Southwest Gas 10-Q 7 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 27, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Trading Symbol	SWX
Entity Registrant Name	SOUTHWEST GAS HOLDINGS, INC.
Entity Central Index Key	1,692,115
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		47,731,840

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Utility plant:
Gas plant	$ 6,440,547	$ 6,193,564
Less: accumulated depreciation	(2,218,796)	(2,172,966)
Acquisition adjustments, net	81	196
Construction work in progress	164,030	111,177
Net utility plant	4,385,862	4,131,971
Other property and investments	369,303	342,343
Current assets:
Cash and cash equivalents	59,152	28,066
Accounts receivable, net of allowances	301,792	285,145
Accrued utility revenue	34,100	76,200
Income taxes receivable, net	5,462	4,455
Deferred purchased gas costs	6,230	2,608
Prepaids and other current assets	132,182	136,833
Total current assets	538,918	533,307
Noncurrent assets:
Goodwill	147,865	139,983
Deferred income taxes	1,467	1,288
Deferred charges and other assets	411,655	432,234
Total noncurrent assets	560,987	573,505
Total assets	5,855,070	5,581,126
Capitalization:
Common stock	49,362	49,112
Additional paid-in capital	924,213	903,123
Accumulated other comprehensive income (loss), net	(42,818)	(48,008)
Retained earnings	784,934	759,263
Total Southwest Gas Holdings, Inc./Total Southwest Gas Corporation equity	1,715,691	1,663,490
Noncontrolling interest	(2,295)	(2,217)
Total equity	1,713,396	1,661,273
Redeemable noncontrolling interest		22,590
Long-term debt, less current maturities	1,731,981	1,549,983
Total capitalization	3,445,377	3,233,846
Current liabilities:
Current maturities of long-term debt	28,453	50,101
Short-term debt	110,500
Accounts payable	159,382	184,669
Customer deposits	70,162	72,296
Income taxes payable	1,543	1,909
Accrued general taxes	48,998	42,921
Accrued interest	24,543	17,939
Deferred purchased gas costs	14,971	90,476
Other current liabilities	197,854	168,064
Total current liabilities	656,406	628,375
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	894,011	840,653
Accumulated removal costs	312,000	308,000
Other deferred credits and other long-term liabilities	547,276	570,252
Total deferred income taxes and other credits	1,753,287	1,718,905
Total capitalization and liabilities	5,855,070	5,581,126
Southwest Gas Corporation [Member]
Utility plant:
Gas plant	6,440,547	6,193,564
Less: accumulated depreciation	(2,218,796)	(2,172,966)
Acquisition adjustments, net	81	196
Construction work in progress	164,030	111,177
Net utility plant	4,385,862	4,131,971
Other property and investments	115,841	108,569
Current assets:
Cash and cash equivalents	46,467	19,024
Accounts receivable, net of allowances	68,028	111,845
Accrued utility revenue	34,100	76,200
Income taxes receivable, net	6,440	4,455
Deferred purchased gas costs	6,230	2,608
Prepaids and other current assets	118,587	126,363
Total current assets	279,852	340,495
Noncurrent assets:
Goodwill	10,095	10,095
Deferred charges and other assets	393,942	410,625
Discontinued operations-construction services-assets		579,371
Total noncurrent assets	404,037	1,000,091
Total assets	5,185,592	5,581,126
Capitalization:
Common stock	49,112	49,112
Additional paid-in capital	917,581	897,346
Accumulated other comprehensive income (loss), net	(42,299)	(45,639)
Retained earnings	606,007	767,061
Total Southwest Gas Holdings, Inc./Total Southwest Gas Corporation equity	1,530,401	1,667,880
Discontinued operations-construction services non-owner equity		15,983
Total equity	1,530,401	1,667,880
Long-term debt, less current maturities	1,520,790	1,375,080
Total capitalization	3,051,191	3,058,943
Current liabilities:
Current maturities of long-term debt		25,000
Short-term debt	83,000
Accounts payable	92,257	138,229
Customer deposits	70,162	72,296
Accrued general taxes	48,998	42,921
Accrued interest	24,406	17,395
Deferred purchased gas costs	14,971	90,476
Payable to parent	2,560
Other current liabilities	109,705	95,999
Total current liabilities	446,059	482,316
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	853,682	806,109
Accumulated removal costs	312,000	308,000
Other deferred credits and other long-term liabilities	522,660	545,143
Discontinued operations-construction services-liabilities		380,615
Total deferred income taxes and other credits	1,688,342	2,039,867
Total capitalization and liabilities	$ 5,185,592	$ 5,581,126

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Common stock, par	$ 1	$ 1
Common stock, authorized	60,000,000	60,000,000
Common stock, issued	47,731,840	47,482,068
Common stock, outstanding	47,731,840	47,482,068

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating revenues:
Gas operating revenues	$ 213,059	$ 200,179	$ 935,823	$ 980,927	$ 1,276,308	$ 1,376,388
Construction revenues	380,094	339,790	872,536	838,038	1,173,576	1,127,982
Total operating revenues	593,153	539,969	1,808,359	1,818,965	2,449,884	2,504,370
Gas operating revenues	213,059	200,179	935,823	980,927	1,276,308	1,376,388
Operating expenses:
Net cost of gas sold	45,539	39,056	261,839	324,072	334,888	460,836
Operations and maintenance	102,278	102,438	314,488	301,979	414,233	400,222
Depreciation and amortization	58,529	69,845	189,089	217,764	260,457	286,977
Taxes other than income taxes	14,046	12,480	43,325	39,480	56,221	51,810
Construction expenses	342,629	300,611	806,586	757,919	1,073,090	1,009,188
Total operating expenses	563,021	524,430	1,615,327	1,641,214	2,138,889	2,209,033
Operating income (loss)	30,132	15,539	193,032	177,751	310,995	295,337
Other income and (expenses):
Net interest deductions	(19,494)	(18,158)	(56,863)	(54,100)	(76,423)	(71,884)
Other income (deductions)	2,876	2,565	8,788	6,756	11,501	10,861
Total other income and (expenses)	(16,618)	(15,593)	(48,075)	(47,344)	(64,922)	(61,023)
Income (loss) before income taxes	13,514	(54)	144,957	130,407	246,073	234,314
Income tax expense (benefit)	3,094	(2,961)	47,411	43,046	82,833	80,255
Net income	10,420	2,907	97,546	87,361	163,240	154,059
Net income attributable to noncontrolling interests	216	435	170	500	684	1,079
Net income (loss)	$ 10,204	$ 2,472	$ 97,376	$ 86,861	$ 162,556	$ 152,980
Basic earnings per share	$ 0.21	$ 0.05	$ 2.05	$ 1.83	$ 3.42	$ 3.22
Diluted earnings per share	0.21	0.05	2.03	1.82	3.39	3.20
Dividends declared per share	$ 0.495	$ 0.450	$ 1.485	$ 1.350	$ 1.935	$ 1.755
Average number of common shares outstanding	47,628	47,481	47,577	47,464	47,553	47,442
Average shares outstanding (assuming dilution)	47,986	47,830	47,912	47,802	47,896	47,787
Southwest Gas Corporation [Member]
Operating revenues:
Gas operating revenues	$ 213,059	$ 200,179	$ 935,823	$ 980,927	$ 1,276,308	$ 1,376,388
Gas operating revenues	213,059	200,179	935,823	980,927	1,276,308	1,376,388
Operating expenses:
Net cost of gas sold	45,539	39,056	261,839	324,072	334,888	460,836
Operations and maintenance	102,215	102,438	313,395	301,979	413,140	400,222
Depreciation and amortization	46,194	56,436	153,643	174,413	212,693	228,609
Taxes other than income taxes	14,046	12,480	43,325	39,480	56,221	51,810
Total operating expenses	207,994	210,410	772,202	839,944	1,016,942	1,141,477
Operating income (loss)	5,065	(10,231)	163,621	140,983	259,366	234,911
Other income and (expenses):
Net interest deductions	(17,421)	(16,364)	(51,622)	(49,155)	(69,464)	(65,146)
Other income (deductions)	3,081	2,521	8,744	6,712	10,308	9,615
Total other income and (expenses)	(14,340)	(13,843)	(42,878)	(42,443)	(59,156)	(55,531)
Income (loss) before income taxes	(9,275)	(24,074)	120,743	98,540	200,210	179,380
Income tax expense (benefit)	(5,251)	(11,669)	38,307	31,004	65,887	59,544
Net income			82,436	87,361	148,130	154,059
Net income (loss) from continuing operations	(4,024)	(12,405)	82,436	67,536	134,323	119,836
Discontinued operations-construction services, income before income taxes		24,020		31,867	21,649	54,934
Discontinued operations-construction services, Income tax expense		8,708		12,042	7,842	20,711
Discontinued operations-construction services, Income		15,312		19,825	13,807	34,223
Discontinued operations-construction services - Noncontrolling interests		435		500	514	1,079
Income-discontinued operations		14,877		19,325	13,293	33,144
Net income (loss)	$ (4,024)	$ 2,472	$ 82,436	$ 86,861	$ 147,616	$ 152,980

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income	$ 10,420	$ 2,907	$ 97,546	$ 87,361	$ 163,240	$ 154,059
Defined benefit pension plans:
Net actuarial gain (loss)					(14,118)	(18,922)
Amortization of prior service cost	207	207	621	621	828	828
Amortization of net actuarial loss	3,944	4,196	11,832	12,586	16,027	17,915
Regulatory adjustment	(3,555)	(3,796)	(10,667)	(11,388)	(2,741)	(404)
Net defined benefit pension plans	596	607	1,786	1,819	(4)	(583)
Forward-starting interest rate swaps:
Amounts reclassified into net income	518	518	1,554	1,556	2,073	2,073
Net forward-starting interest rate swaps	518	518	1,554	1,556	2,073	2,073
Foreign currency translation adjustments	1,012	(238)	1,861	614	1,408	233
Total other comprehensive income (loss), net of tax	2,126	887	5,201	3,989	3,477	1,723
Comprehensive income	12,546	3,794	102,747	91,350	166,717	155,782
Comprehensive income attributable to noncontrolling interests	198	427	181	521	679	1,089
Comprehensive income (loss)	12,348	3,367	102,566	90,829	166,038	154,693
Southwest Gas Corporation [Member]
Net income (loss) from continuing operations	(4,024)	(12,405)	82,436	67,536	134,323	119,836
Net income			82,436	87,361	148,130	154,059
Defined benefit pension plans:
Net actuarial gain (loss)					(14,118)	(18,922)
Amortization of prior service cost	207	207	621	621	828	828
Amortization of net actuarial loss	3,944	4,196	11,832	12,586	16,027	17,915
Regulatory adjustment	(3,555)	(3,796)	(10,667)	(11,388)	(2,741)	(404)
Net defined benefit pension plans	596	607	1,786	1,819	(4)	(583)
Forward-starting interest rate swaps:
Amounts reclassified into net income	518	518	1,554	1,556	2,073	2,073
Net forward-starting interest rate swaps	518	518	1,554	1,556	2,073	2,073
Total other comprehensive income (loss), net of tax	1,114	1,125	3,340	3,375	2,069	1,490
Comprehensive income	(2,910)	(11,280)	85,776	70,911	136,392	121,326
Discontinued operations-construction services, net income		14,877		19,325	13,293	33,144
Discontinued operations-construction services, foreign currency translation adjustments		(238)		614	(453)	233
Discontinued operations-construction services, comprehensive income		14,639		19,939	12,840	33,377
Discontinued operations-construction services, comprehensive income (loss) attributable to noncontrolling interests		(8)		21	(16)	10
Comprehensive income attributable to discontinued operations-construction services		14,647		19,918	12,856	33,367
Comprehensive income (loss)	$ (2,910)	$ 3,367	$ 85,776	$ 90,829	$ 149,248	$ 154,693

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$ 97,546	$ 87,361	$ 163,240	$ 154,059
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	189,089	217,764	260,457	286,977
Deferred income taxes	49,409	43,702	74,439	86,526
Changes in current assets and liabilities:
Accounts receivable, net of allowances	(15,330)	28,531	(13,765)	(17,889)
Accrued utility revenue	42,100	41,700	(1,100)	(800)
Deferred purchased gas costs	(79,127)	81,389	(114,658)	79,460
Accounts payable	(26,771)	(24,942)	19,866	10,445
Accrued taxes	4,689	(7,055)	38,084	(11,033)
Other current assets and liabilities	43,044	12,022	3,590	22,034
Gains on sale	(1,452)	(4,117)	(4,483)	(4,200)
Changes in undistributed stock compensation	9,199	4,347	10,308	5,142
AFUDC	(2,077)	(1,893)	(2,473)	(2,890)
Changes in other assets and deferred charges	(14,470)	3,926	(1,436)	4,183
Changes in other liabilities and deferred credits	3,395	(4,813)	(10,239)	702
Net cash provided by operating activities	299,244	477,922	421,830	612,716
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(449,998)	(404,388)	(575,141)	(555,819)
Acquisition of businesses, net of cash acquired		(17,000)		(17,000)
Changes in customer advances	(1,951)	5,445	504	9,445
Miscellaneous inflows	9,160	7,965	14,234	4,726
Net cash used in investing activities	(442,789)	(407,978)	(560,403)	(558,648)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	11,563	530	11,505	507
Dividends paid	(68,503)	(61,950)	(89,870)	(81,138)
Centuri distribution to redeemable noncontrolling interest	(204)	(99)	(544)	(198)
Issuance of long-term debt, net	104,308	408,946	119,308	420,946
Retirement of long-term debt	(100,240)	(196,351)	(159,162)	(240,999)
Change in credit facility and commercial paper	145,000	(150,000)	150,000	(97,000)
Change in short-term debt	110,500	(18,000)	110,500
Principal payments on capital lease obligations	(796)	(1,125)	(1,025)	(1,449)
Redemption of Centuri shares from noncontrolling parties	(23,000)		(23,000)
Withholding remittance-share-based compensation	(3,176)	(2,119)	(3,176)	(2,164)
Other	(1,104)	(605)	(2,068)	(60)
Net cash provided by (used in) financing activities	174,348	(20,773)	112,468	(1,555)
Effects of currency translation on cash and cash equivalents	283	(14)	103	(318)
Change in cash and cash equivalents	31,086	49,157	(26,002)	52,195
Cash and cash equivalents at beginning of period	28,066	35,997	85,154	32,959
Cash and cash equivalents at end of period	59,152	85,154	59,152	85,154
Supplemental information:
Interest paid, net of amounts capitalized	45,771	47,134	66,077	68,445
Income taxes paid (received)	3,687	6,530	(21,875)	9,899
Southwest Gas Corporation [Member]
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	82,436	87,361	148,130	154,059
Income (loss) from discontinued operations		19,825	13,807	34,223
Net income (loss) from continuing operations	82,436	67,536	134,323	119,836
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	153,643	174,413	212,693	228,609
Deferred income taxes	44,621	39,953	72,627	76,837
Changes in current assets and liabilities:
Accounts receivable, net of allowances	43,818	91,680	(7,131)	8,543
Accrued utility revenue	42,100	41,700	(1,100)	(800)
Deferred purchased gas costs	(79,127)	81,389	(114,658)	79,460
Accounts payable	(45,972)	(47,060)	17,271	1,467
Accrued taxes	4,092	(5,660)	29,143	4,567
Other current assets and liabilities	32,453	(819)	(224)	9,135
Changes in undistributed stock compensation	7,999	4,347	9,108	5,142
AFUDC	(2,077)	(1,893)	(2,473)	(2,890)
Changes in other assets and deferred charges	(14,861)	3,664	(1,914)	3,834
Changes in other liabilities and deferred credits	2,883	(4,813)	(10,751)	702
Net cash provided by operating activities	272,008	444,437	336,914	534,442
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(395,463)	(337,921)	(514,661)	(485,665)
Changes in customer advances	(1,951)	5,445	504	9,445
Miscellaneous inflows	2,407	2,464	2,925	3,506
Dividends received		2,801	9,660	5,602
Net cash used in investing activities	(395,007)	(327,211)	(501,572)	(467,112)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net		530	(58)	507
Contributions from parent	11,659		11,659
Dividends paid	(60,497)	(61,950)	(81,864)	(81,138)
Issuance of long-term debt, net		296,469		296,469
Retirement of long-term debt	(25,000)	(124,855)	(25,000)	(124,855)
Change in credit facility and commercial paper	145,000	(150,000)	150,000	(97,000)
Change in short-term debt	83,000	(18,000)	83,000
Withholding remittance-share-based compensation	(3,176)	(2,119)	(3,176)	(2,164)
Other	(544)	(605)	(1,508)	(9)
Net cash provided by (used in) financing activities	150,442	(60,530)	133,053	(8,190)
Net cash provided by discontinued operating activities		33,485	57,680	78,274
Net cash used in discontinued investing activities		(80,767)	(11,049)	(91,536)
Net cash provided by (used in) discontinued financing activities		39,757	(44,491)	6,635
Effects of currency translation on cash and cash equivalents		(14)	(180)	(318)
Change in cash and cash equivalents	27,443	49,157	(29,645)	52,195
Change in cash and cash equivalents of discontinued operations included in discontinued operations construction services assets		7,539	(1,960)	6,945
Change in cash and cash equivalents of continuing operations	27,443	56,696	(31,605)	59,140
Cash and cash equivalents at beginning of period	19,024	21,376	78,072	18,932
Cash and cash equivalents at end of period	46,467	78,072	46,467	78,072
Supplemental information:
Interest paid, net of amounts capitalized	40,751	42,804	59,448	63,031
Income taxes paid (received)	$ 4	$ (3,055)	$ (27,952)	$ (16,600)

