019 , and no significant impacts to earnings resulted from the measurement-period adjustments reflected above. The preliminary allocation of the purchase price of Linetec was accounted for in accordance with applicable accounting guidance. Goodwill consists of the value associated with the assembled workforce, consolidation of operations, and the estimated economic value attributable to future opportunities related to the transaction. As the business of Linetec was deemed an asset purchase for tax purposes, the tax-basis goodwill is expected to be deductible for tax purposes. During 2019, values at the acquisition date were adjusted, as reflected in the table above, on the Company’s Condensed Consolidated Balance Sheets.

Background, Organization, and_2

Background, Organization, and Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
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). At the annual meeting of shareholders of Southwest Gas Holdings, Inc., held on May 2, 2019, shareholders voted to approve changing the state of incorporation for Southwest Gas Holdings, Inc. from California to Delaware. The reincorporation remains subject to certain regulatory approvals, which are currently pending. 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 from installation, replacement, repair, and maintenance of energy distribution systems, and developing industrial construction solutions. 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. In November 2017, Centuri acquired Neuco, thereby expanding its core services in the northeast region of the U.S. Additionally, in November 2018, Centuri expanded its operations in the southeast region of the U.S. through the acquisition of an 80% interest in Linetec. See Note 12 – Business Acquisitions for more information.
Basis of Presentation	Basis of Presentation. The condensed consolidated financial statements for 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”). The year-end condensed balance sheet data was derived from audited financial statements. 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, or in the primary businesses comprising those segments as a result of the foregoing acquisitions of Neuco and Linetec. 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. It is suggested that these condensed consolidated financial statements be read in conjunction with the consolidated financial statements and the notes thereto included in the 2018 Annual Report to Shareholders, which is incorporated by reference into the 2018 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. The Company primarily used quoted market prices and other observable market pricing information in valuing cash and cash equivalents, derivatives, long-term debt outstanding, and assets of the qualified pension plan and postretirement benefit plans required to be recorded and/or disclosed at fair value.
Cash and Cash Equivalents	Cash and Cash Equivalents. For purposes of reporting consolidated cash flows, cash and cash equivalents include cash on hand and financial instruments with original maturities of three months or less. Such investments are carried at cost, which approximates market value. Cash and cash equivalents for Southwest and the Company also include money market fund investments totaling approximately $19.6 million and $23.8 million , respectively, at June 30, 2019 , and $18 million and $59.9 million , respectively, at December 31, 2018 , which fall within Level 2 of the fair value hierarchy, due to the asset valuation methods used by money market funds. Typical non-cash investing activities for Southwest include customer advances applied as contributions toward utility construction activity and capital expenditures that were not paid as of quarter end that are included in accounts payable. Amounts related to such activities were immaterial for the periods presented herein. Non-cash investing activities for the twelve months ended June 30, 2019 included $31.3 million of purchase consideration related to the Linetec acquisition by Centuri, in the form of liabilities incurred that remained unpaid as of June 30, 2019
Income Taxes	Income Taxes. In 2017, the Tax Cuts and Jobs Act (the “TCJA”) was enacted. The TCJA had significant impacts on the taxation of business entities, including specific provisions related to regulated public utilities. The more significant changes that impacted the Company and Southwest include the reduction in the corporate federal income tax rate from 35% to 21%, and limiting the utilization of net operating losses (“NOLs”) to 80% of taxable income, with the ability to indefinitely carryforward unutilized NOLs to reduce future taxable income.
Goodwill	Goodwill. Goodwill is assessed as of October 1 st
Recent Accounting Standards Updates	Recent Accounting Standards Updates. Accounting pronouncements adopted in 2019: In February 2016, the Financial Accounting Standards Board (“FASB”) issued the update “Leases (Topic 842).” Under the update, lessees were required to recognize a lease liability for the obligation to make lease payments, measured on a discounted basis; and a right-of-use asset for the right to use, or control the use of, a specified asset for the lease term. The Company and Southwest adopted Topic 842 in the first quarter of 2019 through an optional transition method, which was elected, permitting the application of the provisions of the standard at the adoption date, rather than to earlier comparative periods. As a result, the Company and Southwest have not recast prior periods to reflect the adoption of this standard. See Note 4 – Leases . Accounting pronouncements that will be effective after 2019: In June 2016, the FASB issued Accounting Standards Update (“ASU”) 2016-13 update “Financial Instruments — Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” The update requires the measurement of all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. The inputs currently used to estimate credit losses will still be used; however, they may be adapted to reflect the full amount of expected losses. The update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. All entities may adopt the amendments in this update earlier as of fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Management is evaluating what impact, if any, this update might have on the Company’s and Southwest’s consolidated financial statements and disclosures. In January 2017, the FASB issued ASU 2017-04 “Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment.” Currently, unless meeting the criteria for qualitative assessment only, an entity is required to perform a two-step test to determine the amount, if any, of goodwill impairment. In Step 1, an entity compares the fair value of a reporting unit with its carrying amount, including goodwill. If the carrying amount of the reporting unit exceeds its fair value, the entity performs Step 2 and compares the implied fair value of goodwill with the carrying amount of that goodwill for that reporting unit, requiring a hypothetical purchase price allocation to measure the amount of a goodwill impairment. An impairment charge equal to the amount by which the carrying amount of goodwill for the reporting unit exceeds the implied fair value of that goodwill is recorded, limited to the amount of goodwill allocated to that reporting unit. Under the update, an entity will apply a one-step quantitative test and record the amount of goodwill impairment as the excess of a reporting unit's carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit. The new guidance does not amend the optional qualitative assessment. The amount of any goodwill impairment calculated under the update could vary from the calculation under existing guidance, largely due to the consideration to be given to unrecognized differences between the fair value and carrying values of the other assets and liabilities in the reporting unit under the new guidance. The amendments should be applied on a prospective basis. The update is effective for fiscal and interim periods beginning after December 15, 2019. Early adoption has been permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. Management is evaluating the impacts this update might have on the Company’s and Southwest’s consolidated financial statements and disclosures. In August 2018, the FASB issued ASU 2018-15 “Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract.” The update generally aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement (that is a service contract) with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. Once capitalized, the update requires the entity to expense the amount capitalized over the term of the hosting arrangement, including reasonably certain renewal periods. The update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption of the amendments in this update is permitted for interim and related annual fiscal periods after December 15, 2018. Management is evaluating the impacts this update might have on the Company’s and Southwest’s consolidated financial statements and disclosures. In August 2018, the FASB issued ASU 2018-14 “Compensation—Retirement Benefits—Defined Benefit Plans—General (Subtopic 715-20): Disclosure Framework—Changes to the Disclosure Requirements for Defined Benefit Plans.” This update removes disclosures that are no longer considered cost-beneficial, clarifies the specific requirements of disclosures, and adds disclosure requirements identified as relevant. The update applies to all employers that sponsor defined benefit pension or other postretirement plans. The update is effective for fiscal years ending after December 15, 2020. Upon adoption, the Company and Southwest will modify their disclosures to conform to the requirements of the update. In August 2018, the FASB issued ASU 2018-13 “Fair Value Measurement: Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement.” The update is intended to improve the effectiveness of fair value measurement disclosures and removes the following disclosure requirements: the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy; the policy for timing of transfers between levels; and the valuation processes for Level 3 fair value measurements. The update also modifies or clarifies for investments in certain entities that calculate net asset value, a requirement to disclose the timing of liquidation of an investee’s assets and the date when restrictions from redemption might lapse (in cases when the timing has been communicated or announced publicly). It also clarifies communication requirements about measurement uncertainty as of the reporting date. For certain unobservable inputs, an entity may disclose other quantitative information in lieu of the weighted average if it would be a more reasonable and rational method to reflect the distribution of inputs to the measurements. The update is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Management is evaluating the impacts this update might have on its disclosures.

Background, Organization, and_3

Background, Organization, and Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2019
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): June 30, 2019 December 31, 2018 Southwest Gas Corporation: Net cash surrender value of COLI policies $ 125,491 $ 114,405 Other property 1,720 1,741 Total Southwest Gas Corporation 127,211 116,146 Centuri property, equipment, and intangibles 952,120 792,191 Centuri accumulated depreciation/amortization (332,683 ) (298,939 ) Other property 15,004 14,153 Total Southwest Gas Holdings, Inc. $ 761,652 $ 623,551
Accounts Receivable for Services	Centuri’s accounts receivable for these services are presented in the table below (thousands of dollars): June 30, 2019 December 31, 2018 Centuri accounts receivable for services provided to Southwest $ 19,260 $ 18,830
Schedule of Goodwill	(Thousands of dollars) Natural Gas Operations Utility Infrastructure Services Total Company December 31, 2018 $ 10,095 $ 348,950 $ 359,045 Measurement-period adjustments - Linetec acquisition (a) — (17,511 ) (17,511 ) Foreign currency translation adjustment — 4,338 4,338 June 30, 2019 $ 10,095 $ 335,777 $ 345,872 (a) See Note 12 – Business Acquisitions for details regarding Linetec measurement-period adjustments.
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 (thousands of dollars): Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2019 2018 2019 2018 2019 2018 Southwest Gas Corporation - natural gas operations segment: Change in COLI policies $ 3,400 $ 2,000 $ 11,000 $ 1,300 $ 6,500 $ 6,900 Interest income 1,822 1,377 3,419 2,795 6,644 4,401 Equity AFUDC 1,007 357 1,967 586 5,008 1,773 Other components of net periodic benefit cost (3,765 ) (5,264 ) (7,530 ) (10,529 ) (18,060 ) (20,241 ) Miscellaneous income and (expense) (872 ) (564 ) (1,318 ) (849 ) (3,097 ) (1,869 ) Southwest Gas Corporation - total other income (deductions) 1,592 (2,094 ) 7,538 (6,697 ) (3,005 ) (9,036 ) Utility infrastructure services segment: Interest income — 1 — 2 86 4 Foreign transaction gain (loss) 21 202 552 349 (19 ) (207 ) Miscellaneous income and (expense) (498 ) (835 ) (154 ) (720 ) 462 (69 ) Centuri - total other income (deductions) (477 ) (632 ) 398 (369 ) 529 (272 ) Corporate and administrative 31 20 49 26 75 38 Consolidated Southwest Gas Holdings, Inc. - total other income (deductions) $ 1,146 $ (2,706 ) $ 7,985 $ (7,040 ) $ (2,401 ) $ (9,270 )