Nature of Operations and Basis

Nature of Operations and Basis of Presentation	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations and Basis of Presentation	Note 1 – Nature of Operations and Basis of Presentation Nature of Operations. Note 9 – Construction Services Redeemable Noncontrolling Interest In January 2017, a previously contemplated and approved reorganization under a holding company structure was made effective. The reorganization was designed to provide further separation between regulated and unregulated businesses, and to provide additional financing flexibility. Coincident with the effective date of the reorganization, existing shareholders of Southwest Gas Corporation became shareholders of Southwest Gas Holdings, Inc., on a one-for-one Southwest Gas Corporation (“Southwest” or the “natural gas operations segment”) 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 construction services enterprise dedicated to meeting the growing demands of North American utilities, energy and industrial markets. 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”, formerly Link-Line Contractors Ltd.), W.S. Nicholls Construction, Inc. (“W.S. Nicholls”), and Brigadier Pipelines Inc. (“Brigadier”). Typically, Centuri revenues are lowest during the first quarter of the year due to unfavorable winter weather conditions. Operating revenues typically improve as more favorable weather conditions occur during the summer and fall months. Basis of Presentation. Form 10-Q, Note 10 – Reorganization Impacts – Discontinued Operations Solely Related to Southwest Gas Corporation non-current 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 organizational changes. Centuri operations continue to be part of continuing operations and included in the consolidated financial statements of Southwest Gas Holdings, Inc. 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 statement 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 2016 Annual Report to Shareholders, which is incorporated by reference into the 2016 Form 10-K. Prepaids and other current assets. Other current liabilities Cash and Cash Equivalents. Significant non-cash non-cash Adoption of Accounting Standards Update (“ASU”) No. 2016-09. No. 2016-09 Under the new guidance, the Company can withhold any amount between the minimum and maximum individual statutory tax rates and still treat the entire award as equity. The Company intends to administer withholding such that awards under stock compensation programs will continue to be treated as equity awards. In addition to the above, the update requires all income tax-related Amendments related to the timing of when excess tax benefits are recognized, minimum statutory withholding requirements, forfeitures, and intrinsic value are required to be applied using a modified retrospective transition method by means of a cumulative-effect adjustment to equity as of the beginning of the period in which the guidance is adopted. The Company had no previously unrecognized tax benefits as a result of these changes; therefore, no cumulative effect adjustment to the Company’s opening retained earnings was required. Goodwill. (In thousands of dollars) Natural Gas Construction Consolidated December 31, 2016 $ 10,095 $ 129,888 $ 139,983 Foreign currency translation adjustment — 7,882 7,882 September 30, 2017 $ 10,095 $ 137,770 $ 147,865 Intercompany Transactions Note 3— Segment Information September 30, 2017 December 31, 2016 Centuri accounts receivable for services provided to Southwest $ 11,486 $ 10,585 The accounts receivable balance, revenues, and associated profits are included in the condensed consolidated financial statements of the Company and were not eliminated during consolidation in accordance with accounting treatment for rate-regulated entities. Other Property and Investments. September 30, 2017 December 31, 2016 Centuri property and equipment $ 493,599 $ 451,114 Centuri accumulated provision for depreciation and amortization (251,831 ) (228,374 ) Net cash surrender value of COLI policies 114,052 106,744 Other property 13,483 12,859 Total $ 369,303 $ 342,343 Other Income (Deductions). Three Months Ended Nine Months Ended Twelve Months Ended September 30 September 30 September 30 2017 2016 2017 2016 2017 2016 Southwest Gas Corporation—natural gas operations segment: Change in COLI policies $ 2,100 $ 2,300 $ 6,800 $ 5,400 $ 8,800 $ 7,500 Interest income 670 522 1,848 1,279 2,417 1,664 Equity AFUDC 968 611 2,077 1,893 2,473 2,890 Miscellaneous income and (expense) (657 ) (912 ) (1,981 ) (1,860 ) (3,382 ) (2,439 ) Southwest Gas Corporation—total other income (deductions) 3,081 2,521 8,744 6,712 10,308 9,615 Construction services segment: Interest income 1 — 2 1 2 414 Foreign transaction gain (loss) (442 ) (3 ) (640 ) (22 ) (640 ) 28 Miscellaneous income and (expense) 231 47 676 65 1,825 804 Centuri—total other income (deductions) (210 ) 44 38 44 1,187 1,246 Corporate and administrative 5 — 6 — 6 — Consolidated Southwest Gas Holdings, Inc.—total other income (deductions) $ 2,876 $ 2,565 $ 8,788 $ 6,756 $ 11,501 $ 10,861 Included in the table above is the change in cash surrender values of company-owned life insurance (“COLI”) policies (including net death benefits recognized). 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. Current tax regulations provide for tax-free Recently Issued Accounting Standards Updates. one-year Deliberations have been ongoing by the utility industry, notably in connection with efforts to produce an accounting guide intended to be developed by the American Institute of Certified Public Accountants (“AICPA”). In association with this undertaking, the AICPA formed a number of industry task forces, including a Power & Utilities (“P&U”) Task Force, on which Company personnel actively participate via formal membership. Industry representatives and organizations, the largest auditing firms, the AICPA’s Revenue Recognition Working Group and its Financial Reporting Executive Committee have undertaken, and continue to undertake, consideration of several items relevant to the utility industry. Where applicable or necessary, the FASB’s Transition Resource Group (“TRG”) has also participated. Through the P&U Task Force undertakings, general determinations were made that contributions received in aid of construction (“CIAC”) efforts related to the industry’s pipe distribution and transmission systems are reimbursements of expenditures rather than revenue (consistent with current accounting practices). Furthermore, regarding the “collectibility” criterion in the update that must be met for revenue recognition, general determinations have been made that contracts for utility service (including service to lower income or lower credit quality customers) represent genuine and valid contracts for which revenue is able to be recognized when service is rendered (consistent with current accounting practices). These determinations by the P&U industry are based on the various measures the industry takes to help ensure collectibility (e.g., proof of creditworthiness, customer deposits, late fee assessment, disconnection, service re-establishment With regard to the construction services segment, the principles of the new revenue recognition guidance are very similar to existing guidance for construction contractors. Similar to the P&U Task Force noted above, the AICPA formed the Engineering and Construction Contractors Task Force to assist the construction industry with implementing the new guidance. The accounting guide the AICPA intends to release is expected to provide implementation guidance related to several issues including 1) combining contracts and separating performance obligations; 2) estimating change orders, incentives, penalties, liquidated damages and other variable consideration items and 3) acceptable measures of progress when recognizing revenue over time. Management of both segments of the Company has substantially completed assessments of sources of revenue and the effects that adoption of the new guidance will have on the Company’s (and Southwest’s in the case of utility operations) financial position, results of operations, and cash flows. Based on assessments completed to date, management believes that such impacts will not be material overall. Presentation and disclosure requirements of the new guidance will have the most impact on the Company’s financial statements and note disclosures. The Company is currently planning to adopt the new guidance in 2018 under the modified retrospective transition method, as permissible. In January 2016, the FASB issued the update “Financial Instruments – Overall (Subtopic 825-10): In February 2016, the FASB issued the update “Leases (Topic 842).” Under the update, lessees will be required to recognize the following for all leases (with the exception of short-term leases) at the commencement date: • A lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and • A right-of-use Under the new guidance, lessor accounting is largely unchanged. Certain targeted improvements were made to align, where necessary, lessor accounting with the lessee accounting model and Topic 606, Revenue from Contracts with Customers. Though companies have historically been required to make disclosures regarding leases and of associated contractual obligations, leases (with terms longer than a year) will no longer exist off-balance In June 2016, the FASB issued the update “Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” The update amends guidance on reporting credit losses for financial assets held at amortized cost basis and available for sale debt securities. For assets held at amortized cost basis, the update eliminates the “probable” threshold for initial recognition of credit losses in current U.S. GAAP and, instead, requires an entity to reflect its current estimate of all expected credit losses. The allowance for credit losses is a valuation account that is deducted from the amortized cost basis of the financial asset to present the net amount expected to be collected. For available for sale debt securities, credit losses should be measured in a manner similar to current U.S. GAAP, however the update will require that credit losses be presented as an allowance rather than as a write-down. This update affects entities holding financial assets and net investment in leases that are not accounted for at fair value through net income. The update affects loans, debt securities, trade receivables, net investments in leases, off-balance In August 2016, the FASB issued the update “Classification of Certain Cash Receipts and Cash Payments.” This update addresses the following specific cash flow issues: debt prepayment or debt extinguishment costs; settlement of zero-coupon In October 2016, the FASB issued the update “Accounting for Income Taxes: Intra-Entity Asset Transfers of Assets Other than Inventory.” This update eliminates the current U.S. GAAP exception for all intra-entity sales of assets other than inventory. As a result, a reporting entity would recognize the tax expense from the sale of the asset in the seller’s tax jurisdiction when the transfer occurs, even though the pre-tax In January 2017, the FASB issued the update “Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment.” The update eliminates Step 2 from the goodwill impairment test. The annual, or interim, goodwill impairment test is performed by comparing the fair value of a reporting unit with its carrying amount. An impairment charge should be recognized for the amount by which the carrying amount exceeds the reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. In addition, income tax effects from any tax-deductible In March 2017, the FASB issued the update “Compensation – Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost.” The update applies to all employers that offer employee benefits under defined benefit pension plans, other postretirement benefit plans, or other types of benefits accounted for under Topic 715, Compensation – Retirement Benefits. The update requires that an employer report the service cost component in the same line item or items as other compensation costs arising from services rendered by the employees during the period. The other components of net benefit cost are required to be presented in the income statement separately from the service cost component and outside a subtotal of income from operations, and be appropriately described. The update also allows only the service cost component (and not the other components of periodic benefit costs) to be eligible for capitalization when applicable, making no exception for specialized industries, including rate-regulated Southwest is a rate-regulated utility offering pension and postretirement benefits to retired employees. It is anticipated that Southwest would continue to request recovery of the total costs of defined benefit plans in rate applications filed with its various regulatory bodies. Rate-regulated entities providing utility and transmission services have historically capitalized a portion of periodic benefit costs (including non-service non-service non-service non-service

Components of Net Periodic Bene

Components of Net Periodic Benefit Cost	9 Months Ended
	Sep. 30, 2017
Text Block [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. 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 net periodic benefit costs to the same accounts to which productive labor is charged. As a result, net periodic benefit 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. Qualified Retirement Plan Period Ended September 30, Three Months Nine Months Twelve Months 2017 2016 2017 2016 2017 2016 (Thousands of dollars) Service cost $ 5,848 $ 5,708 $ 17,544 $ 17,125 $ 23,252 $ 23,406 Interest cost 11,520 11,507 34,561 34,520 46,068 45,577 Expected return on plan assets (13,799 ) (14,140 ) (41,397 ) (42,419 ) (55,536 ) (56,871 ) Amortization of net actuarial loss 6,001 6,317 18,003 18,950 24,319 27,136 Net periodic benefit cost $ 9,570 $ 9,392 $ 28,711 $ 28,176 $ 38,103 $ 39,248 SERP Period Ended September 30, Three Months Nine Months Twelve Months 2017 2016 2017 2016 2017 2016 (Thousands of dollars) Service cost $ 77 $ 83 $ 232 $ 248 $ 315 $ 328 Interest cost 471 464 1,413 1,394 1,878 1,818 Amortization of net actuarial loss 361 346 1,081 1,038 1,426 1,361 Net periodic benefit cost $ 909 $ 893 $ 2,726 $ 2,680 $ 3,619 $ 3,507 PBOP Period Ended September 30, Three Months Nine Months Twelve Months 2017 2016 2017 2016 2017 2016 (Thousands of dollars) Service cost $ 367 $ 375 $ 1,101 $ 1,124 $ 1,476 $ 1,534 Interest cost 808 795 2,424 2,386 3,218 3,136 Expected return on plan assets (839 ) (787 ) (2,518 ) (2,362 ) (3,305 ) (3,228 ) Amortization of prior service costs 333 333 1,001 1,001 1,335 1,335 Amortization of net actuarial loss — 104 — 312 105 398 Net periodic benefit cost $ 669 $ 820 $ 2,008 $ 2,461 $ 2,829 $ 3,175