Components of Net Periodic Be_2

Components of Net Periodic Benefit Cost (Tables)	6 Months Ended
	Jun. 30, 2019
Retirement Benefits [Abstract]
Schedule of Net Periodic Benefit Costs	The service cost component of net periodic benefit costs included in the table below are components of an overhead loading process associated with the cost of labor. The overhead process ultimately results in allocation of that portion of overall net periodic benefit costs to the same accounts to which productive labor is charged. As a result, service costs become components of various accounts, primarily operations and maintenance expense, net utility plant, and deferred charges and other assets for both the Company and Southwest. The 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 Period Ended June 30, Three Months Six Months Twelve Months 2019 2018 2019 2018 2019 2018 (Thousands of dollars) Service cost $ 6,466 $ 7,139 $ 12,932 $ 14,278 $ 27,209 $ 25,974 Interest cost 12,251 11,044 24,503 22,087 46,590 45,129 Expected return on plan assets (15,061 ) (14,688 ) (30,122 ) (29,377 ) (59,500 ) (56,975 ) Amortization of net actuarial loss 5,589 8,028 11,178 16,057 27,236 28,059 Net periodic benefit cost $ 9,245 $ 11,523 $ 18,491 $ 23,045 $ 41,535 $ 42,187 SERP Period Ended June 30, Three Months Six Months Twelve Months 2019 2018 2019 2018 2019 2018 (Thousands of dollars) Service cost $ 67 $ 61 $ 133 $ 122 $ 256 $ 276 Interest cost 440 414 880 829 1,709 1,770 Amortization of net actuarial loss 255 376 510 751 1,261 1,472 Net periodic benefit cost $ 762 $ 851 $ 1,523 $ 1,702 $ 3,226 $ 3,518 PBOP Period Ended June 30, Three Months Six Months Twelve Months 2019 2018 2019 2018 2019 2018 (Thousands of dollars) Service cost $ 319 $ 369 $ 638 $ 737 $ 1,374 $ 1,471 Interest cost 762 687 1,524 1,374 2,898 2,990 Expected return on plan assets (789 ) (930 ) (1,578 ) (1,860 ) (3,436 ) (3,539 ) Amortization of prior service costs 318 334 635 668 1,302 1,335 Net periodic benefit cost $ 610 $ 460 $ 1,219 $ 919 $ 2,138 $ 2,257

Revenue (Tables)

Revenue (Tables)	6 Months Ended
	Jun. 30, 2019
Revenue from Contract with Customer [Abstract]
Summary of Revenue Disaggregated by Service Type and Contract Type	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: (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2019 2018 2019 2018 2019 2018 Service Types: Gas infrastructure services $ 323,817 $ 300,346 $ 521,709 $ 493,894 $ 1,205,197 $ 1,072,082 Electric power infrastructure services 61,366 3,994 113,667 9,396 136,900 19,350 Other 69,117 90,864 131,786 151,931 292,129 317,831 Total Utility infrastructure services revenues $ 454,300 $ 395,204 $ 767,162 $ 655,221 $ 1,634,226 $ 1,409,263 (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2019 2018 2019 2018 2019 2018 Contract Types: Master services agreement $ 375,860 $ 290,075 $ 611,515 $ 484,539 $ 1,229,388 $ 1,014,517 Bid contract 78,440 105,129 155,647 170,682 404,838 394,746 Total Utility infrastructure services revenues $ 454,300 $ 395,204 $ 767,162 $ 655,221 $ 1,634,226 $ 1,409,263 Unit priced contracts $ 355,487 $ 329,236 $ 591,172 $ 526,557 $ 1,323,034 $ 1,102,335 Fixed priced contracts 4,426 27,025 42,965 52,566 107,697 138,658 Time and materials contracts 94,387 38,943 133,025 76,098 203,495 168,270 Total Utility infrastructure services revenues $ 454,300 $ 395,204 $ 767,162 $ 655,221 $ 1,634,226 $ 1,409,263 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 Six Months Ended Twelve Months Ended June 30, June 30, June 30, (Thousands of dollars) 2019 2018 2019 2018 2019 2018 Residential $ 164,873 $ 166,702 $ 582,101 $ 511,313 $ 958,008 $ 867,338 Small commercial 50,184 55,653 139,794 143,596 251,281 254,966 Large commercial 11,123 13,134 25,085 28,574 49,703 54,577 Industrial/other 5,413 5,491 11,891 12,001 23,379 23,533 Transportation 21,815 20,719 46,717 44,773 88,934 88,842 Revenue from contracts with customers 253,408 261,699 805,588 740,257 1,371,305 1,289,256 Alternative revenue program revenues (deferrals) 3,392 10,393 (31,153 ) 37,602 (22,776 ) 67,046 Other revenues (a) 1,911 3,587 4,953 (7,867 ) 18,595 (6,766 ) Total Gas operating revenues $ 258,711 $ 275,679 $ 779,388 $ 769,992 $ 1,367,124 $ 1,349,536 (a) Comprised of various other revenue impacts, including $(1.1) million , $(2.9) million , and $(3.9) million for the three, six, and twelve months ending June 30, 2019 , respectively; and, $1.6 million during the three months, and $(12.5) million in both the six and twelve months ending June 30, 2018 related to tax reform savings reserves/adjustments.
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 June 30, 2019 and December 31, 2018 on the Company’s Condensed Consolidated Balance Sheets: (Thousands of dollars) June 30, 2019 December 31, 2018 Contracts receivable, net $ 205,571 $ 186,249 Revenue earned on contracts in progress in excess of billings 118,529 87,520 Amounts billed in excess of revenue earned on contracts 6,786 4,211
Schedule of Construction Services Contracts Receivable	Utility infrastructure services contracts receivable consists of the following: (Thousands of dollars) June 30, 2019 December 31, 2018 Billed on completed contracts and contracts in progress $ 207,057 $ 184,100 Other receivables 1,863 2,588 Contracts receivable, gross 208,920 186,688 Allowance for doubtful accounts (3,349 ) (439 ) Contracts receivable, net $ 205,571 $ 186,249

Leases (Tables)

Leases (Tables)	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Components of Lease Expense, and Supplemental Cash Flow Information Related to Leases	The components of lease expense were as follows: (Thousands of dollars) Three Months Ended Six Months Ended June 30, 2019 June 30, 2019 Southwest: Operating lease cost $ 409 $ 815 Centuri: Operating lease cost $ 3,238 $ 6,043 Finance lease cost: Amortization of ROU assets $ 34 $ 69 Interest on lease liabilities 7 14 Total finance lease cost 41 83 Short-term lease cost 3,357 5,932 Total lease cost $ 7,045 $ 12,873 Supplemental cash flow information related to leases for the six months ended June 30, 2019 was as follows: (Thousands of dollars) Southwest Centuri Consolidated Total Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 671 $ 5,293 $ 5,964 Operating cash flows from finance leases — 14 14 Financing cash flows from finance leases — 107 107 ROU assets obtained in exchange for lease obligations: Operating leases $ 901 $ 8,968 $ 9,869 Finance leases — 339 339
Supplemental Information Related to Leases Included in Balance Sheet	Supplemental information related to leases, including location in the Condensed Consolidated Balance Sheets, is as follows: (Thousands of dollars) June 30, 2019 Southwest: Operating leases: Net Utility Plant $ 2,072 Other current liabilities $ 897 Other deferred credits and other long-term liabilities 1,200 Total operating lease liabilities $ 2,097 Weighted average remaining lease term (in years) 3.81 Weighted average discount rate 3.36 % Centuri: Operating leases: Other property and investments $ 60,301 Other current liabilities 7,965 Other deferred credits and other long-term liabilities 55,185 Total operating lease liabilities $ 63,150 Finance leases: Other property and investments $ 854 Other current liabilities 314 Other deferred credits and other long-term liabilities 394 Total finance lease liabilities $ 708 Weighted average remaining lease term (in years) Operating leases 9.07 Finance leases 1.84 Weighted average discount rate Operating leases 4.08 % Finance leases 5.90 %
Schedule of Maturities of Operating Lease Liabilities	The following are schedules of maturities of lease liabilities as of June 30, 2019 : (Thousands of dollars) Operating leases Southwest: 2020 $ 1,018 2021 534 2022 213 2023 102 2024 78 Thereafter 287 Total lease payments 2,232 Less imputed interest 135 Total $ 2,097 (Thousands of dollars) Operating leases Finance leases Centuri: 2020 $ 10,714 $ 349 2021 9,551 157 2022 8,736 168 2023 7,548 72 2024 5,992 35 Thereafter 34,239 — Total lease payments 76,780 781 Less imputed interest 13,630 73 Total $ 63,150 $ 708
Schedule of Maturities of Finance Lease Liabilities	The following are schedules of maturities of lease liabilities as of June 30, 2019 : (Thousands of dollars) Operating leases Southwest: 2020 $ 1,018 2021 534 2022 213 2023 102 2024 78 Thereafter 287 Total lease payments 2,232 Less imputed interest 135 Total $ 2,097 (Thousands of dollars) Operating leases Finance leases Centuri: 2020 $ 10,714 $ 349 2021 9,551 157 2022 8,736 168 2023 7,548 72 2024 5,992 35 Thereafter 34,239 — Total lease payments 76,780 781 Less imputed interest 13,630 73 Total $ 63,150 $ 708
Schedule of Rental and Lease Payments	As the Company and Southwest adopted Topic 842 using the optional transition method referred to in Note 1 – Background, Organization, and Summary of Significant Accounting Policies , the recent annual disclosure of rental and lease payments as of December 31, 2018 in accordance with Topic 840 is presented in the table below (thousands of dollars): 2018 2017 Southwest Gas Corporation $ 4,556 $ 4,926 Centuri 59,491 62,310 Consolidated rental payments/lease expense $ 64,047 $ 67,236
Schedule of Future Minimum Rental Payments for Operating Leases	The following is a schedule of future minimum lease payments for operating leases (with initial or remaining terms in excess of one year) as of December 31, 2018 (thousands of dollars): Southwest Centuri Consolidated Total 2019 $ 898 $ 10,053 $ 10,951 2020 363 7,656 8,019 2021 299 5,760 6,059 2022 163 5,163 5,326 2023 79 3,681 3,760 Thereafter 177 10,511 10,688 Total minimum lease payments $ 1,979 $ 42,824 $ 44,803