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Segment Information	Note 3 – Segment Information The Company has two reportable segments: natural gas operations and construction 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 related to 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 Construction Other Total Three months ended September 30, 2017 Revenues from external customers $ 213,059 $ 351,850 $ — $ 564,909 Intersegment revenues — 28,244 — 28,244 Total $ 213,059 $ 380,094 $ — $ 593,153 Segment net income (loss) $ (4,024 ) $ 14,335 $ (107 ) $ 10,204 Three months ended September 30, 2016 Revenues from external customers $ 200,179 $ 312,531 $ — $ 512,710 Intersegment revenues — 27,259 — 27,259 Total $ 200,179 $ 339,790 $ — $ 539,969 Segment net income (loss) $ (12,405 ) $ 14,877 $ — $ 2,472 Natural Gas Construction Other Total Nine months ended September 30, 2017 Revenues from external customers $ 935,823 $ 800,073 $ — $ 1,735,896 Intersegment revenues — 72,463 — 72,463 Total $ 935,823 $ 872,536 $ — $ 1,808,359 Segment net income (loss) $ 82,436 $ 15,717 $ (777 ) $ 97,376 Nine months ended September 30, 2016 Revenues from external customers $ 980,927 $ 762,835 $ — $ 1,743,762 Intersegment revenues — 75,203 — 75,203 Total $ 980,927 $ 838,038 $ — $ 1,818,965 Segment net income $ 67,536 $ 19,325 $ — $ 86,861 Natural Gas Construction Other Total Twelve months ended September 30, 2017 Revenues from external customers $ 1,276,308 $ 1,078,195 $ — $ 2,354,503 Intersegment revenues — 95,381 — 95,381 Total $ 1,276,308 $ 1,173,576 $ — $ 2,449,884 Segment net income (loss) $ 134,323 $ 29,010 $ (777 ) $ 162,556 Twelve months ended September 30, 2016 Revenues from external customers $ 1,376,388 $ 1,022,416 $ — $ 2,398,804 Intersegment revenues — 105,566 — 105,566 Total $ 1,376,388 $ 1,127,982 $ — $ 2,504,370 Segment net income $ 119,836 $ 33,144 $ — $ 152,980

Derivatives and Fair Value Meas

Derivatives and Fair Value Measurements	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Derivatives and Fair Value Measurements	Note 4 – Derivatives and Fair Value Measurements Derivatives. fixed-for-floating 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, during time frames ranging from October 2017 through March 2019. 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): September 30, 2017 December 31, 2016 Contract notional amounts 10,936 10,543 Southwest does not utilize derivative financial instruments for speculative purposes, nor does it have trading operations. The following table sets forth the gains and (losses) recognized on the Swaps (derivatives) for the three-, nine-, and twelve-month periods ended September 30, 2017 and 2016 and their location in the Condensed Consolidated Statements of Income for both the Company and Southwest: Gains (losses) recognized in income for derivatives not designated as hedging instruments: (Thousands of dollars) Three Months Ended Nine Months Ended Twelve Months Ended Location of Gain or (Loss) September 30 September 30 September 30 Instrument Recognized in Income on Derivative 2017 2016 2017 2016 2017 2016 Swaps Net cost of gas sold $ (546 ) $ (2,072 ) $ (6,851 ) $ 2,253 $ (4,098 ) $ (656 ) Swaps Net cost of gas sold 546 * 2,072 * 6,851 * (2,253 )* 4,098 * 656 * Total $ — $ — $ — $ — $ — $ — * Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities. No gains (losses) were recognized in net income or other comprehensive income during the periods presented for derivatives designated as cash flow hedging instruments. Previously, Southwest entered into two forward-starting interest rate swaps (“FSIRS”), both of which were designated cash flow hedges, to partially hedge the risk of interest rate variability during the period leading up to the planned issuance of debt. The first FSIRS terminated in December 2010. The second FSIRS terminated in March 2012. Losses on both FSIRS are being amortized over ten-year The following table sets forth the fair values of the Swaps and their location in the Condensed Consolidated Balance Sheets for both the Company and Southwest (thousands of dollars): Fair values of derivatives not designated as hedging instruments: September 30, 2017 Instrument Balance Sheet Location Asset Liability Net Swaps Prepaids and other current assets $ 56 $ (22 ) $ 34 Swaps Other current liabilities 27 (1,899 ) (1,872 ) Swaps Other deferred credits 1 (768 ) (767 ) Total $ 84 $ (2,689 ) $ (2,605 ) December 31, 2016 Instrument Balance Sheet Location Asset Liability Net Swaps Deferred charges and other assets $ 899 $ (54 ) $ 845 Swaps Prepaids and other current assets 3,551 (19 ) 3,532 Total $ 4,450 $ (73 ) $ 4,377 The estimated fair values of the natural gas derivatives were determined using future natural gas index prices (as more fully described below). 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. 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 purchased gas adjustment (“PGA”) mechanism in determining its deferred PGA balances. Neither changes in fair value, nor settled amounts, of Swaps have a direct effect on earnings or other comprehensive income. The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps. Three Months Ended Nine Months Ended Twelve Months Ended (Thousands of dollars) September 30, 2017 September 30, 2017 September 30, 2017 Paid to counterparties $ 143 $ 1,555 $ 2,655 Received from counterparties $ — $ 1,685 $ 2,060 The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Condensed Consolidated Balance Sheets for both the Company and Southwest (thousands of dollars). September 30, 2017 Instrument Balance Sheet Location Net Total Swaps Other current liabilities $ (34 ) Swaps Prepaids and other current assets 1,872 Swaps Deferred charges and other assets 767 December 31, 2016 Instrument Balance Sheet Location Net Total Swaps Other deferred credits $ (845 ) Swaps Other current liabilities (3,532 ) Fair Value Measurements. The following table sets forth, by level within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability, the financial assets and liabilities that were accounted for at fair value by both the Company and Southwest: Level 2—Significant other observable inputs (Thousands of dollars) September 30, 2017 December 31, 2016 Assets at fair value: Prepaids and other current assets—Swaps $ 34 $ 3,532 Deferred charges and other assets—Swaps — 845 Liabilities at fair value: Other current liabilities—Swaps (1,872 ) — Other deferred credits—Swaps (767 ) — Net Assets (Liabilities) $ (2,605 ) $ 4,377 No financial assets or liabilities associated with the Swaps, which were accounted for at fair value, fell within Level 1 (quoted prices in active markets for identical financial assets) or Level 3 (significant unobservable inputs) of the fair value hierarchy. With regard to the fair values of assets associated with pension and postretirement benefit plans, asset values were last updated as required as of December 2016. Refer to Note 10 – Pension and Other Post Retirement Benefits in the 2016 Annual Report to Shareholders on Form 10-K.

Common Stock

Common Stock	9 Months Ended
	Sep. 30, 2017
Text Block [Abstract]
Common Stock	Note 5 – Common Stock In January 2017, the holding company reorganization was made effective and each outstanding share of Southwest Gas Corporation common stock was converted into a share of common stock in Southwest Gas Holdings, Inc., on a one-for-one On March 29, 2017, the Company filed with the Securities Exchange Commission (“SEC”) an automatic shelf registration statement on Form S-3 No. 333-217018), at-the-market rd During the nine months ended September 30, 2017, the Company issued approximately 103,000 shares of common stock through the Restricted Stock/Unit Plan and Management Incentive Plan.

Long-Term Debt

Long-Term Debt	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Long-Term Debt	Note 6 – Long-Term Debt Carrying amounts of long-term debt and related estimated fair values as of September 30, 2017 and December 31, 2016 are disclosed in the following table. Southwest’s revolving credit facility (including commercial paper) and the variable-rate Industrial Development Revenue Bonds (“IDRBs”) approximate their carrying values, as they are repaid quickly (in the case of credit facility borrowings) and have interest rates that reset frequently. These 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. The fair values of Southwest’s debentures, senior notes, and fixed-rate IDRBs 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 and fixed-rate IDRBs 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 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. September 30, 2017 December 31, 2016 Carrying Market Carrying Market Amount Value Amount Value (Thousands of dollars) Southwest Gas Corporation: Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 130,325 $ 125,000 $ 129,703 Notes, 6.1%, due 2041 125,000 154,434 125,000 149,734 Notes, 3.875%, due 2022 250,000 258,943 250,000 254,900 Notes, 4.875%, due 2043 250,000 275,168 250,000 266,793 Notes, 3.8%, due 2046 300,000 292,578 300,000 283,029 8% Series, due 2026 75,000 97,218 75,000 94,691 Medium-term notes, 7.59% series, due 2017 — — 25,000 25,040 Medium-term notes, 7.78% series, due 2022 25,000 29,174 25,000 29,290 Medium-term notes, 7.92% series, due 2027 25,000 31,964 25,000 31,905 Medium-term notes, 6.76% series, due 2027 7,500 8,920 7,500 8,769 Unamortized discount and debt issuance costs (9,498 ) (9,931 ) 1,173,002 1,197,569 Revolving credit facility and commercial paper 150,000 150,000 5,000 5,000 Industrial development revenue bonds: Variable-rate bonds: Tax-exempt 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 (2,212 ) (2,489 ) 197,788 197,511 Less: current maturities — (25,000 ) Long-term debt, less current maturities - Southwest Gas Corporation $ 1,520,790 $ 1,375,080 Centuri: Centuri term loan facility $ 107,250 107,403 $ 106,700 106,819 Unamortized debt issuance costs (383 ) (516 ) 106,867 106,184 Centuri secured revolving credit facility 81,250 81,402 41,185 41,292 Centuri other debt obligations 51,527 51,978 52,635 52,840 Less: current maturities (28,453 ) (25,101 ) Long-term debt, less current maturities - Centuri $ 211,191 $ 174,903 Consolidated Southwest Gas Holdings, Inc.: Southwest Gas Corporation long-term debt $ 1,520,790 $ 1,400,080 Centuri long-term debt 239,644 200,004 Less: current maturities (28,453 ) (50,101 ) Long-term debt, less current maturities - Southwest Gas Holdings, Inc. $ 1,731,981 $ 1,549,983 In March 2017, Southwest amended its credit facility, increasing the borrowing capacity from $300 million to $400 million. Also, the facility was previously scheduled to expire in March 2021 and was extended to March 2022. Southwest continues to designate $150 million of capacity related to the facility as long-term debt and with the total capacity now available, 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 the Southwest’s senior unsecured debt rating. At September 30, 2017, 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 September 30, 2017, $150 million was outstanding on the long-term portion and $83 million was outstanding on the short-term portion of this credit facility (See Note 7 – Short-Term Debt At September 30, 2017, Centuri has a $300 million secured revolving credit and term loan facility that is scheduled to expire in October 2019. This facility includes a revolving credit facility and a term loan facility. The term loan facility portion had an initial limit of approximately $150 million, which was reached in 2014 and had $107 million outstanding (after repayments) at September 30, 2017. The $300 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 assets securing the facility at September 30, 2017 totaled $526 million. At September 30, 2017, $189 million in borrowings were outstanding under the Centuri facility.

Short-Term Debt

Short-Term Debt	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Short-Term Debt	Note 7 – Short-Term Debt In March 2017, Southwest Gas Holdings, Inc. entered into a credit facility with a borrowing capacity of $100 million that expires in March 2022. The Company intends to utilize this facility for short-term financing needs. Interest rates for this facility are calculated at either the LIBOR or the “alternate base rate,” plus in each case an applicable margin that is determined based on the Company’s senior unsecured debt rating. The applicable margin ranges from 0.75% to 1.50% for loans bearing interest with reference to LIBOR and from 0% to 0.5% for loans bearing interest with reference to the alternative base rate. The Company is also required to pay a commitment fee on the unfunded portion of the commitments based on its senior unsecured long-term debt rating. The commitment fee ranges from 0.075% to 0.200% per annum. At September 30, 2017, $27.5 million was outstanding under this facility. As discussed in Note 6 – Long-Term Debt

Equity, Other Comprehensive Inc

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	Note 8 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income The table below provides details of activity in equity and the redeemable noncontrolling interest for Southwest Gas Holdings, Inc. on a consolidated basis during the nine months ended September 30, 2017. Southwest Gas Holdings, Inc. Equity Accumulated Redeemable Additional Other Non- Noncontrolling Common Stock Paid-in Comprehensive Retained controlling Interest (In thousands, except per share amounts) Shares Amount Capital Income (Loss) Earnings Interest Total (Temporary DECEMBER 31, 2016 47,482 $ 49,112 $ 903,123 $ (48,008 ) $ 759,263 $ (2,217 ) $ 1,661,273 $ 22,590 Common stock issuances 250 250 21,090 21,340 Net income (loss) 97,376 (78 ) 97,298 248 Redemption value adjustments (355 ) (355 ) 355 Foreign currency exchange translation adj. 1,850 1,850 11 Redemption of Centuri shares from noncontrolling parties (23,000 ) Other comprehensive income (loss): Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax 1,786 1,786 Amounts reclassified to net income, net of tax (FSIRS) 1,554 1,554 Centuri dividend to redeemable noncontrolling interest (204 ) Dividends declared Common: $1.485 per share (71,350 ) (71,350 ) SEPTEMBER 30, 2017 47,732 $ 49,362 $ 924,213 $ (42,818 ) $ 784,934 $ (2,295 ) $ 1,713,396 $ — The table below provides details of activity in equity for Southwest Gas Corporation during the nine months ended September 30, 2017. Effective in January 2017, Southwest became a subsidiary of Southwest Gas Holdings, Inc., and only equity shares of the latter are publicly traded, under the ticker symbol “SWX.” Southwest Gas Corporation Equity Accumulated Additional Other Common Stock Paid-in Comprehensive Retained (In thousands, except per share amounts) Shares Amount Capital Income (Loss) Earnings Total DECEMBER 31, 2016 47,482 $ 49,112 $ 897,346 $ (45,639 ) $ 767,061 $ 1,667,880 Net income 82,436 82,436 Other comprehensive income (loss): Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax 1,786 1,786 Amounts reclassified to net income, net of tax (FSIRS) 1,554 1,554 Distribution to Southwest Gas Holdings, Inc. investment in discontinued operations (182,773 ) (182,773 ) Stock-based compensation (a) 8,576 (587 ) 7,989 Dividends declared to Southwest Gas Holdings, Inc. (60,130 ) (60,130 ) Contributions from Southwest Gas Holdings, Inc. 11,659 11,659 SEPTEMBER 30, 2017 47,482 $ 49,112 $ 917,581 $ (42,299 ) $ 606,007 $ 1,530,401 (a) Stock-based compensation is based on stock awards of Southwest Gas Corporation to be issued in shares of Southwest Gas Holdings, Inc. The table above gives effect to the holding company reorganization whereby Southwest and Centuri became subsidiaries of the Company. The historic investment in Centuri was distributed to the parent holding company. This presentation is only applicable to Southwest and not to the Company overall, as Centuri continues to be included in the continuing operations of the Company. Also in connection with the holding company creation, compensation plans of Southwest include programs that will be settled with equity shares issued by Southwest Gas Holdings, Inc. Management has determined that when no consideration is directly exchanged for these programs between Southwest and the Company, the accounting impact at Southwest for these programs is reflected both as compensation expense and as an equity contribution (of the parent) in Southwest. 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 in the Condensed Consolidated Balance Sheets and the associated column in the equity table above, as well as the Redeemable Noncontrolling Interest. See Note 4 – Derivatives and Fair Value Measurements Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Thousands of dollars) Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 Before- Tax Net-of- Before- Tax Net-of- Tax (Expense) Tax Tax (Expense) Tax Amount or Benefit (1) Amount Amount or Benefit (1) Amount Defined benefit pension plans: Amortization of prior service cost $ 333 $ (126 ) $ 207 $ 333 $ (126 ) $ 207 Amortization of net actuarial (gain)/loss 6,362 (2,418 ) 3,944 6,767 (2,571 ) 4,196 Regulatory adjustment (5,734 ) 2,179 (3,555 ) (6,122 ) 2,326 (3,796 ) Pension plans other comprehensive income (loss) 961 (365 ) 596 978 (371 ) 607 FSIRS (designated hedging activities): Amounts reclassifed into net income 835 (317 ) 518 835 (317 ) 518 FSIRS other comprehensive income 835 (317 ) 518 835 (317 ) 518 Total other comprehensive income (loss) - Southwest Gas Corporation 1,796 (682 ) 1,114 1,813 (688 ) 1,125 Foreign currency translation adjustments: Translation adjustments 1,012 — 1,012 (238 ) — (238 ) Foreign currency other comprehensive income (loss) 1,012 — 1,012 (238 ) — (238 ) Total other comprehensive income (loss) - Southwest Gas Holdings, Inc. $ 2,808 $ (682 ) $ 2,126 $ 1,575 $ (688 ) $ 887 Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2017 Before- Tax Net-of- Before- Tax Net-of- Tax (Expense) Tax Tax (Expense) Tax Amount or Benefit (1) Amount Amount or Benefit (1) Amount Defined benefit pension plans: Amortization of prior service cost $ 1,001 $ (380 ) $ 621 $ 1,001 $ (380 ) $ 621 Amortization of net actuarial (gain)/loss 19,084 (7,252 ) 11,832 20,300 (7,714 ) 12,586 Regulatory adjustment (17,204 ) 6,537 (10,667 ) (18,368 ) 6,980 (11,388 ) Pension plans other comprehensive income (loss) 2,881 (1,095 ) 1,786 2,933 (1,114 ) 1,819 FSIRS (designated hedging activities): Amounts reclassifed into net income 2,507 (953 ) 1,554 2,508 (952 ) 1,556 FSIRS other comprehensive income 2,507 (953 ) 1,554 2,508 (952 ) 1,556 Total other comprehensive income (loss)—Southwest Gas Corporation 5,388 (2,048 ) 3,340 5,441 (2,066 ) 3,375 Foreign currency translation adjustments: Translation adjustments 1,861 — 1,861 614 — 614 Foreign currency other comprehensive income (loss) 1,861 — 1,861 614 — 614 Total other comprehensive income (loss) $ 7,249 $ (2,048 ) $ 5,201 $ 6,055 $ (2,066 ) $ 3,989 Twelve Months Ended September 30, 2017 Twelve Months Ended September 30, 2016 Before- Tax Net-of- Before- Tax Net-of- Tax (Expense) Tax Tax (Expense) Tax Amount or Benefit (1) Amount Amount or Benefit (1) Amount Defined benefit pension plans: Net actuarial gain/(loss) $ (22,770 ) $ 8,652 $ (14,118 ) $ (30,519 ) $ 11,597 $ (18,922 ) Amortization of prior service cost 1,335 (507 ) 828 1,335 (507 ) 828 Amortization of net actuarial (gain)/loss 25,850 (9,823 ) 16,027 28,895 (10,980 ) 17,915 Regulatory adjustment (4,420 ) 1,679 (2,741 ) (653 ) 249 (404 ) Pension plans other comprehensive income (loss) (5 ) 1 (4 ) (942 ) 359 (583 ) FSIRS (designated hedging activities): Amounts reclassifed into net income 3,344 (1,271 ) 2,073 3,344 (1,271 ) 2,073 FSIRS other comprehensive income (loss) 3,344 (1,271 ) 2,073 3,344 (1,271 ) 2,073 Total other comprehensive income (loss)—Southwest Gas Corporation 3,339 (1,270 ) 2,069 2,402 (912 ) 1,490 Foreign currency translation adjustments: Translation adjustments 1,408 — 1,408 233 — 233 Foreign currency other comprehensive income (loss) 1,408 — 1,408 233 — 233 Total other comprehensive income (loss)—Southwest Gas Holdings, Inc. $ 4,747 $ (1,270 ) $ 3,477 $ 2,635 $ (912 ) $ 1,723 (1) Tax amounts are calculated using a 38% rate. 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, the Company is not recognizing any tax effect or presenting a tax expense or benefit for the currency translation adjustment amount reported in Other Comprehensive Income, as repatriation of earnings is not anticipated. Approximately $2.1 million of realized losses (net of tax) related to the FSIRS, reported in Accumulated other comprehensive income (“AOCI”) at September 30, 2017, 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: AOCI—Rollforward (Thousands of dollars) Defined Benefit Plans FSIRS Foreign Currency Items Before-Tax Tax After-Tax Before-Tax Tax After-Tax Before-Tax Tax After-Tax AOCI Beginning Balance AOCI December 31, 2016 $ (57,613 ) $ 21,893 $ (35,720 ) $ (15,999 ) $ 6,080 $ (9,919 ) $ (2,369 ) $ — $ (2,369 ) $ (48,008 ) Translation adjustments — — — — — — 1,861 — 1,861 1,861 Other comprehensive income before reclassifications — — — — — — 1,861 — 1,861 1,861 FSIRS amounts reclassified from AOCI (1) — — — 2,507 (953 ) 1,554 — — — 1,554 Amortization of prior service cost (2) 1,001 (380 ) 621 — — — — — — 621 Amortization of net actuarial loss (2) 19,084 (7,252 ) 11,832 — — — — — — 11,832 Regulatory adjustment (3) (17,204 ) 6,537 (10,667 ) — — — — — — (10,667 ) Net current period other comprehensive income (loss) 2,881 (1,095 ) 1,786 2,507 (953 ) 1,554 1,861 — 1,861 5,201 Less: Translation adjustment attributable to redeemable noncontrolling interest — — — — — — 11 — 11 11 Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. 2,881 (1,095 ) 1,786 2,507 (953 ) 1,554 1,850 — 1,850 5,190 Ending Balance AOCI September 30, 2017 $ (54,732 ) $ 20,798 $ (33,934 ) $ (13,492 ) $ 5,127 $ (8,365 ) $ (519 ) $ — $ (519 ) $ (42,818 ) (1) The FSIRS reclassification amounts are included in the Net interest deductions line item 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 (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 the Deferred charges and other assets line item on the Company’s Condensed Consolidated Balance Sheets). (4) Tax amounts are calculated using a 38% rate. The following table represents a rollforward of AOCI, presented on Southwest’s Condensed Consolidated Balance Sheets: AOCI—Rollforward (Thousands of dollars) Defined Benefit Plans FSIRS Before-Tax Tax After-Tax Before-Tax Tax After-Tax AOCI Beginning Balance AOCI December 31, 2016 $ (57,613 ) $ 21,893 $ (35,720 ) $ (15,999 ) $ 6,080 $ (9,919 ) $ (45,639 ) FSIRS amounts reclassified from AOCI (5) — — — 2,507 (953 ) 1,554 1,554 Amortization of prior service cost (6) 1,001 (380 ) 621 — — — 621 Amortization of net actuarial loss (6) 19,084 (7,252 ) 11,832 — — — 11,832 Regulatory adjustment (7) (17,204 ) 6,537 (10,667 ) — — — (10,667 ) Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation 2,881 (1,095 ) 1,786 2,507 (953 ) 1,554 3,340 Ending Balance AOCI September 30, 2017 $ (54,732 ) $ 20,798 $ (33,934 ) $ (13,492 ) $ 5,127 $ (8,365 ) $ (42,299 ) (5) The FSIRS reclassification amounts are included in the Net interest deductions line item on Southwest’s Condensed Consolidated Statements of Income. (6) These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost (7) The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on Southwest’s Condensed Consolidated Balance Sheets). (8) Tax amounts are calculated using a 38% rate. The following table represents amounts (before income tax impacts) included in AOCI (in the tables above), that have not yet been recognized in net periodic benefit cost: Amounts Recognized in AOCI (Before Tax) (Thousands of dollars) September 30, 2017 December 31, 2016 Net actuarial (loss) gain $ (411,889 ) $ (430,973 ) Prior service cost (4,702 ) (5,703 ) Less: amount recognized in regulatory assets 361,859 379,063 Recognized in AOCI $ (54,732 ) $ (57,613 )

Construction Services Redeemabl

Construction Services Redeemable Noncontrolling Interest	9 Months Ended
	Sep. 30, 2017
Noncontrolling Interest [Abstract]
Construction Services Redeemable Noncontrolling Interest	Note 9 – Construction Services Redeemable Noncontrolling Interest In conjunction with the acquisition of the Canadian construction businesses in October 2014, the previous owners of the acquired companies retained a 3.4% equity interest in Centuri, which, subject to an eligibility timeline, would have been redeemable at the election of the noncontrolling parties (in its entirety) beginning in July 2022. In August 2017, in advance of when otherwise eligible, the parties agreed to a current redemption. Southwest Gas Holdings, Inc. paid $23 million to the previous owners, thereby acquiring the remaining 3.4% equity interest in Centuri in accordance with an early redemption agreement. Accordingly, Centuri is now a wholly owned subsidiary of the Company. The following depicts changes to the balance of the redeemable noncontrolling interest between the indicated periods. Redeemable (Thousands of dollars): Balance, December 31, 2016 $ 22,590 Net income attributable to redeemable noncontrolling interest 248 Foreign currency exchange translation adjustment 11 Centuri dividend to redeemable noncontrolling interest (204 ) Adjustment to redemption value 355 Redemption of Centuri shares from noncontrolling parties (23,000 ) Balance, September 30, 2017 $ —

Reorganization Impacts - Discon

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation	9 Months Ended
	Sep. 30, 2017
Restructuring and Related Activities [Abstract]
Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation	Note 10 – Reorganization Impacts – Discontinued Operations Solely Related to Southwest Gas Corporation No substantive change has occurred with regard to the Company’s business segments on the whole, or in the primary businesses comprising those segments (Centuri operations continue to be part of continuing operations of the controlled group of companies), and financial information related to Centuri continues to be included in condensed consolidated financial statements of Southwest Gas Holdings, Inc. However, as part of the holding company reorganization effective January 2017, Centuri is no longer a subsidiary of Southwest; whereas historically, Centuri had been a direct subsidiary of Southwest. To give effect to this change, the condensed consolidated financial statements related to Southwest Gas Corporation, which are separately included in this Form 10-Q, Due to the discontinued operations accounting reflection, the following disclosures provide additional information regarding the assets, liabilities, equity, revenues, and expenses of Centuri which are shown as discontinued operations on the condensed consolidated financial statements of Southwest Gas Corporation for periods prior to the beginning of 2017. The following table presents the major categories of assets and liabilities within the amounts reported as discontinued operations – construction services in the Condensed Consolidated Balance Sheet of Southwest Gas Corporation: (Thousands of dollars) December 31, 2016 Assets: Other property and investments $ 233,774 Cash and cash equivalents 9,042 Accounts receivable, net of allowances 173,300 Prepaids and other current assets 10,470 Goodwill 129,888 Other noncurrent assets 22,897 Discontinued operations - construction services - assets $ 579,371 Liabilities: Current maturities of long-term debt $ 25,101 Accounts payable 46,440 Other current liabilities 74,518 Long-term debt, less current maturities 174,903 Deferred income taxes and other deferred credits 59,653 Discontinued operations—construction services—liabilities $ 380,615 The following table presents the components of the Discontinued operations – construction services non-owner (Thousands of dollars) December 31, 2016 Construction services equity $ (4,390 ) Construction services noncontrolling interest (2,217 ) Construction services redeemable noncontrolling interest 22,590 Discontinued operations - construction services non-owner $ 15,983 The following table presents the major income statement components of discontinued operations – construction services reported in the Condensed Consolidated Income Statements of Southwest Gas Corporation: Results of Construction Services Three Nine Twelve Twelve Months Ended Months Ended Months Ended Months Ended (Thousands of dollars) September 30, 2016 September 30, 2016 September 30, 2017 September 30, 2016 Construction revenues $ 339,790 $ 838,038 $ 301,040 $ 1,127,982 Operating expenses: Construction expenses 300,611 757,919 266,504 1,009,188 Depreciation and amortization 13,409 43,351 12,318 58,368 Operating income 25,770 36,768 22,218 60,426 Other income (deductions) 44 44 1,149 1,246 Net interest deductions 1,794 4,945 1,718 6,738 Income before income taxes 24,020 31,867 21,649 54,934 Income tax expense 8,708 12,042 7,842 20,711 Net income 15,312 19,825 13,807 34,223 Net income attributable to noncontrolling interests 435 500 514 1,079 Discontinued operations - construction services - net income $ 14,877 $ 19,325 $ 13,293 $ 33,144