Derivatives (Tables)

Derivatives (Tables)	6 Months Ended
	Jun. 30, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Notional Amounts under Swaps Contracts	The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms): June 30, 2019 December 31, 2018 Contract notional amounts 17,309 13,387
Paid to and Received from Counterparties for Settlements of Matured Swaps	The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps: Three Months Ended Six Months Ended Twelve Months Ended (Thousands of dollars) June 30, 2019 June 30, 2019 June 30, 2019 Paid to counterparties $ 4,265 $ 6,147 $ 8,997 Received from counterparties $ — $ 1,047 $ 1,647
Fair Values of Swaps in Consolidated Balance Sheets	Fair values of derivatives not designated as hedging instruments: June 30, 2019 Swap Position Instrument Balance Sheet Location Asset Derivatives Liability Derivatives Net Total Offsetting Balance Sheet Location (Regulatory Asset/(Liability)) Swaps Deferred charges and other assets $ 37 $ (6 ) $ 31 Other deferred credits Swaps Other current liabilities 669 (6,309 ) (5,640 ) Prepaid and other current assets Swaps Other deferred credits — (1,314 ) (1,314 ) Deferred charges and other assets Total $ 706 $ (7,629 ) $ (6,923 ) December 31, 2018 Swap Position Instrument Balance Sheet Location Asset Derivatives Liability Derivatives Net Total Offsetting Balance Sheet Location (Regulatory Asset/(Liability)) Swaps Prepaid and other current assets $ 243 $ (99 ) $ 144 Other current liabilities Swaps Other current liabilities 1,595 (3,347 ) (1,752 ) Prepaid and other current assets Swaps Other deferred credits 141 (251 ) (110 ) Deferred charges and other assets Total $ 1,979 $ (3,697 ) $ (1,718 )

Common Stock (Tables)

Common Stock (Tables)	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Schedule of Common Stock Activity	The following table provides the activity under the Equity Shelf Program for the quarter and life-to-date ended June 30, 2019 : Gross proceeds $ 74,999,771 Less: agent commissions (749,998 ) Net proceeds $ 74,249,773 Number of shares sold 873,361 Weighted average price per share $ 85.87

Long-Term Debt (Tables)

Long-Term Debt (Tables)	6 Months Ended
	Jun. 30, 2019
Debt Disclosure [Abstract]
Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt	The Centuri secured revolving credit and term loan facility and Centuri’s other debt obligations (not actively traded) are categorized as Level 3, based on significant unobservable inputs to their fair values. Because Centuri’s debt is not publicly traded, fair values for the secured revolving credit and term loan facility and other debt obligations were based on a conventional discounted cash flow methodology and utilized current market pricing yield curves, across Centuri’s debt maturity spectrum, of other industrial bonds with an assumed credit rating comparable to the Company’s. June 30, 2019 December 31, 2018 Carrying Amount Market Value Carrying Amount Market Value (Thousands of dollars) Southwest Gas Corporation: Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 127,030 $ 125,000 $ 126,213 Notes, 6.1%, due 2041 125,000 158,916 125,000 150,728 Notes, 3.875%, due 2022 250,000 257,128 250,000 254,195 Notes, 4.875%, due 2043 250,000 287,903 250,000 268,985 Notes, 3.8%, due 2046 300,000 296,868 300,000 267,030 Notes, 3.7%, due 2028 300,000 316,170 300,000 298,926 Notes, 4.15%, due 2049 300,000 315,426 — — 8% Series, due 2026 75,000 96,803 75,000 93,827 Medium-term notes, 7.78% series, due 2022 25,000 27,774 25,000 27,497 Medium-term notes, 7.92% series, due 2027 25,000 31,660 25,000 30,016 Medium-term notes, 6.76% series, due 2027 7,500 9,064 7,500 8,651 Unamortized discount and debt issuance costs (15,102 ) (11,807 ) 1,767,398 1,470,693 Revolving credit facility and commercial paper 46,000 46,000 150,000 150,000 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 (1,839 ) (2,024 ) 198,161 197,976 Less: current maturities — — Long-term debt, less current maturities - Southwest Gas Corporation $ 2,011,559 $ 1,818,669 Centuri: Centuri term loan facility $ 253,120 $ 259,002 $ 255,959 $ 260,135 Unamortized debt issuance costs (1,270 ) (1,414 ) 251,850 254,545 Centuri secured revolving credit facility 88,174 88,213 — — Centuri other debt obligations 58,227 59,440 67,104 67,053 Less: current maturities (36,807 ) (33,060 ) Long-term debt, less current maturities - Centuri $ 361,444 $ 288,589 Consolidated Southwest Gas Holdings, Inc.: Southwest Gas Corporation long-term debt $ 2,011,559 $ 1,818,669 Centuri long-term debt 398,251 321,649 Less: current maturities (36,807 ) (33,060 ) Long-term debt, less current maturities - Southwest Gas Holdings, Inc. $ 2,373,003 $ 2,107,258

Other Comprehensive Income an_2

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	6 Months Ended
	Jun. 30, 2019
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) (Thousands of dollars) Three Months Ended June 30, 2019 Three Months Ended June 30, 2018 Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Defined benefit pension plans: Amortization of prior service cost $ 318 $ (76 ) $ 242 $ 334 $ (80 ) $ 254 Amortization of net actuarial (gain)/loss 5,844 (1,403 ) 4,441 8,404 (2,017 ) 6,387 Regulatory adjustment (5,348 ) 1,284 (4,064 ) (7,559 ) 1,815 (5,744 ) Pension plans other comprehensive income 814 (195 ) 619 1,179 (282 ) 897 FSIRS (designated hedging activities): Amounts reclassified into net income 836 (200 ) 636 836 (200 ) 636 FSIRS other comprehensive income 836 (200 ) 636 836 (200 ) 636 Total other comprehensive income - Southwest Gas Corporation 1,650 (395 ) 1,255 2,015 (482 ) 1,533 Foreign currency translation adjustments: Translation adjustments 787 — 787 (690 ) — (690 ) Foreign currency other comprehensive income (loss) 787 — 787 (690 ) — (690 ) Total other comprehensive income - Southwest Gas Holdings, Inc. $ 2,437 $ (395 ) $ 2,042 $ 1,325 $ (482 ) $ 843 Six Months Ended June 30, 2019 Six Months Ended June 30, 2018 Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Defined benefit pension plans: Amortization of prior service cost $ 635 $ (152 ) $ 483 $ 668 $ (160 ) $ 508 Amortization of net actuarial (gain)/loss 11,688 (2,805 ) 8,883 16,808 (4,034 ) 12,774 Regulatory adjustment (10,695 ) 2,567 (8,128 ) (15,119 ) 3,629 (11,490 ) Pension plans other comprehensive income 1,628 (390 ) 1,238 2,357 (565 ) 1,792 FSIRS (designated hedging activities): Amounts reclassified into net income 1,672 (401 ) 1,271 1,673 (402 ) 1,271 FSIRS other comprehensive income 1,672 (401 ) 1,271 1,673 (402 ) 1,271 Total other comprehensive income - Southwest Gas Corporation 3,300 (791 ) 2,509 4,030 (967 ) 3,063 Foreign currency translation adjustments: Translation adjustments 1,578 — 1,578 (1,601 ) — (1,601 ) Foreign currency other comprehensive income (loss) 1,578 — 1,578 (1,601 ) — (1,601 ) Total other comprehensive income - Southwest Gas Holdings, Inc. $ 4,878 $ (791 ) $ 4,087 $ 2,429 $ (967 ) $ 1,462 Twelve Months Ended June 30, 2019 Twelve Months Ended June 30, 2018 Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Defined benefit pension plans: Net actuarial gain/(loss) $ (20,426 ) $ 4,902 $ (15,524 ) $ (43,027 ) $ 10,326 $ (32,701 ) Amortization of prior service cost 1,302 (312 ) 990 1,335 (413 ) 922 Amortization of net actuarial (gain)/loss 28,497 (6,839 ) 21,658 29,531 (8,869 ) 20,662 Regulatory adjustment (3,809 ) 914 (2,895 ) 8,691 (479 ) 8,212 Pension plans other comprehensive income (loss) 5,564 (1,335 ) 4,229 (3,470 ) 565 (2,905 ) FSIRS (designated hedging activities): Amounts reclassified into net income 3,344 (803 ) 2,541 3,345 (1,037 ) 2,308 FSIRS other comprehensive income 3,344 (803 ) 2,541 3,345 (1,037 ) 2,308 Total other comprehensive income (loss) - Southwest Gas Corporation 8,908 (2,138 ) 6,770 (125 ) (472 ) (597 ) Foreign currency translation adjustments: Translation adjustments 169 — 169 (679 ) — (679 ) Foreign currency other comprehensive income (loss) 169 — 169 (679 ) — (679 ) Total other comprehensive income (loss) - Southwest Gas Holdings, Inc. $ 9,077 $ (2,138 ) $ 6,939 $ (804 ) $ (472 ) $ (1,276 ) (1) Tax amounts are calculated using a 24% rate following the December 22, 2017 enactment date of U.S. tax reform. For periods prior to the enactment date (and included in specific line items of the tables for the twelve months ended June 30, 2018 ), tax amounts were calculated using a 38% rate. The tax effect of before-tax amounts remaining in the balance of Accumulated other comprehensive income (loss) as of June 30, 2019 is effectively computed using a 24% tax rate overall. With regard to foreign currency translation adjustments, the Company has elected to indefinitely reinvest the earnings of Centuri’s Canadian subsidiaries in Canada, thus preventing deferred taxes on such earnings. As a result of this assertion, and no repatriation of earnings anticipated, the Company is not recognizing a tax effect or presenting a tax expense or benefit for currency translation adjustments 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 (thousands of dollars): Defined Benefit Plans FSIRS Foreign Currency Items Before-Tax Tax (Expense) Benefit (4,5) After-Tax (5) Before-Tax Tax (Expense) Benefit (4,5) After-Tax (5) Before-Tax Tax (Expense) Benefit After-Tax AOCI Beginning Balance AOCI December 31, 2018 $ (55,227 ) $ 13,254 $ (41,973 ) $ (9,310 ) $ 2,234 $ (7,076 ) $ (3,619 ) $ — $ (3,619 ) $ (52,668 ) Translation adjustments — — — — — — 1,578 — 1,578 1,578 Other comprehensive income (loss) before reclassifications — — — — — — 1,578 — 1,578 1,578 FSIRS amounts reclassified from AOCI (1) — — — 1,672 (401 ) 1,271 — — — 1,271 Amortization of prior service cost (2) 635 (152 ) 483 — — — — — — 483 Amortization of net actuarial loss (2) 11,688 (2,805 ) 8,883 — — — — — — 8,883 Regulatory adjustment (3) (10,695 ) 2,567 (8,128 ) — — — — — — (8,128 ) Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. 1,628 (390 ) 1,238 1,672 (401 ) 1,271 1,578 — 1,578 4,087 Ending Balance AOCI June 30, 2019 $ (53,599 ) $ 12,864 $ (40,735 ) $ (7,638 ) $ 1,833 $ (5,805 ) $ (2,041 ) $ — $ (2,041 ) $ (48,581 ) (1) The FSIRS reclassification amounts are 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. (5) The beginning balances depict amounts attributable to the individual components of AOCI (Defined Benefit Plans and FSIRS) following the adoption of ASU No. 2018-02, with no impact to the total balance of AOCI resulting from the depiction. The following table represents a rollforward of AOCI, presented on Southwest’s Condensed Consolidated Balance Sheets (thousands of dollars): Defined Benefit Plans FSIRS Before-Tax Tax (Expense) Benefit (9,10) After-Tax (10) Before-Tax Tax (Expense) Benefit (9,10) After-Tax (10) AOCI Beginning Balance AOCI December 31, 2018 $ (55,227 ) $ 13,254 $ (41,973 ) $ (9,310 ) $ 2,234 $ (7,076 ) $ (49,049 ) FSIRS amounts reclassified from AOCI (6) — — — 1,672 (401 ) 1,271 1,271 Amortization of prior service cost (7) 635 (152 ) 483 — — — 483 Amortization of net actuarial loss (7) 11,688 (2,805 ) 8,883 — — — 8,883 Regulatory adjustment (8) (10,695 ) 2,567 (8,128 ) — — — (8,128 ) Net current period other comprehensive income attributable to Southwest Gas Corporation 1,628 (390 ) 1,238 1,672 (401 ) 1,271 2,509 Ending Balance AOCI June 30, 2019 $ (53,599 ) $ 12,864 $ (40,735 ) $ (7,638 ) $ 1,833 $ (5,805 ) $ (46,540 ) (6) The FSIRS reclassification amounts are included in Net interest deductions on Southwest’s Condensed Consolidated Statements of Income. (7) 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). (8) The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in Deferred charges and other assets on Southwest’s Condensed Consolidated Balance Sheets). (9) Tax amounts are calculated using a 24% rate. (10) The beginning balances depict amounts attributable to the individual components of AOCI (Defined Benefit Plans and FSIRS) following the adoption of ASU No. 2018-02, with no impact to the total balance of AOCI resulting from the depiction.
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): June 30, 2019 December 31, 2018 Net actuarial (loss) gain $ (423,676 ) $ (435,364 ) Prior service cost (2,398 ) (3,033 ) Less: amount recognized in regulatory assets 372,475 383,170 Recognized in AOCI $ (53,599 ) $ (55,227 )

Segment Information (Tables)

Segment Information (Tables)	6 Months Ended
	Jun. 30, 2019
Segment Reporting [Abstract]
Schedule of Segment Reporting Information	The following tables present revenues from external customers, intersegment revenues, and segment net income for the two reportable segments (thousands of dollars): Natural Gas Operations Utility Infrastructure Services Other Total Three Months Ended June 30, 2019 Revenues from external customers $ 258,711 $ 405,218 $ — $ 663,929 Intersegment revenues — 49,082 — 49,082 Total $ 258,711 $ 454,300 $ — $ 713,011 Segment net income (loss) $ 3,369 $ 18,918 $ (231 ) $ 22,056 Three Months Ended June 30, 2018 Revenues from external customers $ 275,679 $ 362,132 $ — $ 637,811 Intersegment revenues — 33,072 — 33,072 Total $ 275,679 $ 395,204 $ — $ 670,883 Segment net income (loss) $ 2,622 $ 19,236 $ (307 ) $ 21,551 Natural Gas Operations Utility Infrastructure Services Other Total Six Months Ended June 30, 2019 Revenues from external customers $ 779,388 $ 679,407 $ — $ 1,458,795 Intersegment revenues — 87,755 — 87,755 Total $ 779,388 $ 767,162 $ — $ 1,546,550 Segment net income (loss) $ 106,758 $ 10,887 $ (780 ) $ 116,865 Six Months Ended June 30, 2018 Revenues from external customers $ 769,992 $ 594,991 $ — $ 1,364,983 Intersegment revenues — 60,230 — 60,230 Total $ 769,992 $ 655,221 $ — $ 1,425,213 Segment net income (loss) $ 92,971 $ 8,235 $ (564 ) $ 100,642 Natural Gas Operations Utility Infrastructure Services Other Total Twelve Months Ended June 30, 2019 Revenues from external customers $ 1,367,124 $ 1,470,787 $ — $ 2,837,911 Intersegment revenues — 163,439 — 163,439 Total $ 1,367,124 $ 1,634,226 $ — $ 3,001,350 Segment net income (loss) $ 152,629 $ 47,629 $ (1,758 ) $ 198,500 Twelve Months Ended June 30, 2018 Revenues from external customers $ 1,349,536 $ 1,296,093 $ — $ 2,645,629 Intersegment revenues — 113,170 — 113,170 Total $ 1,349,536 $ 1,409,263 $ — $ 2,758,799 Segment net income (loss) $ 163,329 $ 45,213 $ (1,231 ) $ 207,311

Redeemable Noncontrolling Int_2

Redeemable Noncontrolling Interests (Tables)	6 Months Ended
	Jun. 30, 2019
Noncontrolling Interest [Abstract]
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, 2018 $ 81,831 Net income attributable to redeemable noncontrolling interest 1,351 Balance, June 30, 2019 $ 83,182

Business Acquisitions (Tables)

Business Acquisitions (Tables)	6 Months Ended
	Jun. 30, 2019
Business Combinations [Abstract]
Summary of Fair Values of Assets Acquired and Liabilities Assumed	The preliminary estimated fair values of assets acquired and liabilities assumed as of November 30, 2018, are as follows (millions of dollars): Acquisition Date Measurement Period Adjustments Revised Acquisition Date Cash and cash equivalents $ 3.9 $ — $ 3.9 Accounts receivable 32.8 (0.5 ) 32.3 Revenue earned on contracts in progress in excess of billings 21.6 (0.2 ) 21.4 Prepaid expenses and other current assets 1.1 0.2 1.3 Property and equipment 89.4 (0.5 ) 88.9 Intangible assets 89.3 — 89.3 Goodwill 188.5 (17.5 ) 171.0 Total assets acquired 426.6 (18.5 ) 408.1 Accounts payable 8.0 — 8.0 Accrued liabilities 6.9 1.6 8.5 Deferred compensation and related accrued taxes 3.4 — 3.4 Redeemable noncontrolling interest 81.7 — 81.7 Total liabilities assumed and noncontrolling interest 100.0 1.6 101.6 Net assets acquired $ 326.6 $ (20.1 ) $ 306.5

Background, Organization, and_4

Background, Organization, and Summary of Significant Accounting Policies - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Other property	$ 15,004	$ 14,153
Total	761,652	623,551
Southwest Gas Corporation
Investment [Line Items]
Net cash surrender value of COLI policies	125,491	114,405
Other property	1,720	1,741
Total	127,211	116,146
Centuri
Investment [Line Items]
Centuri property, equipment, and intangibles	952,120	792,191
Centuri accumulated depreciation/amortization	$ (332,683)	$ (298,939)

Background, Organization, and_5

Background, Organization, and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Nov. 30, 2018	Mar. 31, 2018
Significant Accounting Policies [Line Items]
Goodwill impairment		$ 0
Gas Pipe Materials and Supplies | Prepaid and other current assets
Significant Accounting Policies [Line Items]
Gas pipe materials and operating supplies		62,000,000	$ 62,000,000	$ 56,000,000
Prepaid and other current assets		52,000,000	52,000,000	74,000,000
Pooled Funds and Mutual Funds | Level 2 - Significant Other Observable Inputs
Significant Accounting Policies [Line Items]
Money market fund investments		23,800,000	23,800,000	59,900,000
Centuri
Significant Accounting Policies [Line Items]
Additional capital contributed	$ 1,000,000
Centuri | Linetec
Significant Accounting Policies [Line Items]
Percentage of voting interests acquired					80.00%
Purchase consideration			31,300,000
Southwest Gas Corporation
Significant Accounting Policies [Line Items]
Dividends declared but not yet paid		24,700,000	24,700,000	23,500,000
Southwest Gas Corporation | Pooled Funds and Mutual Funds | Level 2 - Significant Other Observable Inputs
Significant Accounting Policies [Line Items]
Money market fund investments		$ 19,600,000	$ 19,600,000	$ 18,000,000
Intellichoice Energy LLC | Centuri
Significant Accounting Policies [Line Items]
Ownership percentage by parent	95.00%					65.00%
Intellichoice Energy LLC | NPL
Significant Accounting Policies [Line Items]
Ownership percentage by noncontrolling owners		5.00%	5.00%

Background, Organization, and_6

Background, Organization, and Summary of Significant Accounting Policies - Accounts Receivable for Services (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Centuri
Segment Reporting Information [Line Items]
Centuri accounts receivable for services provided to Southwest	$ 19,260	$ 18,830

Background, Organization, and_7

Background, Organization, and Summary of Significant Accounting Policies - Schedule of Goodwill (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 359,045
Foreign currency translation adjustment	4,338
Goodwill, ending balance	345,872
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	348,950
Foreign currency translation adjustment	4,338
Goodwill, ending balance	335,777
Linetec
Goodwill [Roll Forward]
Measurement-period adjustments - Linetec acquisition (a)	(17,511)
Goodwill, ending balance	171,000
Linetec | Natural Gas Operations
Goodwill [Roll Forward]
Measurement-period adjustments - Linetec acquisition (a)	0
Linetec | Utility Infrastructure Services
Goodwill [Roll Forward]
Measurement-period adjustments - Linetec acquisition (a)	$ (17,511)