Nature of Operations and Basi17

Nature of Operations and Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations. Note 9 – Construction Services Redeemable Noncontrolling Interest In January 2017, a previously contemplated and approved reorganization under a holding company structure was made effective. The reorganization was designed to provide further separation between regulated and unregulated businesses, and to provide additional financing flexibility. Coincident with the effective date of the reorganization, existing shareholders of Southwest Gas Corporation became shareholders of Southwest Gas Holdings, Inc., on a one-for-one Southwest Gas Corporation (“Southwest” or the “natural gas operations segment”) 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 construction services enterprise dedicated to meeting the growing demands of North American utilities, energy and industrial markets. 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”, formerly Link-Line Contractors Ltd.), W.S. Nicholls Construction, Inc. (“W.S. Nicholls”), and Brigadier Pipelines Inc. (“Brigadier”). Typically, Centuri revenues are lowest during the first quarter of the year due to unfavorable winter weather conditions. Operating revenues typically improve as more favorable weather conditions occur during the summer and fall months.
Basis of Presentation	Basis of Presentation. Form 10-Q, Note 10 – Reorganization Impacts – Discontinued Operations Solely Related to Southwest Gas Corporation non-current 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 organizational changes. Centuri operations continue to be part of continuing operations and included in the consolidated financial statements of Southwest Gas Holdings, Inc. 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 statement 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 2016 Annual Report to Shareholders, which is incorporated by reference into the 2016 Form 10-K.
Prepaids and Other Current Assets	Prepaids and other current assets.
Cash and Cash Equivalents	Cash and Cash Equivalents. Significant non-cash non-cash
Adoption of Accounting Standards Update	Adoption of Accounting Standards Update (“ASU”) No. 2016-09. No. 2016-09 Under the new guidance, the Company can withhold any amount between the minimum and maximum individual statutory tax rates and still treat the entire award as equity. The Company intends to administer withholding such that awards under stock compensation programs will continue to be treated as equity awards. In addition to the above, the update requires all income tax-related Amendments related to the timing of when excess tax benefits are recognized, minimum statutory withholding requirements, forfeitures, and intrinsic value are required to be applied using a modified retrospective transition method by means of a cumulative-effect adjustment to equity as of the beginning of the period in which the guidance is adopted. The Company had no previously unrecognized tax benefits as a result of these changes; therefore, no cumulative effect adjustment to the Company’s opening retained earnings was required.
Goodwill	Goodwill. (In thousands of dollars) Natural Gas Construction Consolidated December 31, 2016 $ 10,095 $ 129,888 $ 139,983 Foreign currency translation adjustment — 7,882 7,882 September 30, 2017 $ 10,095 $ 137,770 $ 147,865
Intercompany Transactions	Intercompany Transactions Note 3— Segment Information September 30, 2017 December 31, 2016 Centuri accounts receivable for services provided to Southwest $ 11,486 $ 10,585 The accounts receivable balance, revenues, and associated profits are included in the condensed consolidated financial statements of the Company and were not eliminated during consolidation in accordance with accounting treatment for rate-regulated entities.
Other Property and Investments	Other Property and Investments. September 30, 2017 December 31, 2016 Centuri property and equipment $ 493,599 $ 451,114 Centuri accumulated provision for depreciation and amortization (251,831 ) (228,374 ) Net cash surrender value of COLI policies 114,052 106,744 Other property 13,483 12,859 Total $ 369,303 $ 342,343
Other Income (Deductions)	Other Income (Deductions). Three Months Ended Nine Months Ended Twelve Months Ended September 30 September 30 September 30 2017 2016 2017 2016 2017 2016 Southwest Gas Corporation—natural gas operations segment: Change in COLI policies $ 2,100 $ 2,300 $ 6,800 $ 5,400 $ 8,800 $ 7,500 Interest income 670 522 1,848 1,279 2,417 1,664 Equity AFUDC 968 611 2,077 1,893 2,473 2,890 Miscellaneous income and (expense) (657 ) (912 ) (1,981 ) (1,860 ) (3,382 ) (2,439 ) Southwest Gas Corporation—total other income (deductions) 3,081 2,521 8,744 6,712 10,308 9,615 Construction services segment: Interest income 1 — 2 1 2 414 Foreign transaction gain (loss) (442 ) (3 ) (640 ) (22 ) (640 ) 28 Miscellaneous income and (expense) 231 47 676 65 1,825 804 Centuri—total other income (deductions) (210 ) 44 38 44 1,187 1,246 Corporate and administrative 5 — 6 — 6 — Consolidated Southwest Gas Holdings, Inc.—total other income (deductions) $ 2,876 $ 2,565 $ 8,788 $ 6,756 $ 11,501 $ 10,861 Included in the table above is the change in cash surrender values of company-owned life insurance (“COLI”) policies (including net death benefits recognized). 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. Current tax regulations provide for tax-free
Recently Issued Accounting Standards Updates	Recently Issued Accounting Standards Updates. one-year Deliberations have been ongoing by the utility industry, notably in connection with efforts to produce an accounting guide intended to be developed by the American Institute of Certified Public Accountants (“AICPA”). In association with this undertaking, the AICPA formed a number of industry task forces, including a Power & Utilities (“P&U”) Task Force, on which Company personnel actively participate via formal membership. Industry representatives and organizations, the largest auditing firms, the AICPA’s Revenue Recognition Working Group and its Financial Reporting Executive Committee have undertaken, and continue to undertake, consideration of several items relevant to the utility industry. Where applicable or necessary, the FASB’s Transition Resource Group (“TRG”) has also participated. Through the P&U Task Force undertakings, general determinations were made that contributions received in aid of construction (“CIAC”) efforts related to the industry’s pipe distribution and transmission systems are reimbursements of expenditures rather than revenue (consistent with current accounting practices). Furthermore, regarding the “collectibility” criterion in the update that must be met for revenue recognition, general determinations have been made that contracts for utility service (including service to lower income or lower credit quality customers) represent genuine and valid contracts for which revenue is able to be recognized when service is rendered (consistent with current accounting practices). These determinations by the P&U industry are based on the various measures the industry takes to help ensure collectibility (e.g., proof of creditworthiness, customer deposits, late fee assessment, disconnection, service re-establishment With regard to the construction services segment, the principles of the new revenue recognition guidance are very similar to existing guidance for construction contractors. Similar to the P&U Task Force noted above, the AICPA formed the Engineering and Construction Contractors Task Force to assist the construction industry with implementing the new guidance. The accounting guide the AICPA intends to release is expected to provide implementation guidance related to several issues including 1) combining contracts and separating performance obligations; 2) estimating change orders, incentives, penalties, liquidated damages and other variable consideration items and 3) acceptable measures of progress when recognizing revenue over time. Management of both segments of the Company has substantially completed assessments of sources of revenue and the effects that adoption of the new guidance will have on the Company’s (and Southwest’s in the case of utility operations) financial position, results of operations, and cash flows. Based on assessments completed to date, management believes that such impacts will not be material overall. Presentation and disclosure requirements of the new guidance will have the most impact on the Company’s financial statements and note disclosures. The Company is currently planning to adopt the new guidance in 2018 under the modified retrospective transition method, as permissible. In January 2016, the FASB issued the update “Financial Instruments – Overall (Subtopic 825-10): In February 2016, the FASB issued the update “Leases (Topic 842).” Under the update, lessees will be required to recognize the following for all leases (with the exception of short-term leases) at the commencement date: • A lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and • A right-of-use Under the new guidance, lessor accounting is largely unchanged. Certain targeted improvements were made to align, where necessary, lessor accounting with the lessee accounting model and Topic 606, Revenue from Contracts with Customers. Though companies have historically been required to make disclosures regarding leases and of associated contractual obligations, leases (with terms longer than a year) will no longer exist off-balance In June 2016, the FASB issued the update “Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” The update amends guidance on reporting credit losses for financial assets held at amortized cost basis and available for sale debt securities. For assets held at amortized cost basis, the update eliminates the “probable” threshold for initial recognition of credit losses in current U.S. GAAP and, instead, requires an entity to reflect its current estimate of all expected credit losses. The allowance for credit losses is a valuation account that is deducted from the amortized cost basis of the financial asset to present the net amount expected to be collected. For available for sale debt securities, credit losses should be measured in a manner similar to current U.S. GAAP, however the update will require that credit losses be presented as an allowance rather than as a write-down. This update affects entities holding financial assets and net investment in leases that are not accounted for at fair value through net income. The update affects loans, debt securities, trade receivables, net investments in leases, off-balance In August 2016, the FASB issued the update “Classification of Certain Cash Receipts and Cash Payments.” This update addresses the following specific cash flow issues: debt prepayment or debt extinguishment costs; settlement of zero-coupon In October 2016, the FASB issued the update “Accounting for Income Taxes: Intra-Entity Asset Transfers of Assets Other than Inventory.” This update eliminates the current U.S. GAAP exception for all intra-entity sales of assets other than inventory. As a result, a reporting entity would recognize the tax expense from the sale of the asset in the seller’s tax jurisdiction when the transfer occurs, even though the pre-tax In January 2017, the FASB issued the update “Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment.” The update eliminates Step 2 from the goodwill impairment test. The annual, or interim, goodwill impairment test is performed by comparing the fair value of a reporting unit with its carrying amount. An impairment charge should be recognized for the amount by which the carrying amount exceeds the reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. In addition, income tax effects from any tax-deductible In March 2017, the FASB issued the update “Compensation – Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost.” The update applies to all employers that offer employee benefits under defined benefit pension plans, other postretirement benefit plans, or other types of benefits accounted for under Topic 715, Compensation – Retirement Benefits. The update requires that an employer report the service cost component in the same line item or items as other compensation costs arising from services rendered by the employees during the period. The other components of net benefit cost are required to be presented in the income statement separately from the service cost component and outside a subtotal of income from operations, and be appropriately described. The update also allows only the service cost component (and not the other components of periodic benefit costs) to be eligible for capitalization when applicable, making no exception for specialized industries, including rate-regulated Southwest is a rate-regulated utility offering pension and postretirement benefits to retired employees. It is anticipated that Southwest would continue to request recovery of the total costs of defined benefit plans in rate applications filed with its various regulatory bodies. Rate-regulated entities providing utility and transmission services have historically capitalized a portion of periodic benefit costs (including non-service non-service non-service non-service

Nature of Operations and Basi18

Nature of Operations and Basis of Presentation (Tables)	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Goodwill	(In thousands of dollars) Natural Gas Construction Consolidated December 31, 2016 $ 10,095 $ 129,888 $ 139,983 Foreign currency translation adjustment — 7,882 7,882 September 30, 2017 $ 10,095 $ 137,770 $ 147,865
Accounts Receivable for Services	Centuri’s accounts receivable for these services are presented in the table below (thousands of dollars): September 30, 2017 December 31, 2016 Centuri accounts receivable for services provided to Southwest $ 11,486 $ 10,585
Schedule of Other Property and Investments	Other property and investments on the Southwest Gas Holdings, Inc. Condensed Consolidated Balance Sheets includes (thousands of dollars): September 30, 2017 December 31, 2016 Centuri property and equipment $ 493,599 $ 451,114 Centuri accumulated provision for depreciation and amortization (251,831 ) (228,374 ) Net cash surrender value of COLI policies 114,052 106,744 Other property 13,483 12,859 Total $ 369,303 $ 342,343
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 Nine Months Ended Twelve Months Ended September 30 September 30 September 30 2017 2016 2017 2016 2017 2016 Southwest Gas Corporation—natural gas operations segment: Change in COLI policies $ 2,100 $ 2,300 $ 6,800 $ 5,400 $ 8,800 $ 7,500 Interest income 670 522 1,848 1,279 2,417 1,664 Equity AFUDC 968 611 2,077 1,893 2,473 2,890 Miscellaneous income and (expense) (657 ) (912 ) (1,981 ) (1,860 ) (3,382 ) (2,439 ) Southwest Gas Corporation—total other income (deductions) 3,081 2,521 8,744 6,712 10,308 9,615 Construction services segment: Interest income 1 — 2 1 2 414 Foreign transaction gain (loss) (442 ) (3 ) (640 ) (22 ) (640 ) 28 Miscellaneous income and (expense) 231 47 676 65 1,825 804 Centuri—total other income (deductions) (210 ) 44 38 44 1,187 1,246 Corporate and administrative 5 — 6 — 6 — Consolidated Southwest Gas Holdings, Inc.—total other income (deductions) $ 2,876 $ 2,565 $ 8,788 $ 6,756 $ 11,501 $ 10,861

Components of Net Periodic Be19

Components of Net Periodic Benefit Cost (Tables)	9 Months Ended
	Sep. 30, 2017
Text Block [Abstract]
Schedule of Net Periodic Benefit Costs	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 net periodic benefit costs to the same accounts to which productive labor is charged. As a result, net periodic benefit 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. Qualified Retirement Plan Period Ended September 30, Three Months Nine Months Twelve Months 2017 2016 2017 2016 2017 2016 (Thousands of dollars) Service cost $ 5,848 $ 5,708 $ 17,544 $ 17,125 $ 23,252 $ 23,406 Interest cost 11,520 11,507 34,561 34,520 46,068 45,577 Expected return on plan assets (13,799 ) (14,140 ) (41,397 ) (42,419 ) (55,536 ) (56,871 ) Amortization of net actuarial loss 6,001 6,317 18,003 18,950 24,319 27,136 Net periodic benefit cost $ 9,570 $ 9,392 $ 28,711 $ 28,176 $ 38,103 $ 39,248 SERP Period Ended September 30, Three Months Nine Months Twelve Months 2017 2016 2017 2016 2017 2016 (Thousands of dollars) Service cost $ 77 $ 83 $ 232 $ 248 $ 315 $ 328 Interest cost 471 464 1,413 1,394 1,878 1,818 Amortization of net actuarial loss 361 346 1,081 1,038 1,426 1,361 Net periodic benefit cost $ 909 $ 893 $ 2,726 $ 2,680 $ 3,619 $ 3,507 PBOP Period Ended September 30, Three Months Nine Months Twelve Months 2017 2016 2017 2016 2017 2016 (Thousands of dollars) Service cost $ 367 $ 375 $ 1,101 $ 1,124 $ 1,476 $ 1,534 Interest cost 808 795 2,424 2,386 3,218 3,136 Expected return on plan assets (839 ) (787 ) (2,518 ) (2,362 ) (3,305 ) (3,228 ) Amortization of prior service costs 333 333 1,001 1,001 1,335 1,335 Amortization of net actuarial loss — 104 — 312 105 398 Net periodic benefit cost $ 669 $ 820 $ 2,008 $ 2,461 $ 2,829 $ 3,175

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2017
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 Construction Other Total Three months ended September 30, 2017 Revenues from external customers $ 213,059 $ 351,850 $ — $ 564,909 Intersegment revenues — 28,244 — 28,244 Total $ 213,059 $ 380,094 $ — $ 593,153 Segment net income (loss) $ (4,024 ) $ 14,335 $ (107 ) $ 10,204 Three months ended September 30, 2016 Revenues from external customers $ 200,179 $ 312,531 $ — $ 512,710 Intersegment revenues — 27,259 — 27,259 Total $ 200,179 $ 339,790 $ — $ 539,969 Segment net income (loss) $ (12,405 ) $ 14,877 $ — $ 2,472 Natural Gas Construction Other Total Nine months ended September 30, 2017 Revenues from external customers $ 935,823 $ 800,073 $ — $ 1,735,896 Intersegment revenues — 72,463 — 72,463 Total $ 935,823 $ 872,536 $ — $ 1,808,359 Segment net income (loss) $ 82,436 $ 15,717 $ (777 ) $ 97,376 Nine months ended September 30, 2016 Revenues from external customers $ 980,927 $ 762,835 $ — $ 1,743,762 Intersegment revenues — 75,203 — 75,203 Total $ 980,927 $ 838,038 $ — $ 1,818,965 Segment net income $ 67,536 $ 19,325 $ — $ 86,861 Natural Gas Construction Other Total Twelve months ended September 30, 2017 Revenues from external customers $ 1,276,308 $ 1,078,195 $ — $ 2,354,503 Intersegment revenues — 95,381 — 95,381 Total $ 1,276,308 $ 1,173,576 $ — $ 2,449,884 Segment net income (loss) $ 134,323 $ 29,010 $ (777 ) $ 162,556 Twelve months ended September 30, 2016 Revenues from external customers $ 1,376,388 $ 1,022,416 $ — $ 2,398,804 Intersegment revenues — 105,566 — 105,566 Total $ 1,376,388 $ 1,127,982 $ — $ 2,504,370 Segment net income $ 119,836 $ 33,144 $ — $ 152,980

Derivatives and Fair Value Me21

Derivatives and Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [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): September 30, 2017 December 31, 2016 Contract notional amounts 10,936 10,543
Amount of Gain or Losses Recognized in Income on Derivatives	The following table sets forth the gains and (losses) recognized on the Swaps (derivatives) for the three-, nine-, and twelve-month periods ended September 30, 2017 and 2016 and their location in the Condensed Consolidated Statements of Income for both the Company and Southwest: Gains (losses) recognized in income for derivatives not designated as hedging instruments: (Thousands of dollars) Three Months Ended Nine Months Ended Twelve Months Ended Location of Gain or (Loss) September 30 September 30 September 30 Instrument Recognized in Income on Derivative 2017 2016 2017 2016 2017 2016 Swaps Net cost of gas sold $ (546 ) $ (2,072 ) $ (6,851 ) $ 2,253 $ (4,098 ) $ (656 ) Swaps Net cost of gas sold 546 * 2,072 * 6,851 * (2,253 )* 4,098 * 656 * Total $ — $ — $ — $ — $ — $ — * Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities.
Fair Values of Swaps in Consolidated Balance Sheets	The following table sets forth the fair values of the Swaps and their location in the Condensed Consolidated Balance Sheets for both the Company and Southwest (thousands of dollars): Fair values of derivatives not designated as hedging instruments: September 30, 2017 Instrument Balance Sheet Location Asset Liability Net Swaps Prepaids and other current assets $ 56 $ (22 ) $ 34 Swaps Other current liabilities 27 (1,899 ) (1,872 ) Swaps Other deferred credits 1 (768 ) (767 ) Total $ 84 $ (2,689 ) $ (2,605 ) December 31, 2016 Instrument Balance Sheet Location Asset Liability Net Swaps Deferred charges and other assets $ 899 $ (54 ) $ 845 Swaps Prepaids and other current assets 3,551 (19 ) 3,532 Total $ 4,450 $ (73 ) $ 4,377
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 Nine Months Ended Twelve Months Ended (Thousands of dollars) September 30, 2017 September 30, 2017 September 30, 2017 Paid to counterparties $ 143 $ 1,555 $ 2,655 Received from counterparties $ — $ 1,685 $ 2,060
Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets	The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Condensed Consolidated Balance Sheets for both the Company and Southwest (thousands of dollars). September 30, 2017 Instrument Balance Sheet Location Net Total Swaps Other current liabilities $ (34 ) Swaps Prepaids and other current assets 1,872 Swaps Deferred charges and other assets 767 December 31, 2016 Instrument Balance Sheet Location Net Total Swaps Other deferred credits $ (845 ) Swaps Other current liabilities (3,532 )
Summary of Financial Assets and Liabilities at Fair Value	The following table sets forth, by level within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability, the financial assets and liabilities that were accounted for at fair value by both the Company and Southwest: Level 2—Significant other observable inputs (Thousands of dollars) September 30, 2017 December 31, 2016 Assets at fair value: Prepaids and other current assets—Swaps $ 34 $ 3,532 Deferred charges and other assets—Swaps — 845 Liabilities at fair value: Other current liabilities—Swaps (1,872 ) — Other deferred credits—Swaps (767 ) — Net Assets (Liabilities) $ (2,605 ) $ 4,377

Long-Term Debt (Tables)

Long-Term Debt (Tables)	9 Months Ended
	Sep. 30, 2017
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 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. September 30, 2017 December 31, 2016 Carrying Market Carrying Market Amount Value Amount Value (Thousands of dollars) Southwest Gas Corporation: Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 130,325 $ 125,000 $ 129,703 Notes, 6.1%, due 2041 125,000 154,434 125,000 149,734 Notes, 3.875%, due 2022 250,000 258,943 250,000 254,900 Notes, 4.875%, due 2043 250,000 275,168 250,000 266,793 Notes, 3.8%, due 2046 300,000 292,578 300,000 283,029 8% Series, due 2026 75,000 97,218 75,000 94,691 Medium-term notes, 7.59% series, due 2017 — — 25,000 25,040 Medium-term notes, 7.78% series, due 2022 25,000 29,174 25,000 29,290 Medium-term notes, 7.92% series, due 2027 25,000 31,964 25,000 31,905 Medium-term notes, 6.76% series, due 2027 7,500 8,920 7,500 8,769 Unamortized discount and debt issuance costs (9,498 ) (9,931 ) 1,173,002 1,197,569 Revolving credit facility and commercial paper 150,000 150,000 5,000 5,000 Industrial development revenue bonds: Variable-rate bonds: Tax-exempt 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 (2,212 ) (2,489 ) 197,788 197,511 Less: current maturities — (25,000 ) Long-term debt, less current maturities - Southwest Gas Corporation $ 1,520,790 $ 1,375,080 Centuri: Centuri term loan facility $ 107,250 107,403 $ 106,700 106,819 Unamortized debt issuance costs (383 ) (516 ) 106,867 106,184 Centuri secured revolving credit facility 81,250 81,402 41,185 41,292 Centuri other debt obligations 51,527 51,978 52,635 52,840 Less: current maturities (28,453 ) (25,101 ) Long-term debt, less current maturities - Centuri $ 211,191 $ 174,903 Consolidated Southwest Gas Holdings, Inc.: Southwest Gas Corporation long-term debt $ 1,520,790 $ 1,400,080 Centuri long-term debt 239,644 200,004 Less: current maturities (28,453 ) (50,101 ) Long-term debt, less current maturities - Southwest Gas Holdings, Inc. $ 1,731,981 $ 1,549,983

Equity, Other Comprehensive I23

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2017
Detailed Activities of Equity and Redeemable Noncontrolling Interest	The table below provides details of activity in equity and the redeemable noncontrolling interest for Southwest Gas Holdings, Inc. on a consolidated basis during the nine months ended September 30, 2017. Southwest Gas Holdings, Inc. Equity Accumulated Redeemable Additional Other Non- Noncontrolling Common Stock Paid-in Comprehensive Retained controlling Interest (In thousands, except per share amounts) Shares Amount Capital Income (Loss) Earnings Interest Total (Temporary DECEMBER 31, 2016 47,482 $ 49,112 $ 903,123 $ (48,008 ) $ 759,263 $ (2,217 ) $ 1,661,273 $ 22,590 Common stock issuances 250 250 21,090 21,340 Net income (loss) 97,376 (78 ) 97,298 248 Redemption value adjustments (355 ) (355 ) 355 Foreign currency exchange translation adj. 1,850 1,850 11 Redemption of Centuri shares from noncontrolling parties (23,000 ) Other comprehensive income (loss): Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax 1,786 1,786 Amounts reclassified to net income, net of tax (FSIRS) 1,554 1,554 Centuri dividend to redeemable noncontrolling interest (204 ) Dividends declared Common: $1.485 per share (71,350 ) (71,350 ) SEPTEMBER 30, 2017 47,732 $ 49,362 $ 924,213 $ (42,818 ) $ 784,934 $ (2,295 ) $ 1,713,396 $ —
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 September 30, 2017 Three Months Ended September 30, 2016 Before- Tax Net-of- Before- Tax Net-of- Tax (Expense) Tax Tax (Expense) Tax Amount or Benefit (1) Amount Amount or Benefit (1) Amount Defined benefit pension plans: Amortization of prior service cost $ 333 $ (126 ) $ 207 $ 333 $ (126 ) $ 207 Amortization of net actuarial (gain)/loss 6,362 (2,418 ) 3,944 6,767 (2,571 ) 4,196 Regulatory adjustment (5,734 ) 2,179 (3,555 ) (6,122 ) 2,326 (3,796 ) Pension plans other comprehensive income (loss) 961 (365 ) 596 978 (371 ) 607 FSIRS (designated hedging activities): Amounts reclassifed into net income 835 (317 ) 518 835 (317 ) 518 FSIRS other comprehensive income 835 (317 ) 518 835 (317 ) 518 Total other comprehensive income (loss) - Southwest Gas Corporation 1,796 (682 ) 1,114 1,813 (688 ) 1,125 Foreign currency translation adjustments: Translation adjustments 1,012 — 1,012 (238 ) — (238 ) Foreign currency other comprehensive income (loss) 1,012 — 1,012 (238 ) — (238 ) Total other comprehensive income (loss) - Southwest Gas Holdings, Inc. $ 2,808 $ (682 ) $ 2,126 $ 1,575 $ (688 ) $ 887 Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2017 Before- Tax Net-of- Before- Tax Net-of- Tax (Expense) Tax Tax (Expense) Tax Amount or Benefit (1) Amount Amount or Benefit (1) Amount Defined benefit pension plans: Amortization of prior service cost $ 1,001 $ (380 ) $ 621 $ 1,001 $ (380 ) $ 621 Amortization of net actuarial (gain)/loss 19,084 (7,252 ) 11,832 20,300 (7,714 ) 12,586 Regulatory adjustment (17,204 ) 6,537 (10,667 ) (18,368 ) 6,980 (11,388 ) Pension plans other comprehensive income (loss) 2,881 (1,095 ) 1,786 2,933 (1,114 ) 1,819 FSIRS (designated hedging activities): Amounts reclassifed into net income 2,507 (953 ) 1,554 2,508 (952 ) 1,556 FSIRS other comprehensive income 2,507 (953 ) 1,554 2,508 (952 ) 1,556 Total other comprehensive income (loss)—Southwest Gas Corporation 5,388 (2,048 ) 3,340 5,441 (2,066 ) 3,375 Foreign currency translation adjustments: Translation adjustments 1,861 — 1,861 614 — 614 Foreign currency other comprehensive income (loss) 1,861 — 1,861 614 — 614 Total other comprehensive income (loss) $ 7,249 $ (2,048 ) $ 5,201 $ 6,055 $ (2,066 ) $ 3,989 Twelve Months Ended September 30, 2017 Twelve Months Ended September 30, 2016 Before- Tax Net-of- Before- Tax Net-of- Tax (Expense) Tax Tax (Expense) Tax Amount or Benefit (1) Amount Amount or Benefit (1) Amount Defined benefit pension plans: Net actuarial gain/(loss) $ (22,770 ) $ 8,652 $ (14,118 ) $ (30,519 ) $ 11,597 $ (18,922 ) Amortization of prior service cost 1,335 (507 ) 828 1,335 (507 ) 828 Amortization of net actuarial (gain)/loss 25,850 (9,823 ) 16,027 28,895 (10,980 ) 17,915 Regulatory adjustment (4,420 ) 1,679 (2,741 ) (653 ) 249 (404 ) Pension plans other comprehensive income (loss) (5 ) 1 (4 ) (942 ) 359 (583 ) FSIRS (designated hedging activities): Amounts reclassifed into net income 3,344 (1,271 ) 2,073 3,344 (1,271 ) 2,073 FSIRS other comprehensive income (loss) 3,344 (1,271 ) 2,073 3,344 (1,271 ) 2,073 Total other comprehensive income (loss)—Southwest Gas Corporation 3,339 (1,270 ) 2,069 2,402 (912 ) 1,490 Foreign currency translation adjustments: Translation adjustments 1,408 — 1,408 233 — 233 Foreign currency other comprehensive income (loss) 1,408 — 1,408 233 — 233 Total other comprehensive income (loss)—Southwest Gas Holdings, Inc. $ 4,747 $ (1,270 ) $ 3,477 $ 2,635 $ (912 ) $ 1,723 (1) Tax amounts are calculated using a 38% rate. 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, the Company is not recognizing any tax effect or presenting a tax expense or benefit for the currency translation adjustment amount reported in Other Comprehensive Income, as repatriation of earnings is not anticipated.
Rollforward of Accumulated Other Comprehensive Income	The following table represents a rollforward of AOCI, presented on the Company’s Condensed Consolidated Balance Sheets: AOCI—Rollforward (Thousands of dollars) Defined Benefit Plans FSIRS Foreign Currency Items Before-Tax Tax After-Tax Before-Tax Tax After-Tax Before-Tax Tax After-Tax AOCI Beginning Balance AOCI December 31, 2016 $ (57,613 ) $ 21,893 $ (35,720 ) $ (15,999 ) $ 6,080 $ (9,919 ) $ (2,369 ) $ — $ (2,369 ) $ (48,008 ) Translation adjustments — — — — — — 1,861 — 1,861 1,861 Other comprehensive income before reclassifications — — — — — — 1,861 — 1,861 1,861 FSIRS amounts reclassified from AOCI (1) — — — 2,507 (953 ) 1,554 — — — 1,554 Amortization of prior service cost (2) 1,001 (380 ) 621 — — — — — — 621 Amortization of net actuarial loss (2) 19,084 (7,252 ) 11,832 — — — — — — 11,832 Regulatory adjustment (3) (17,204 ) 6,537 (10,667 ) — — — — — — (10,667 ) Net current period other comprehensive income (loss) 2,881 (1,095 ) 1,786 2,507 (953 ) 1,554 1,861 — 1,861 5,201 Less: Translation adjustment attributable to redeemable noncontrolling interest — — — — — — 11 — 11 11 Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. 2,881 (1,095 ) 1,786 2,507 (953 ) 1,554 1,850 — 1,850 5,190 Ending Balance AOCI September 30, 2017 $ (54,732 ) $ 20,798 $ (33,934 ) $ (13,492 ) $ 5,127 $ (8,365 ) $ (519 ) $ — $ (519 ) $ (42,818 ) (1) The FSIRS reclassification amounts are included in the Net interest deductions line item 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 (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 the Deferred charges and other assets line item on the Company’s Condensed Consolidated Balance Sheets). (4) Tax amounts are calculated using a 38% rate.
Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income	The following table represents amounts (before income tax impacts) included in AOCI (in the tables above), that have not yet been recognized in net periodic benefit cost: Amounts Recognized in AOCI (Before Tax) (Thousands of dollars) September 30, 2017 December 31, 2016 Net actuarial (loss) gain $ (411,889 ) $ (430,973 ) Prior service cost (4,702 ) (5,703 ) Less: amount recognized in regulatory assets 361,859 379,063 Recognized in AOCI $ (54,732 ) $ (57,613 )
Southwest Gas Corporation [Member]
Detailed Activities of Equity and Redeemable Noncontrolling Interest	The table below provides details of activity in equity for Southwest Gas Corporation during the nine months ended September 30, 2017. Effective in January 2017, Southwest became a subsidiary of Southwest Gas Holdings, Inc., and only equity shares of the latter are publicly traded, under the ticker symbol “SWX.” Southwest Gas Corporation Equity Accumulated Additional Other Common Stock Paid-in Comprehensive Retained (In thousands, except per share amounts) Shares Amount Capital Income (Loss) Earnings Total DECEMBER 31, 2016 47,482 $ 49,112 $ 897,346 $ (45,639 ) $ 767,061 $ 1,667,880 Net income 82,436 82,436 Other comprehensive income (loss): Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax 1,786 1,786 Amounts reclassified to net income, net of tax (FSIRS) 1,554 1,554 Distribution to Southwest Gas Holdings, Inc. investment in discontinued operations (182,773 ) (182,773 ) Stock-based compensation (a) 8,576 (587 ) 7,989 Dividends declared to Southwest Gas Holdings, Inc. (60,130 ) (60,130 ) Contributions from Southwest Gas Holdings, Inc. 11,659 11,659 SEPTEMBER 30, 2017 47,482 $ 49,112 $ 917,581 $ (42,299 ) $ 606,007 $ 1,530,401 (a) Stock-based compensation is based on stock awards of Southwest Gas Corporation to be issued in shares of Southwest Gas Holdings, Inc. The table above gives effect to the holding company reorganization whereby Southwest and Centuri became subsidiaries of the Company. The historic investment in Centuri was distributed to the parent holding company. This presentation is only applicable to Southwest and not to the Company overall, as Centuri continues to be included in the continuing operations of the Company. Also in connection with the holding company creation, compensation plans of Southwest include programs that will be settled with equity shares issued by Southwest Gas Holdings, Inc. Management has determined that when no consideration is directly exchanged for these programs between Southwest and the Company, the accounting impact at Southwest for these programs is reflected both as compensation expense and as an equity contribution (of the parent) in Southwest.
Rollforward of Accumulated Other Comprehensive Income	The following table represents a rollforward of AOCI, presented on Southwest’s Condensed Consolidated Balance Sheets: AOCI—Rollforward (Thousands of dollars) Defined Benefit Plans FSIRS Before-Tax Tax After-Tax Before-Tax Tax After-Tax AOCI Beginning Balance AOCI December 31, 2016 $ (57,613 ) $ 21,893 $ (35,720 ) $ (15,999 ) $ 6,080 $ (9,919 ) $ (45,639 ) FSIRS amounts reclassified from AOCI (5) — — — 2,507 (953 ) 1,554 1,554 Amortization of prior service cost (6) 1,001 (380 ) 621 — — — 621 Amortization of net actuarial loss (6) 19,084 (7,252 ) 11,832 — — — 11,832 Regulatory adjustment (7) (17,204 ) 6,537 (10,667 ) — — — (10,667 ) Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation 2,881 (1,095 ) 1,786 2,507 (953 ) 1,554 3,340 Ending Balance AOCI September 30, 2017 $ (54,732 ) $ 20,798 $ (33,934 ) $ (13,492 ) $ 5,127 $ (8,365 ) $ (42,299 ) (5) The FSIRS reclassification amounts are included in the Net interest deductions line item on Southwest’s Condensed Consolidated Statements of Income. (6) These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost (7) The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on Southwest’s Condensed Consolidated Balance Sheets). (8) Tax amounts are calculated using a 38% rate.