Background, Organization, and_8

Background, Organization, and Summary of Significant Accounting Policies- Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Other income (deductions)
Total other income (deductions)	$ 1,146	$ (2,706)	$ 7,985	$ (7,040)	$ (2,401)	$ (9,270)
Southwest Gas Corporation
Other income (deductions)
Total other income (deductions)	1,592	(2,094)	7,538	(6,697)	(3,005)	(9,036)
Operating Segments | Natural Gas Operations | Southwest Gas Corporation
Other income (deductions)
Change in COLI policies	3,400	2,000	11,000	1,300	6,500	6,900
Interest income	1,822	1,377	3,419	2,795	6,644	4,401
Equity AFUDC	1,007	357	1,967	586	5,008	1,773
Other components of net periodic benefit cost	(3,765)	(5,264)	(7,530)	(10,529)	(18,060)	(20,241)
Miscellaneous income and (expense)	(872)	(564)	(1,318)	(849)	(3,097)	(1,869)
Total other income (deductions)	1,592	(2,094)	7,538	(6,697)	(3,005)	(9,036)
Operating Segments | Utility Infrastructure Services | Centuri
Other income (deductions)
Interest income	0	1	0	2	86	4
Foreign transaction gain (loss)	21	202	552	349	(19)	(207)
Miscellaneous income and (expense)	(498)	(835)	(154)	(720)	462	(69)
Total other income (deductions)	(477)	(632)	398	(369)	529	(272)
Corporate and administrative
Other income (deductions)
Total other income (deductions)	$ 31	$ 20	$ 49	$ 26	$ 75	$ 38

Components of Net Periodic Be_3

Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,466	$ 7,139	$ 12,932	$ 14,278	$ 27,209	$ 25,974
Interest cost	12,251	11,044	24,503	22,087	46,590	45,129
Expected return on plan assets	(15,061)	(14,688)	(30,122)	(29,377)	(59,500)	(56,975)
Amortization of net actuarial loss	5,589	8,028	11,178	16,057	27,236	28,059
Net periodic benefit cost	9,245	11,523	18,491	23,045	41,535	42,187
SERP
Defined Benefit Plan Disclosure [Line Items]
Service cost	67	61	133	122	256	276
Interest cost	440	414	880	829	1,709	1,770
Amortization of net actuarial loss	255	376	510	751	1,261	1,472
Net periodic benefit cost	762	851	1,523	1,702	3,226	3,518
PBOP
Defined Benefit Plan Disclosure [Line Items]
Service cost	319	369	638	737	1,374	1,471
Interest cost	762	687	1,524	1,374	2,898	2,990
Expected return on plan assets	(789)	(930)	(1,578)	(1,860)	(3,436)	(3,539)
Amortization of prior service costs	318	334	635	668	1,302	1,335
Net periodic benefit cost	$ 610	$ 460	$ 1,219	$ 919	$ 2,138	$ 2,257

Revenue - Additional Informatio

Revenue - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2019USD ($)SegmentContract
Southwest Gas Corporation
Segment Reporting Information [Line Items]
Number of operating segment | Segment	1
Centuri
Segment Reporting Information [Line Items]
Change in contract liability, revenue recognized | $	$ 4.2
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]: 2019-07-01 $ in Millions	Jun. 30, 2019USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to unsatisfied performance obligations of contracts	$ 64.8
Revenue, remaining performance obligation, expected timing of satisfaction, period	3 years

Revenue - Schedule of Gas Opera

Revenue - Schedule of Gas Operating Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 253,408	$ 261,699	$ 805,588	$ 740,257	$ 1,371,305	$ 1,289,256
Alternative revenue program revenues (deferrals)	3,392	10,393	(31,153)	37,602	(22,776)	67,046
Other revenues	1,911	3,587	4,953	(7,867)	18,595	(6,766)
Total Gas operating revenues	258,711	275,679	779,388	769,992	1,367,124	1,349,536
Tax Reform Savings Revenue Adjustments
Disaggregation of Revenue [Line Items]
Other revenues	(1,100)	1,600	(2,900)	(12,500)	(3,900)	(12,500)
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	164,873	166,702	582,101	511,313	958,008	867,338
Small commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	50,184	55,653	139,794	143,596	251,281	254,966
Large commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	11,123	13,134	25,085	28,574	49,703	54,577
Industrial/other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5,413	5,491	11,891	12,001	23,379	23,533
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 21,815	$ 20,719	$ 46,717	$ 44,773	$ 88,934	$ 88,842

Revenue - Summary of Revenue Di

Revenue - Summary of Revenue Disaggregated by Service Type, and Contract Type (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 253,408,000	$ 261,699,000	$ 805,588,000	$ 740,257,000	$ 1,371,305,000	$ 1,289,256,000
Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	454,300,000	395,204,000	767,162,000	655,221,000	1,634,226,000	1,409,263,000
Master services agreement | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	375,860,000	290,075,000	611,515,000	484,539,000	1,229,388,000	1,014,517,000
Bid contract | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	78,440,000	105,129,000	155,647,000	170,682,000	404,838,000	394,746,000
Unit priced contracts | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	355,487,000	329,236,000	591,172,000	526,557,000	1,323,034,000	1,102,335,000
Fixed priced contracts | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4,426,000	27,025,000	42,965,000	52,566,000	107,697,000	138,658,000
Time and materials contracts | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	94,387,000	38,943,000	133,025	76,098	203,495,000	168,270,000
Gas infrastructure services | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	323,817,000	300,346,000	521,709,000	493,894,000	1,205,197,000	1,072,082,000
Electric power infrastructure services | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	61,366,000	3,994,000	113,667,000	9,396,000	136,900,000	19,350,000
Other | Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 69,117,000	$ 90,864,000	$ 131,786,000	$ 151,931,000	$ 292,129,000	$ 317,831,000

Revenue - Summary of Informatio

Revenue - Summary of Information about Receivables (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Contracts receivable, net	$ 205,571	$ 186,249
Revenue earned on contracts in progress in excess of billings	118,529	87,520
Amounts billed in excess of revenue earned on contracts	$ 6,786	$ 4,211

Revenue - Schedule of Construct

Revenue - Schedule of Construction Services Contracts Receivable (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Billed on completed contracts and contracts in progress	$ 207,057	$ 184,100
Other receivables	1,863	2,588
Contracts receivable, gross	208,920	186,688
Allowance for doubtful accounts	(3,349)	(439)
Contracts receivable, net	$ 205,571	$ 186,249

Leases - Additional Information

Leases - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Jan. 01, 2019
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Operating lease ROU assets	$ 2,072	$ 2,072
Lease liabilities	$ 2,097	$ 2,097
Lease renewal term (up to)	3 years	3 years
Southwest Gas Corporation | Minimum
Lessee, Lease, Description [Line Items]
Lease remaining lease term		1 year
Southwest Gas Corporation | Maximum
Lessee, Lease, Description [Line Items]
Lease remaining lease term		10 years
Centuri
Lessee, Lease, Description [Line Items]
Operating lease ROU assets	$ 60,301	$ 60,301
Lease liabilities	$ 63,150	$ 63,150
Lease renewal term (up to)	5 years	5 years
Lease termination period		1 year
Short-term lease cost	$ 3,357	$ 5,932
Centuri | Minimum
Lessee, Lease, Description [Line Items]
Lease remaining lease term		1 year
Centuri | Maximum
Lessee, Lease, Description [Line Items]
Lease remaining lease term		14 years
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Operating lease ROU assets			$ 58,400
Lease liabilities			60,800
Accounting Standards Update 2016-02 | Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Operating lease ROU assets			1,900
Lease liabilities			$ 1,900

Leases - Components of Lease Ex

Leases - Components of Lease Expense (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Lessee, Lease, Description [Line Items]
Total lease cost	$ 7,045	$ 12,873
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Operating lease cost	409	815
Centuri
Lessee, Lease, Description [Line Items]
Operating lease cost	3,238	6,043
Amortization of ROU assets	34	69
Interest on lease liabilities	7	14
Total finance lease cost	41	83
Short-term lease cost	$ 3,357	$ 5,932

Leases - Supplemental Cash Flow

Leases - Supplemental Cash Flow Information Related to Leases (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Lessee, Lease, Description [Line Items]
Operating cash flows from operating leases	$ 5,964
Operating cash flows from finance leases	14
Financing cash flows from finance leases	107
ROU assets obtained in exchange for operating leases obligations	9,869
ROU assets obtained in exchange for finance leases obligations	339
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Operating cash flows from operating leases	671
Operating cash flows from finance leases	0
Financing cash flows from finance leases	0
ROU assets obtained in exchange for operating leases obligations	901
ROU assets obtained in exchange for finance leases obligations	0
Centuri
Lessee, Lease, Description [Line Items]
Operating cash flows from operating leases	5,293
Operating cash flows from finance leases	14
Financing cash flows from finance leases	107
ROU assets obtained in exchange for operating leases obligations	8,968
ROU assets obtained in exchange for finance leases obligations	$ 339

Leases - Supplemental Informati

Leases - Supplemental Information Related to Leases Included in Balance Sheet (Details) $ in Thousands	Jun. 30, 2019USD ($)
Lessee, Lease, Description [Line Items]
Operating lease, weighted average remaining lease term (in years)	9 years 25 days
Operating lease, weighted average discount rate	4.08%
Finance lease ROU assets	$ 854
Finance lease liabilities, current	314
Finance lease liabilities, noncurrent	394
Finance lease liabilities	$ 708
Finance lease, weighted average remaining lease term (in years)	1 year 10 months 2 days
Finance lease, weighted average discount rate	5.90%
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Operating lease ROU assets	$ 2,072
Operating lease liabilities, current	897
Operating lease liabilities, noncurrent	1,200
Operating lease liabilities	$ 2,097
Operating lease, weighted average remaining lease term (in years)	3 years 9 months 21 days
Operating lease, weighted average discount rate	3.36%
Centuri
Lessee, Lease, Description [Line Items]
Operating lease ROU assets	$ 60,301
Operating lease liabilities, current	7,965
Operating lease liabilities, noncurrent	55,185
Operating lease liabilities	63,150
Finance lease liabilities	$ 708