Construction Services Redeema24

Construction Services Redeemable Noncontrolling Interest (Tables)	9 Months Ended
	Sep. 30, 2017
Noncontrolling Interest [Abstract]
Summary of Redeemable Noncontrolling Interest	The following depicts changes to the balance of the redeemable noncontrolling interest between the indicated periods. Redeemable (Thousands of dollars): Balance, December 31, 2016 $ 22,590 Net income attributable to redeemable noncontrolling interest 248 Foreign currency exchange translation adjustment 11 Centuri dividend to redeemable noncontrolling interest (204 ) Adjustment to redemption value 355 Redemption of Centuri shares from noncontrolling parties (23,000 ) Balance, September 30, 2017 $ —

Reorganization Impacts - Disc25

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation (Tables)	9 Months Ended
	Sep. 30, 2017
Restructuring and Related Activities [Abstract]
Summary of Discontinued Operations - Construction Services in Condensed Consolidated Balance Sheet and Condensed Consolidated Income Statements	The following table presents the major categories of assets and liabilities within the amounts reported as discontinued operations – construction services in the Condensed Consolidated Balance Sheet of Southwest Gas Corporation: (Thousands of dollars) December 31, 2016 Assets: Other property and investments $ 233,774 Cash and cash equivalents 9,042 Accounts receivable, net of allowances 173,300 Prepaids and other current assets 10,470 Goodwill 129,888 Other noncurrent assets 22,897 Discontinued operations - construction services - assets $ 579,371 Liabilities: Current maturities of long-term debt $ 25,101 Accounts payable 46,440 Other current liabilities 74,518 Long-term debt, less current maturities 174,903 Deferred income taxes and other deferred credits 59,653 Discontinued operations—construction services—liabilities $ 380,615 The following table presents the components of the Discontinued operations – construction services non-owner (Thousands of dollars) December 31, 2016 Construction services equity $ (4,390 ) Construction services noncontrolling interest (2,217 ) Construction services redeemable noncontrolling interest 22,590 Discontinued operations - construction services non-owner $ 15,983 The following table presents the major income statement components of discontinued operations – construction services reported in the Condensed Consolidated Income Statements of Southwest Gas Corporation: Results of Construction Services Three Nine Twelve Twelve Months Ended Months Ended Months Ended Months Ended (Thousands of dollars) September 30, 2016 September 30, 2016 September 30, 2017 September 30, 2016 Construction revenues $ 339,790 $ 838,038 $ 301,040 $ 1,127,982 Operating expenses: Construction expenses 300,611 757,919 266,504 1,009,188 Depreciation and amortization 13,409 43,351 12,318 58,368 Operating income 25,770 36,768 22,218 60,426 Other income (deductions) 44 44 1,149 1,246 Net interest deductions 1,794 4,945 1,718 6,738 Income before income taxes 24,020 31,867 21,649 54,934 Income tax expense 8,708 12,042 7,842 20,711 Net income 15,312 19,825 13,807 34,223 Net income attributable to noncontrolling interests 435 500 514 1,079 Discontinued operations - construction services - net income $ 14,877 $ 19,325 $ 13,293 $ 33,144

Nature of Operations and Basi26

Nature of Operations and Basis of Presentation - Additional Information (Detail) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Aug. 31, 2017
Significant Accounting Policies [Line Items]
Non-cash construction advances - non-cash investing item	$ 1,900,000	$ 3,600,000
Accrued taxes	4,689,000	(7,055,000)	$ 38,084,000		$ (11,033,000)
Goodwill impairment charges	0
Non-service costs				$ 20,000,000
Accounting Standards Update 2016-09 [Member]
Significant Accounting Policies [Line Items]
Accrued taxes		12,000,000			22,000,000
Accounting Standards Update 2016-09 [Member] | Scenario, Previously Reported [Member]
Significant Accounting Policies [Line Items]
Accrued taxes		9,900,000			19,900,000
Other Current Liabilities [Member]
Significant Accounting Policies [Line Items]
Dividends declared but not yet paid	21,000,000		21,000,000
Gas Plant [Member]
Significant Accounting Policies [Line Items]
Non-service costs capitalized				3,000,000
Gas Pipe Materials and Supplies [Member] | Prepaids and Other Current Assets [Member]
Significant Accounting Policies [Line Items]
Gas pipe materials and operating supplies	36,000,000		36,000,000	30,000,000
Prepaids and other current assets	$ 24,000,000		24,000,000	953,000
Centuri Construction Group Inc [Member]
Significant Accounting Policies [Line Items]
Ownership percentage in subsidiary	96.60%
Previous Owners [Member] | Link-Line and WS Nicholls [Member] | Centuri Construction Group Inc [Member]
Significant Accounting Policies [Line Items]
Percentage of earnings attribution by Centuri to previous owners						3.40%
Southwest Gas Corporation [Member]
Significant Accounting Policies [Line Items]
Accrued taxes	$ 4,092,000	(5,660,000)	29,143,000		4,567,000
Southwest Gas Corporation [Member] | Accounting Standards Update 2016-09 [Member]
Significant Accounting Policies [Line Items]
Accrued taxes		819,000			9,100,000
Southwest Gas Corporation [Member] | Accounting Standards Update 2016-09 [Member] | Scenario, Previously Reported [Member]
Significant Accounting Policies [Line Items]
Accrued taxes		$ 2,900,000			$ 7,000,000
Pooled Funds and Mutual Funds [Member] | Level 2 - Significant Other Observable Inputs [Member]
Significant Accounting Policies [Line Items]
Money market fund investments	$ 19,800,000		$ 19,800,000	$ 5,300,000

Nature of Operations and Basi27

Nature of Operations and Basis of Presentation - Schedule of Goodwill (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2017USD ($)
Goodwill [Line Items]
Goodwill, Beginning balance	$ 139,983
Foreign currency translation adjustment	7,882
Goodwill, Ending balance	147,865
Natural Gas Operations [Member]
Goodwill [Line Items]
Goodwill, Beginning balance	10,095
Goodwill, Ending balance	10,095
Construction Services [Member]
Goodwill [Line Items]
Goodwill, Beginning balance	129,888
Foreign currency translation adjustment	7,882
Goodwill, Ending balance	$ 137,770

Nature of Operations and Basi28

Nature of Operations and Basis of Presentation - Accounts Receivable for Services (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Centuri Construction Group Inc [Member]
Segment Reporting Information [Line Items]
Centuri accounts receivable for services provided to Southwest	$ 11,486	$ 10,585

Nature of Operations and Basi29

Nature of Operations and Basis of Presentation - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Net cash surrender value of COLI policies	$ 114,052	$ 106,744
Other property	13,483	12,859
Total	369,303	342,343
Centuri Construction Group Inc [Member]
Investment [Line Items]
Centuri property and equipment	493,599	451,114
Centuri accumulated provision for depreciation and amortization	$ (251,831)	$ (228,374)

Nature of Operations and Basi30

Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other income (deductions)
Corporate and administrative	$ 5		$ 6		$ 6
Total other income (deductions)	2,876	$ 2,565	8,788	$ 6,756	11,501	$ 10,861
Southwest Gas Corporation [Member]
Other income (deductions)
Change in COLI policies	2,100	2,300	6,800	5,400	8,800	7,500
Interest income	670	522	1,848	1,279	2,417	1,664
Equity AFUDC	968	611	2,077	1,893	2,473	2,890
Miscellaneous income and (expense)	(657)	(912)	(1,981)	(1,860)	(3,382)	(2,439)
Total other income (deductions)	3,081	2,521	8,744	6,712	10,308	9,615
Centuri Construction Group Inc [Member]
Other income (deductions)
Interest income	1		2	1	2	414
Foreign transaction gain (loss)	(442)	(3)	(640)	(22)	(640)	28
Miscellaneous income and (expense)	231	47	676	65	1,825	804
Total other income (deductions)	$ (210)	$ 44	$ 38	$ 44	$ 1,187	$ 1,246

Components of Net Periodic Be31

Components of Net Periodic Benefit Cost - Schedule of Net Periodic Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,848	$ 5,708	$ 17,544	$ 17,125	$ 23,252	$ 23,406
Interest cost	11,520	11,507	34,561	34,520	46,068	45,577
Expected return on plan assets	(13,799)	(14,140)	(41,397)	(42,419)	(55,536)	(56,871)
Amortization of net actuarial loss	6,001	6,317	18,003	18,950	24,319	27,136
Net periodic benefit cost	9,570	9,392	28,711	28,176	38,103	39,248
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	77	83	232	248	315	328
Interest cost	471	464	1,413	1,394	1,878	1,818
Amortization of net actuarial loss	361	346	1,081	1,038	1,426	1,361
Net periodic benefit cost	909	893	2,726	2,680	3,619	3,507
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	367	375	1,101	1,124	1,476	1,534
Interest cost	808	795	2,424	2,386	3,218	3,136
Expected return on plan assets	(839)	(787)	(2,518)	(2,362)	(3,305)	(3,228)
Amortization of prior service costs	333	333	1,001	1,001	1,335	1,335
Amortization of net actuarial loss		104		312	105	398
Net periodic benefit cost	$ 669	$ 820	$ 2,008	$ 2,461	$ 2,829	$ 3,175

Segment Information - Additiona

Segment Information - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2017Segment
Segment Reporting Information [Line Items]
Number of segments	2
Southwest Gas Corporation [Member]
Segment Reporting Information [Line Items]
Number of segments	2

Segment Information - Schedule

Segment Information - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ 564,909	$ 512,710	$ 1,735,896	$ 1,743,762	$ 2,354,503	$ 2,398,804
Segment net income (loss)	10,204	2,472	97,376	86,861	162,556	152,980
Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	28,244	27,259	72,463	75,203	95,381	105,566
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	593,153	539,969	1,808,359	1,818,965	2,449,884	2,504,370
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Revenues	213,059	200,179	935,823	980,927	1,276,308	1,376,388
Segment net income (loss)	(4,024)	(12,405)	82,436	67,536	134,323	119,836
Natural Gas Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	213,059	200,179	935,823	980,927	1,276,308	1,376,388
Construction Services [Member]
Segment Reporting Information [Line Items]
Revenues	351,850	312,531	800,073	762,835	1,078,195	1,022,416
Segment net income (loss)	14,335	14,877	15,717	19,325	29,010	33,144
Construction Services [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	28,244	27,259	72,463	75,203	95,381	105,566
Construction Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	380,094	$ 339,790	872,536	$ 838,038	1,173,576	$ 1,127,982
Other [Member]
Segment Reporting Information [Line Items]
Segment net income (loss)	$ (107)		$ (777)		$ (777)

Derivatives and Fair Value Me34

Derivatives and Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$ 518,000	$ 518,000	$ 1,554,000	$ 1,556,000	$ 2,073,000	$ 2,073,000
Derivative instrument loss at settlement amortization period			10 years
Cash Flow Hedging [Member]
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges			$ 0
Minimum [Member]
Derivative [Line Items]
Maturities of natural gas swaps			Oct. 31, 2017
Maximum [Member]
Derivative [Line Items]
Maturities of natural gas swaps			Mar. 31, 2019
Arizona [Member]
Derivative [Line Items]
Natural gas portfolios, maximum % rate			25.00%
California [Member]
Derivative [Line Items]
Natural gas portfolios, maximum % rate			25.00%