Leases - Schedule of Maturities

Leases - Schedule of Maturities of Lease Liabilities (Details) $ in Thousands	Jun. 30, 2019USD ($)
Finance leases
Total	$ 708
Centuri
Operating leases
2020	10,714
2021	9,551
2022	8,736
2023	7,548
2024	5,992
Thereafter	34,239
Total lease payments	76,780
Less imputed interest	13,630
Total	63,150
Finance leases
2020	349
2021	157
2022	168
2023	72
2024	35
Thereafter	0
Total lease payments	781
Less imputed interest	73
Total	708
Southwest Gas Corporation
Operating leases
2020	1,018
2021	534
2022	213
2023	102
2024	78
Thereafter	287
Total lease payments	2,232
Less imputed interest	135
Total	$ 2,097

Leases - Schedule of Rental and

Leases - Schedule of Rental and Lease Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Lessee, Lease, Description [Line Items]
Rental payments/lease expense	$ 64,047	$ 67,236
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
Rental payments/lease expense	4,556	4,926
Centuri
Lessee, Lease, Description [Line Items]
Rental payments/lease expense	$ 59,491	$ 62,310

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Rental Payments for Operating Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Lessee, Lease, Description [Line Items]
2019	$ 10,951
2020	8,019
2021	6,059
2022	5,326
2023	3,760
Thereafter	10,688
Total minimum lease payments	44,803
Southwest Gas Corporation
Lessee, Lease, Description [Line Items]
2019	898
2020	363
2021	299
2022	163
2023	79
Thereafter	177
Total minimum lease payments	1,979
Centuri
Lessee, Lease, Description [Line Items]
2019	10,053
2020	7,656
2021	5,760
2022	5,163
2023	3,681
Thereafter	10,511
Total minimum lease payments	$ 42,824

Derivatives - Additional Inform

Derivatives - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative collateral outstanding	$ 0	$ 0
Arizona
Derivative [Line Items]
Natural gas portfolios, maximum % rate	25.00%
California
Derivative [Line Items]
Natural gas portfolios, maximum % rate	25.00%

Derivatives - Notional Amounts

Derivatives - Notional Amounts under Swaps Contracts (Detail) - MMBTU MMBTU in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Contract notional amounts	17,309	13,387

Derivatives - Paid to and Recei

Derivatives - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - Southwest Gas Corporation - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2019
Derivative [Line Items]
Paid to counterparties	$ 4,265	$ 6,147	$ 8,997
Received from counterparties	$ 0	$ 1,047	$ 1,647

Derivatives - Swaps in Balance

Derivatives - Swaps in Balance Sheet (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	$ 706	$ 1,979
Liability derivatives not designated as hedging instruments	(7,629)	(3,697)
Net total not designated as hedging instruments	(6,923)	(1,718)
Swaps | Deferred charges and other assets
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	37
Liability derivatives not designated as hedging instruments	(6)
Net total not designated as hedging instruments	31
Swaps | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		243
Liability derivatives not designated as hedging instruments		(99)
Net total not designated as hedging instruments		144
Swaps | Other current liabilities
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	669	1,595
Liability derivatives not designated as hedging instruments	(6,309)	(3,347)
Net total not designated as hedging instruments	(5,640)	(1,752)
Swaps | Other deferred credits
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	0	141
Liability derivatives not designated as hedging instruments	(1,314)	(251)
Net total not designated as hedging instruments	$ (1,314)	$ (110)

Common Stock (Detail)

Common Stock (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	May 08, 2019	May 02, 2019
Class of Stock [Line Items]
Net proceeds to company			$ 102,743,000	$ 69,139,000	$ 388,006,000	$ 110,390,000
Common stock available for issuance (in shares)	60,000,000		60,000,000		60,000,000			120,000,000
Restricted Stock/Unit Plan, and Management Incentive Plan
Class of Stock [Line Items]
Common stock issued (in shares)			76,000
Dividend Reinvestment and Stock Purchase Plan
Class of Stock [Line Items]
Common stock issued (in shares)			67,000
Common stock issued, amount			$ 5,500,000
Equity Shelf Program
Class of Stock [Line Items]
Sale of stock, maximum amount of stock allowed to be offered							300,000,000
Sale of stock, common stock available for sale	$ 225,000,229		$ 225,000,229		$ 225,000,229
Sale of stock, number of shares sold (in shares)		278,000
Gross proceeds	74,999,771
Agent commissions	(749,998)
Net proceeds to company	$ 74,249,773
Number of shares sold (in shares)	873,361
Weighted average price per share	$ 85.87

Long-Term Debt - Schedule of Lo

Long-Term Debt - Schedule of Long-Term Debt (Detail) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Line of credit facility	$ 0
Less: current maturities	(36,807,000)	$ (33,060,000)
Long-term debt, less current maturities	2,373,003,000	2,107,258,000
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(36,807,000)	(33,060,000)
Long-term debt, less current maturities	2,373,003,000	2,107,258,000
Southwest Gas Corporation
Debt Instrument [Line Items]
Long-term debt, less current maturities	2,011,559,000	1,818,669,000
Long-term Debt, Current and Noncurrent Abstract
Long-term debt, less current maturities	$ 2,011,559,000	$ 1,818,669,000
Debentures | Notes, 4.45%, due 2020 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	4.45%	4.45%
Debentures | Notes, 6.1%, due 2041 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	6.10%	6.10%
Debentures | Notes, 3.875%, due 2022 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.875%	3.875%
Debentures | Notes, 4.875%, due 2043 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	4.875%	4.875%
Debentures | Notes, 3.8%, due 2046 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.80%	3.80%
Debentures | Notes, 3.7%, due 2028 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.70%	3.70%
Debentures | Notes, 4.15%, due 2049 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	4.15%
Debentures | 8% Series, due 2026 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	8.00%	8.00%
Debentures | Medium-term notes, 7.78% series, due 2022 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	7.78%	7.78%
Debentures | Medium-term notes, 7.92% series, due 2027 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	7.92%	7.92%
Debentures | Medium-term notes, 6.76% series, due 2027 | Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	6.76%	6.76%
Carrying Amount
Debt Instrument [Line Items]
Long-term debt, less current maturities	$ 2,373,003,000	$ 2,107,258,000
Long-term Debt, Current and Noncurrent Abstract
Long-term debt, less current maturities	2,373,003,000	2,107,258,000
Carrying Amount | Southwest Gas Corporation
Debt Instrument [Line Items]
Less: current maturities	0	0
Long-term debt, less current maturities	2,011,559,000	1,818,669,000
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	2,011,559,000	1,818,669,000
Less: current maturities	0	0
Long-term debt, less current maturities	2,011,559,000	1,818,669,000
Carrying Amount | Centuri
Debt Instrument [Line Items]
Less: current maturities	(36,807,000)	(33,060,000)
Long-term debt, less current maturities	361,444,000	288,589,000
Unamortized debt issuance costs	(1,270,000)	(1,414,000)
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	398,251,000	321,649,000
Less: current maturities	(36,807,000)	(33,060,000)
Long-term debt, less current maturities	361,444,000	288,589,000
Centuri secured revolving credit facility	251,850,000	254,545,000
Carrying Amount | Centuri secured revolving credit facility | Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	88,174,000	0
Carrying Amount | Centuri term loan facility | Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	253,120,000	255,959,000
Carrying Amount | Centuri other debt obligations | Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	58,227,000	67,104,000
Carrying Amount | Debentures | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	1,767,398,000	1,470,693,000
Unamortized discount and debt issuance costs	(15,102,000)	(11,807,000)
Carrying Amount | Debentures | Notes, 4.45%, due 2020 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	125,000,000	125,000,000
Carrying Amount | Debentures | Notes, 6.1%, due 2041 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	125,000,000	125,000,000
Carrying Amount | Debentures | Notes, 3.875%, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	250,000,000	250,000,000
Carrying Amount | Debentures | Notes, 4.875%, due 2043 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	250,000,000	250,000,000
Carrying Amount | Debentures | Notes, 3.8%, due 2046 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000,000	300,000,000
Carrying Amount | Debentures | Notes, 3.7%, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000,000	300,000,000
Carrying Amount | Debentures | Notes, 4.15%, due 2049 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000,000	0
Carrying Amount | Debentures | 8% Series, due 2026 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	75,000,000	75,000,000
Carrying Amount | Debentures | Medium-term notes, 7.78% series, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	25,000,000	25,000,000
Carrying Amount | Debentures | Medium-term notes, 7.92% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	25,000,000	25,000,000
Carrying Amount | Debentures | Medium-term notes, 6.76% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	7,500,000	7,500,000
Carrying Amount | Debentures | Revolving credit facility and commercial paper | Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit facility	46,000,000	150,000,000
Carrying Amount | Industrial development revenue bonds | Southwest Gas Corporation
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(1,839,000)	(2,024,000)
Unsecured debt	198,161,000	197,976,000
Carrying Amount | Industrial development revenue bonds | Tax-exempt Series A, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Carrying Amount | Industrial development revenue bonds | 2003 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Carrying Amount | Industrial development revenue bonds | 2008 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Carrying Amount | Industrial development revenue bonds | 2009 Series A, due 2039 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Market Value | Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	259,002,000	260,135,000
Centuri other debt obligations	59,440,000	67,053,000
Market Value | Centuri secured revolving credit facility | Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	88,213,000	0
Market Value | Debentures | Notes, 4.45%, due 2020 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	127,030,000	126,213,000
Market Value | Debentures | Notes, 6.1%, due 2041 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	158,916,000	150,728,000
Market Value | Debentures | Notes, 3.875%, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	257,128,000	254,195,000
Market Value | Debentures | Notes, 4.875%, due 2043 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	287,903,000	268,985,000
Market Value | Debentures | Notes, 3.8%, due 2046 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	296,868,000	267,030,000
Market Value | Debentures | Notes, 3.7%, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	316,170,000	298,926,000
Market Value | Debentures | Notes, 4.15%, due 2049 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	315,426,000	0
Market Value | Debentures | 8% Series, due 2026 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	96,803,000	93,827,000
Market Value | Debentures | Medium-term notes, 7.78% series, due 2022 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	27,774,000	27,497,000
Market Value | Debentures | Medium-term notes, 7.92% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	31,660,000	30,016,000
Market Value | Debentures | Medium-term notes, 6.76% series, due 2027 | Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	9,064,000	8,651,000
Market Value | Debentures | Revolving credit facility and commercial paper | Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit facility, fair value	46,000,000	150,000,000
Market Value | Industrial development revenue bonds | Tax-exempt Series A, due 2028 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Market Value | Industrial development revenue bonds | 2003 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Market Value | Industrial development revenue bonds | 2008 Series A, due 2038 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000,000	50,000,000
Market Value | Industrial development revenue bonds | 2009 Series A, due 2039 | Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 50,000,000	$ 50,000,000

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2019	May 31, 2019	Dec. 31, 2018	Nov. 30, 2018	Oct. 31, 2018
Debt Instrument [Line Items]
Short term borrowings outstanding	$ 0		$ 152,000,000
Borrowings outstanding under facility	$ 0
Percentage of total debt with interest rates reference to LIBOR and due after 2021	11.00%
Southwest Gas Corporation
Debt Instrument [Line Items]
Short term borrowings outstanding	$ 0		$ 152,000,000
Percentage of total debt with interest rates reference to LIBOR and due after 2021	2.00%
LIBOR | Southwest Gas Corporation
Debt Instrument [Line Items]
Borrowings outstanding under facility	$ 46,000,000
LIBOR | Centuri
Debt Instrument [Line Items]
Borrowings outstanding under facility	215,000,000
Southwest Gas Credit Facility | Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit designated as long term debt	150,000,000
Long-term debt	$ 46,000,000
Southwest Gas Credit Facility | LIBOR | Southwest Gas Corporation
Debt Instrument [Line Items]
Applicable margin	1.00%
Southwest Gas Credit Facility | Alternative Base Rate | Southwest Gas Corporation
Debt Instrument [Line Items]
Applicable margin	0.00%
Commercial Paper Program | Southwest Gas Corporation
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	$ 50,000,000
Borrowings outstanding under facility	0
Secured Revolving Credit Facility and Term Loan | Centuri
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity				$ 590,000,000	$ 450,000,000
Borrowings outstanding under facility	341,000,000
Debt secured by assets	$ 1,300,000,000
Line of Credit | Centuri
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity				325,000,000
Centuri term loan facility | Centuri
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity				$ 265,000,000
Debentures | Notes, 4.15%, due 2049 | Southwest Gas Corporation
Debt Instrument [Line Items]
Debt face amount		$ 300,000,000
Debt instrument interest rate	4.15%
Debt discount rate	0.051%

Short-Term Debt (Detail)

Short-Term Debt (Detail) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Short term borrowings outstanding	$ 0	$ 152,000,000
Southwest Gas Corporation
Short-term Debt [Line Items]
Short term borrowings outstanding	0	$ 152,000,000
Southwest Gas Corporation | $100 Million Credit Facility
Short-term Debt [Line Items]
Credit facility maximum borrowing capacity	100,000,000
Short term borrowings outstanding	0
Southwest Gas Corporation | $400 Million Credit Facility
Short-term Debt [Line Items]
Credit facility maximum borrowing capacity	400,000,000
Short term borrowings outstanding	0
Line of credit designated for working capital purposes	$ 250,000,000

Other Comprehensive Income an_3

Other Comprehensive Income and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to OCI (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, net of tax			$ 1,578
Other comprehensive income (loss), before tax	$ 2,437	$ 1,325	4,878	$ 2,429	$ 9,077		$ (804)
Other comprehensive income (loss), tax	(395)	(482)	(791)	(967)	(2,138)		(472)
Total other comprehensive income (loss), net of tax	2,042	843	$ 4,087	1,462	6,939		(1,276)
Effective income tax rate			24.00%			38.00%
Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	1,650	2,015	$ 3,300	4,030	8,908		(125)
Other comprehensive income (loss), tax	(395)	(482)	(791)	(967)	(2,138)		(472)
Total other comprehensive income (loss), net of tax	1,255	1,533	2,509	3,063	6,770		(597)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	814	1,179	1,628	2,357	5,564		(3,470)
Other comprehensive income (loss), tax	(195)	(282)	(390)	(565)	(1,335)		565
Total other comprehensive income (loss), net of tax	619	897	1,238	1,792	4,229		(2,905)
Defined Benefit Plans | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax			1,628
Other comprehensive income (loss), tax			(390)
Total other comprehensive income (loss), net of tax			1,238
Amortization of prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	318	334	635	668	1,302		1,335
Reclassification from AOCI, tax	(76)	(80)	(152)	(160)	(312)		(413)
Reclassification from AOCI, net of tax	242	254	483	508	990		922
Other comprehensive income (loss), before tax			635
Other comprehensive income (loss), tax			(152)
Total other comprehensive income (loss), net of tax			483
Amortization of prior service cost | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax			635
Other comprehensive income (loss), tax			(152)
Total other comprehensive income (loss), net of tax			483
Net actuarial gain/(loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax					(20,426)		(43,027)
Other comprehensive income (loss) before reclassifications, tax					4,902		10,326
Other comprehensive income (loss) before reclassifications, net of tax					(15,524)		(32,701)
Reclassification from AOCI, before Tax	5,844	8,404	11,688	16,808	28,497		29,531
Reclassification from AOCI, tax	(1,403)	(2,017)	(2,805)	(4,034)	(6,839)		(8,869)
Reclassification from AOCI, net of tax	4,441	6,387	8,883	12,774	21,658		20,662
Other comprehensive income (loss), before tax			11,688
Other comprehensive income (loss), tax			(2,805)
Total other comprehensive income (loss), net of tax			8,883
Net actuarial gain/(loss) | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax			11,688
Other comprehensive income (loss), tax			(2,805)
Total other comprehensive income (loss), net of tax			8,883
Regulatory adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	(5,348)	(7,559)	(10,695)	(15,119)	(3,809)		8,691
Reclassification from AOCI, tax	1,284	1,815	2,567	3,629	914		(479)
Reclassification from AOCI, net of tax	(4,064)	(5,744)	(8,128)	(11,490)	(2,895)		8,212
Other comprehensive income (loss), before tax			(10,695)
Other comprehensive income (loss), tax			2,567
Total other comprehensive income (loss), net of tax			(8,128)
Regulatory adjustment | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax			(10,695)
Other comprehensive income (loss), tax			2,567
Total other comprehensive income (loss), net of tax			(8,128)
FSIRS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	836	836	1,672	1,673	3,344		3,345
Reclassification from AOCI, tax	(200)	(200)	(401)	(402)	(803)		(1,037)
Reclassification from AOCI, net of tax	636	636	1,271	1,271	2,541		2,308
Other comprehensive income (loss), before tax	836	836	1,672	1,673	3,344		3,345
Other comprehensive income (loss), tax	(200)	(200)	(401)	(402)	(803)		(1,037)
Total other comprehensive income (loss), net of tax	636	636	1,271	1,271	2,541		2,308
FSIRS | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax			1,672
Reclassification from AOCI, tax			(401)
Reclassification from AOCI, net of tax			1,271
Other comprehensive income (loss), before tax			1,672
Other comprehensive income (loss), tax			(401)
Total other comprehensive income (loss), net of tax			1,271
Foreign Currency Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	787	(690)	1,578	(1,601)	169		(679)
Other comprehensive income (loss) before reclassifications, tax	0	0	0	0	0		0
Other comprehensive income (loss) before reclassifications, net of tax	$ 787	$ (690)	1,578	$ (1,601)	$ 169		$ (679)
Other comprehensive income (loss), before tax			1,578
Total other comprehensive income (loss), net of tax			$ 1,578

Other Comprehensive Income an_4

Other Comprehensive Income and Accumulated Other Comprehensive Income - Additional Information (Detail) $ in Millions	Jun. 30, 2019USD ($)
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$ 2.5

Other Comprehensive Income an_5

Other Comprehensive Income and Accumulated Other Comprehensive Income - AOCI Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 2,251,590		$ 1,932,567
Beginning balance, attributable to parent			2,252,042
Other comprehensive income (loss) before reclassifications, net of tax			1,578
Other comprehensive income (loss), before tax	$ 2,437	$ 1,325	4,878	$ 2,429	9,077		$ (804)
Other comprehensive income (loss), tax	(395)	(482)	(791)	(967)	(2,138)		(472)
Total other comprehensive income (loss), net of tax	2,042	843	4,087	1,462	6,939		(1,276)
Ending balance, attributable to parent	2,419,363		2,419,363		2,419,363	$ 2,252,042
Ending balance	2,419,363	1,932,567	$ 2,419,363	1,932,567	2,419,363	$ 2,251,590	1,932,567
Effective income tax rate			24.00%			38.00%
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			$ (55,227)
Beginning balance, tax			13,254
Beginning balance			(41,973)
Other comprehensive income (loss), before tax	814	1,179	1,628	2,357	5,564		(3,470)
Other comprehensive income (loss), tax	(195)	(282)	(390)	(565)	(1,335)		565
Total other comprehensive income (loss), net of tax	619	897	1,238	1,792	4,229		(2,905)
Ending balance, before tax	(53,599)		(53,599)		(53,599)	$ (55,227)
Ending balance, tax	12,864		12,864		12,864	13,254
Ending balance	(40,735)		(40,735)		(40,735)	(41,973)
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	318	334	635	668	1,302		1,335
Reclassification from AOCI, tax	(76)	(80)	(152)	(160)	(312)		(413)
Reclassification from AOCI, net of tax	242	254	483	508	990		922
Other comprehensive income (loss), before tax			635
Other comprehensive income (loss), tax			(152)
Total other comprehensive income (loss), net of tax			483
Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications, before tax					(20,426)		(43,027)
Other comprehensive income (loss) before reclassifications, net of tax					(15,524)		(32,701)
Reclassification from AOCI, before Tax	5,844	8,404	11,688	16,808	28,497		29,531
Reclassification from AOCI, tax	(1,403)	(2,017)	(2,805)	(4,034)	(6,839)		(8,869)
Reclassification from AOCI, net of tax	4,441	6,387	8,883	12,774	21,658		20,662
Other comprehensive income (loss), before tax			11,688
Other comprehensive income (loss), tax			(2,805)
Total other comprehensive income (loss), net of tax			8,883
Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	(5,348)	(7,559)	(10,695)	(15,119)	(3,809)		8,691
Reclassification from AOCI, tax	1,284	1,815	2,567	3,629	914		(479)
Reclassification from AOCI, net of tax	(4,064)	(5,744)	(8,128)	(11,490)	(2,895)		8,212
Other comprehensive income (loss), before tax			(10,695)
Other comprehensive income (loss), tax			2,567
Total other comprehensive income (loss), net of tax			(8,128)
FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			(9,310)
Beginning balance, tax			2,234
Beginning balance			(7,076)
Reclassification from AOCI, before Tax	836	836	1,672	1,673	3,344		3,345
Reclassification from AOCI, tax	(200)	(200)	(401)	(402)	(803)		(1,037)
Reclassification from AOCI, net of tax	636	636	1,271	1,271	2,541		2,308
Other comprehensive income (loss), before tax	836	836	1,672	1,673	3,344		3,345
Other comprehensive income (loss), tax	(200)	(200)	(401)	(402)	(803)		(1,037)
Total other comprehensive income (loss), net of tax	636	636	1,271	1,271	2,541		2,308
Ending balance, before tax	(7,638)		(7,638)		(7,638)	(9,310)
Ending balance, tax	1,833		1,833		1,833	2,234
Ending balance	(5,805)		(5,805)		(5,805)	(7,076)
Foreign Currency Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			(3,619)
Beginning balance			(3,619)
Other comprehensive income (loss) before reclassifications, before tax	787	(690)	1,578	(1,601)	169		(679)
Other comprehensive income (loss) before reclassifications, net of tax	787	(690)	1,578	(1,601)	169		(679)
Other comprehensive income (loss), before tax			1,578
Total other comprehensive income (loss), net of tax			1,578
Ending balance, before tax	(2,041)		(2,041)		(2,041)	(3,619)
Ending balance	(2,041)		(2,041)		(2,041)	(3,619)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(50,623)	(56,363)	(52,668)	(47,682)	(55,520)	(47,682)
Ending balance	(48,581)	(55,520)	(48,581)	(55,520)	(48,581)	(52,668)	(55,520)
Southwest Gas Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					1,718,993
Beginning balance, attributable to parent			1,782,460
Other comprehensive income (loss), before tax	1,650	2,015	3,300	4,030	8,908		(125)
Other comprehensive income (loss), tax	(395)	(482)	(791)	(967)	(2,138)		(472)
Total other comprehensive income (loss), net of tax	1,255	1,533	2,509	3,063	6,770		(597)
Ending balance, attributable to parent	1,948,222		1,948,222		1,948,222	1,782,460
Ending balance	1,948,222	1,718,993	1,948,222	1,718,993	1,948,222		1,718,993
Southwest Gas Corporation | Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax			(55,227)
Beginning balance, attributable to parent, tax			13,254
Beginning balance, attributable to parent			(41,973)
Other comprehensive income (loss), before tax			1,628
Other comprehensive income (loss), tax			(390)
Total other comprehensive income (loss), net of tax			1,238
Ending balance, attributable to parent, before tax	(53,599)		(53,599)		(53,599)	(55,227)
Ending balance, attributable to parent, tax	12,864		12,864		12,864	13,254
Ending balance, attributable to parent	(40,735)		(40,735)		(40,735)	(41,973)
Southwest Gas Corporation | Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			635
Other comprehensive income (loss), tax			(152)
Total other comprehensive income (loss), net of tax			483
Southwest Gas Corporation | Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			11,688
Other comprehensive income (loss), tax			(2,805)
Total other comprehensive income (loss), net of tax			8,883
Southwest Gas Corporation | Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			(10,695)
Other comprehensive income (loss), tax			2,567
Total other comprehensive income (loss), net of tax			(8,128)
Southwest Gas Corporation | FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax			(9,310)
Beginning balance, attributable to parent, tax			2,234
Beginning balance, attributable to parent			(7,076)
Reclassification from AOCI, before Tax			1,672
Reclassification from AOCI, tax			(401)
Reclassification from AOCI, net of tax			1,271
Other comprehensive income (loss), before tax			1,672
Other comprehensive income (loss), tax			(401)
Total other comprehensive income (loss), net of tax			1,271
Ending balance, attributable to parent, before tax	(7,638)		(7,638)		(7,638)	(9,310)
Ending balance, attributable to parent, tax	1,833		1,833		1,833	2,234
Ending balance, attributable to parent	(5,805)		(5,805)		(5,805)	(7,076)
Southwest Gas Corporation | AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(47,795)	(54,843)	(49,049)	(47,073)	(53,310)	(47,073)
Beginning balance, attributable to parent			(49,049)
Ending balance, attributable to parent	(46,540)		(46,540)		(46,540)	(49,049)
Ending balance	$ (46,540)	$ (53,310)	$ (46,540)	$ (53,310)	$ (46,540)	$ (49,049)	$ (53,310)

Other Comprehensive Income an_6

Other Comprehensive Income and Accumulated Other Comprehensive Income - Amounts Recognized Before Tax, Defined Benefit Plans (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Equity [Abstract]
Net actuarial (loss) gain	$ (423,676)	$ (435,364)
Prior service cost	(2,398)	(3,033)
Less: amount recognized in regulatory assets	372,475	383,170
Recognized in AOCI	$ (53,599)	$ (55,227)

Segment Information - Additiona

Segment Information - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2019Segment
Segment Reporting [Abstract]
Number of segments	2

Segment Information - Schedule

Segment Information - Schedule of Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 713,011	$ 670,883	$ 1,546,550	$ 1,425,213	$ 3,001,350	$ 2,758,799
Segment net income (loss)	22,056	21,551	116,865	100,642	198,500	207,311
Operating Segments
Segment Reporting Information [Line Items]
Revenues	663,929	637,811	1,458,795	1,364,983	2,837,911	2,645,629
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	49,082	33,072	87,755	60,230	163,439	113,170
Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenues	0	0	0	0	0	0
Segment net income (loss)	(231)	(307)	(780)	(564)	(1,758)	(1,231)
Natural Gas Operations
Segment Reporting Information [Line Items]
Revenues	258,711	275,679	779,388	769,992	1,367,124	1,349,536
Segment net income (loss)	3,369	2,622	106,758	92,971	152,629	163,329
Natural Gas Operations | Operating Segments
Segment Reporting Information [Line Items]
Revenues	258,711	275,679	779,388	769,992	1,367,124	1,349,536
Natural Gas Operations | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	0	0	0	0	0	0
Utility Infrastructure Services
Segment Reporting Information [Line Items]
Revenues	454,300	395,204	767,162	655,221	1,634,226	1,409,263
Segment net income (loss)	18,918	19,236	10,887	8,235	47,629	45,213
Utility Infrastructure Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	405,218	362,132	679,407	594,991	1,470,787	1,296,093
Utility Infrastructure Services | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	$ 49,082	$ 33,072	$ 87,755	$ 60,230	$ 163,439	$ 113,170

Redeemable Noncontrolling Int_3

Redeemable Noncontrolling Interests - Additional Information (Detail)	Jun. 30, 2019	Nov. 30, 2018
Noncontrolling Interest [Line Items]
Percentage of interest retained by noncontrolling party subject to election	100.00%
Linetec
Noncontrolling Interest [Line Items]
Ownership percentage by noncontrolling owners		20.00%

Redeemable Noncontrolling Int_4

Redeemable Noncontrolling Interests - Summary of Redeemable Noncontrolling Interest (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Movement In Redeemable Noncontrolling Interest [Roll Forward]
Balance, December 31, 2018	$ 81,831
Net income attributable to redeemable noncontrolling interest	1,351
Balance, June 30, 2019	$ 83,182

Business Acquisitions - Narrati

Business Acquisitions - Narrative (Detail) - USD ($)	3 Months Ended	6 Months Ended	7 Months Ended	12 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2019	Nov. 30, 2018
Linetec
Business Acquisition [Line Items]
Ownership percentage by noncontrolling owners					20.00%
Linetec
Business Acquisition [Line Items]
Reduction to purchase price			$ 20,100,000
Payments to acquisition	$ 19,500,000
Remaining unpaid consideration	31,300,000	$ 31,300,000	$ 31,300,000	$ 31,300,000
Acquisition costs	$ 0	$ 0		$ 6,900,000
Centuri | Linetec
Business Acquisition [Line Items]
Percentage of voting interests acquired					80.00%

Business Acquisitions - Summary

Business Acquisitions - Summary of Fair Values of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	7 Months Ended
	Jun. 30, 2019	Dec. 31, 2018	Nov. 30, 2018
Fair Values of Assets Acquired and Liabilities Assumed
Goodwill	$ 345,872	$ 359,045
Linetec
Fair Values of Assets Acquired and Liabilities Assumed
Cash and cash equivalents	3,900		$ 3,900
Accounts receivable	32,300		32,800
Revenue earned on contracts in progress in excess of billings	21,400		21,600
Prepaid expenses and other current assets	1,300		1,100
Property and equipment	88,900		89,400
Intangible assets	89,300		89,300
Goodwill	171,000		188,500
Total assets acquired	408,100		426,600
Accounts payable	8,000		8,000
Accrued liabilities	8,500		6,900
Deferred compensation and related accrued taxes	3,400		3,400
Redeemable noncontrolling interest	81,700		81,700
Total liabilities assumed and noncontrolling interest	101,600		100,000
Net assets acquired	306,500		$ 326,600
Measurement Period Adjustments
Accounts receivable, measurement period adjustments	(500)
Revenue earned on contracts in progress in excess of billings, measurement period adjustments	(200)
Prepaid expenses and other current assets, measurement period adjustments	200
Property and equipment, measurement period adjustments	(500)
Intangible assets, measurement period adjustments	0
Goodwill, measurement period adjustments	(17,500)
Total assets acquired, measurement period adjustments	(18,500)
Accrued liabilities, measurement period adjustments	1,600
Total liabilities assumed and noncontrolling interest, measurement period adjustments	1,600
Net assets acquired, measurement period adjustments	$ (20,100)