Derivatives and Fair Value Me35

Derivatives and Fair Value Measurements - Notional Amounts under Swaps Contracts (Detail) - MMBTU	Sep. 30, 2017	Dec. 31, 2016
Offsetting [Abstract]
Contract notional amounts	10,936,000	10,543,000

Derivatives and Fair Value Me36

Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - Cash Flow Hedging [Member] - Net Cost of Gas Sold [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ (546)	$ (2,072)	$ (6,851)	$ 2,253	$ (4,098)	$ (656)
Regulatory Deferral Accounting Treatment [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 546	$ 2,072	$ 6,851	$ (2,253)	$ 4,098	$ 656

Derivatives and Fair Value Me37

Derivatives and Fair Value Measurements - Fair Values of Swaps in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	$ 84	$ 4,450
Liability derivatives not designated as hedging instruments	(2,689)	(73)
Net total not designated as hedging instruments	(2,605)	4,377
Swaps [Member] | Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		899
Liability derivatives not designated as hedging instruments		(54)
Net total not designated as hedging instruments		845
Swaps [Member] | Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	56	3,551
Liability derivatives not designated as hedging instruments	(22)	(19)
Net total not designated as hedging instruments	34	$ 3,532
Swaps [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	27
Liability derivatives not designated as hedging instruments	(1,899)
Net total not designated as hedging instruments	(1,872)
Swaps [Member] | Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	1
Liability derivatives not designated as hedging instruments	(768)
Net total not designated as hedging instruments	$ (767)

Derivatives and Fair Value Me38

Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 143	$ 1,555	$ 2,655
Received from counterparties		$ 1,685	$ 2,060

Derivatives and Fair Value Me39

Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) - Swaps [Member] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Net Total		$ (845)
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ (34)	$ (3,532)
Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	1,872
Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 767

Derivatives and Fair Value Me40

Derivatives and Fair Value Measurements - Summary of Financial Assets and Liabilities at Fair Value (Detail) - Level 2 - Significant Other Observable Inputs [Member] - Swaps [Member] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Net Assets (Liabilities)	$ (2,605)	$ 4,377
Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	34	3,532
Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets		$ 845
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	(1,872)
Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	$ (767)

Common Stock - Additional Infor

Common Stock - Additional Information (Detail)	Mar. 29, 2017USD ($)	Jan. 31, 2017	Sep. 30, 2017USD ($)$ / sharesshares	Sep. 30, 2017USD ($)$ / sharesshares	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)
Conversion of stock conversion ratio		1
Value of common stock	$ 150,000,000
Net proceeds to company				$ 11,563,000	$ 530,000	$ 11,505,000	$ 507,000
Common stock issued through Restricted Stock/Unit Plan, and Management Incentive Plan | shares				103,000
BNY Mellon Capital Markets, LLC [Member]
Common stock sold through agent | shares			147,077	147,077
Open market weighted average price per share | $ / shares			$ 80.07	$ 80.07
Net proceeds to company			$ 11,659,104	$ 11,659,104
Agent commissions			117,769	117,769
Common stock available for sale under the program			$ 138,223,127	$ 138,223,127		$ 138,223,127

Long-Term Debt - Schedule of Ca

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Less: current maturities	$ (28,453)	$ (50,101)
Long-term debt, less current maturities	1,731,981	1,549,983
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(28,453)	(50,101)
Long-term debt, less current maturities	1,731,981	1,549,983
Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,520,790	1,375,080
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,520,790	1,375,080
Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Carrying Amount [Member]
Debt Instrument [Line Items]
Less: current maturities	(28,453)	(50,101)
Long-term debt, less current maturities	1,731,981	1,549,983
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(28,453)	(50,101)
Long-term debt, less current maturities	1,731,981	1,549,983
Carrying Amount [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(2,212)	(2,489)
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,520,790	1,375,080
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	1,520,790	1,400,080
Less: current maturities		(25,000)
Long-term debt, less current maturities	1,520,790	1,375,080
Carrying Amount [Member] | Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Less: current maturities	(28,453)	(25,101)
Long-term debt, less current maturities	211,191	174,903
Unamortized debt issuance costs	(383)	(516)
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	239,644	200,004
Less: current maturities	(28,453)	(25,101)
Long-term debt, less current maturities	211,191	174,903
Centuri term loan/secured revolving credit facility	106,867	106,184
Carrying Amount [Member] | Centuri Secured Revolving Credit Facility [Member] | Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	81,250	41,185
Carrying Amount [Member] | IDRBs [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	197,788	197,511
Carrying Amount [Member] | Centuri Term Loan Facility [Member] | Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	107,250	106,700
Carrying Amount [Member] | Centuri Other Debt Obligations [Member] | Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	51,527	52,635
Carrying Amount [Member] | Debentures [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(9,498)	(9,931)
Notes payable	1,173,002	1,197,569
Carrying Amount [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	125,000	125,000
Carrying Amount [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	125,000	125,000
Carrying Amount [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	250,000	250,000
Carrying Amount [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	250,000	250,000
Carrying Amount [Member] | Debentures [Member] | Notes, 3.8%, due 2046 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	300,000	300,000
Carrying Amount [Member] | Debentures [Member] | 8% Series, due 2026 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	75,000	75,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable		25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	25,000	25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	25,000	25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	7,500	7,500
Carrying Amount [Member] | Debentures [Member] | Revolving Credit Facility and Commercial Paper [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Line of credit facility	150,000	5,000
Carrying Amount [Member] | Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Market Value [Member] | Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	107,403	106,819
Centuri other debt obligations	51,978	52,840
Market Value [Member] | Centuri Secured Revolving Credit Facility [Member] | Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	81,402	41,292
Market Value [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	130,325	129,703
Market Value [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	154,434	149,734
Market Value [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	258,943	254,900
Market Value [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	275,168	266,793
Market Value [Member] | Debentures [Member] | Notes, 3.8%, due 2046 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	292,578	283,029
Market Value [Member] | Debentures [Member] | 8% Series, due 2026 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	97,218	94,691
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		25,040
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	29,174	29,290
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	31,964	31,905
Market Value [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	8,920	8,769
Market Value [Member] | Debentures [Member] | Revolving Credit Facility and Commercial Paper [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	150,000	5,000
Market Value [Member] | Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Market Value [Member] | Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Market Value [Member] | Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 50,000	$ 50,000

Long-Term Debt - Schedule of 43

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Parenthetical) (Detail) - Southwest Gas Corporation [Member]	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.59%	7.59%
Debt instrument due date	Jan. 17, 2017	Jan. 17, 2017
Debentures [Member] | Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.45%	4.45%
Debt instrument due date	Dec. 1, 2020	Dec. 1, 2020
Debentures [Member] | Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.10%	6.10%
Debt instrument due date	Feb. 15, 2041	Feb. 15, 2041
Debentures [Member] | Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.875%	3.875%
Debt instrument due date	Apr. 1, 2022	Apr. 1, 2022
Debentures [Member] | Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.875%	4.875%
Debt instrument due date	Oct. 1, 2043	Oct. 1, 2043
Debentures [Member] | Notes, 3.8%, due 2046 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.80%	3.80%
Debt instrument due date	Sep. 30, 2046	Sep. 30, 2046
Debentures [Member] | 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	8.00%	8.00%
Debt instrument due date	Aug. 1, 2026	Aug. 1, 2026
Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.78%	7.78%
Debt instrument due date	Feb. 3, 2022	Feb. 3, 2022
Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.92%	7.92%
Debt instrument due date	Jun. 24, 2027	Jun. 24, 2027
Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.76%	6.76%
Debt instrument due date	Sep. 24, 2027	Sep. 24, 2027
Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 1, 2028	Dec. 1, 2028
Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 1, 2038	Mar. 1, 2038
Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 1, 2038	Mar. 1, 2038
Variable-rate bonds [Member] | Market Value [Member] | 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 1, 2039	Dec. 1, 2039

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail) - USD ($)	1 Months Ended	9 Months Ended
	Mar. 31, 2017	Sep. 30, 2017	Dec. 31, 2014
Centuri Construction Group Inc [Member] | Secured Revolving Credit and Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 300,000,000
Credit facility expiration date		Oct. 31, 2019
Borrowings outstanding under facility		$ 189,000,000
Term loan portion of line of credit			$ 150,000,000
Term loan facility outstanding amount		107,000,000
Debt secured by assets		$ 526,000,000
Southwest Gas Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	$ 400,000,000
Line of credit designated as long term debt	150,000,000
Line of credit designated for working capital purposes	250,000,000
Credit facility expiration date		Mar. 1, 2022
Southwest Gas Credit Facility [Member] | Long-term Debt [Member]
Debt Instrument [Line Items]
Borrowings outstanding under facility		$ 150,000,000
Southwest Gas Credit Facility [Member] | Short-term Debt [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	$ 100,000,000
Credit facility expiration date	Mar. 31, 2022
Borrowings outstanding under facility		$ 83,000,000
Southwest Gas Credit Facility [Member] | Before Amendment [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	$ 300,000,000
Previous credit facility expiration date		Mar. 1, 2021
Southwest Gas Credit Facility [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Applicable margin		1.00%
Southwest Gas Credit Facility [Member] | Alternative Base Rate [Member]
Debt Instrument [Line Items]
Applicable margin		0.00%

Short-Term Debt - Additional In

Short-Term Debt - Additional Information (Detail) - USD ($)	1 Months Ended	9 Months Ended
	Mar. 31, 2017	Sep. 30, 2017
Short-term Debt [Line Items]
Short term borrowings outstanding		$ 110,500,000
Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Credit facility maximum borrowing capacity	$ 400,000,000
Credit facility expiration date		Mar. 1, 2022
Line of credit designated for working capital purposes	250,000,000
Short-term Debt [Member] | Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Credit facility maximum borrowing capacity	$ 100,000,000
Credit facility expiration date	Mar. 31, 2022
Borrowings outstanding under facility		$ 83,000,000
Credit Facility [Member] | Short-term Debt [Member] | Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Short term borrowings outstanding		$ 27,500,000
Minimum [Member] | Short-term Debt [Member] | Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Commitment fee	0.075%
Maximum [Member] | Short-term Debt [Member] | Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Commitment fee	0.20%
Alternative Base Rate [Member] | Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Applicable margin		0.00%
Alternative Base Rate [Member] | Minimum [Member] | Short-term Debt [Member] | Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Applicable margin	0.00%
Alternative Base Rate [Member] | Maximum [Member] | Short-term Debt [Member] | Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Applicable margin	0.50%
LIBOR [Member] | Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Applicable margin		1.00%
LIBOR [Member] | Minimum [Member] | Short-term Debt [Member] | Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Applicable margin	0.75%
LIBOR [Member] | Maximum [Member] | Short-term Debt [Member] | Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Applicable margin	1.50%

Equity, Other Comprehensive I46

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule Of Stockholders Equity [Line Items]
Balance			$ 1,661,273
Balance			$ 22,590
Balance, shares			47,482,068
Common stock issuances			$ 21,340
Net income (loss)			97,298
Net income (loss)	$ 10,420	$ 2,907	97,546	$ 87,361	$ 163,240	$ 154,059
Redemption value adjustments			(355)
Foreign currency exchange translation adj.			1,850
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	596	607	1,786	1,819	(4)	(583)
Amounts reclassified into net income	518	518	1,554	1,556	2,073	2,073
Dividends declared Common			(71,350)
Balance	$ 1,713,396		$ 1,713,396		$ 1,713,396
Balance, shares	47,731,840		47,731,840		47,731,840
Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 1,667,880
Net income (loss)			82,436
Net income (loss)			82,436	87,361	$ 148,130	154,059
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	$ 596	607	1,786	1,819	(4)	(583)
Amounts reclassified into net income	518	$ 518	1,554	$ 1,556	2,073	$ 2,073
Distribution to Southwest Gas Holdings, Inc. investment in discontinued operations			(182,773)
Stock-based compensation			7,989
Dividends declared Common			(60,130)
Contributions from Southwest Gas Holdings, Inc.			11,659
Balance	1,530,401		1,530,401		1,530,401
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 49,112
Balance, shares			47,482,000
Common stock issuances			$ 250
Common stock issuances, Shares			250,000
Other comprehensive income (loss):
Balance	$ 49,362		$ 49,362		$ 49,362
Balance, shares	47,732,000		47,732,000		47,732,000
Common Stock [Member] | Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 49,112
Balance, shares			47,482,000
Other comprehensive income (loss):
Balance	$ 49,112		$ 49,112		$ 49,112
Balance, shares	47,482,000		47,482,000		47,482,000
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 903,123
Common stock issuances			21,090
Other comprehensive income (loss):
Balance	$ 924,213		924,213		$ 924,213
Additional Paid-in Capital [Member] | Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			897,346
Other comprehensive income (loss):
Stock-based compensation			8,576
Contributions from Southwest Gas Holdings, Inc.			11,659
Balance	917,581		917,581		917,581
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(48,008)
Foreign currency exchange translation adj.			1,850
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			1,786
Amounts reclassified into net income			1,554
Balance	(42,818)		(42,818)		(42,818)
Accumulated Other Comprehensive Income (Loss) [Member] | Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(45,639)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			1,786
Amounts reclassified into net income			1,554
Balance	(42,299)		(42,299)		(42,299)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			759,263
Net income (loss)			97,376
Redemption value adjustments			(355)
Other comprehensive income (loss):
Dividends declared Common			(71,350)
Balance	784,934		784,934		784,934
Retained Earnings [Member] | Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			767,061
Net income (loss)			82,436
Other comprehensive income (loss):
Distribution to Southwest Gas Holdings, Inc. investment in discontinued operations			(182,773)
Stock-based compensation			(587)
Dividends declared Common			(60,130)
Balance	606,007		606,007		606,007
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			22,590
Net income (loss) attributable to temporary equity			248
Redemption value adjustments			355
Foreign currency exchange translation adj attributable to temporary equity			11
Redemption of Centuri shares from noncontrolling parties			(23,000)
Other comprehensive income (loss):
Centuri dividend to redeemable noncontrolling interest			(204)
Centuri dividend to redeemable noncontrolling interest			(204)
Balance	0		0		0
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(2,217)
Net income (loss)			(78)
Other comprehensive income (loss):
Balance	$ (2,295)		$ (2,295)		$ (2,295)

Equity, Other Comprehensive I47

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Parenthetical) (Detail) - $ / shares	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Stockholders' Equity [Abstract]
Dividends declared Common, per share	$ 0.495	$ 0.450	$ 1.485	$ 1.350	$ 1.935	$ 1.755

Equity, Other Comprehensive I48

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service cost, Before-Tax Amount	$ 333	$ 333	$ 1,001	$ 1,001	$ 1,335	$ 1,335
Translation adjustments, Before-Tax Amount	1,012	(238)	1,861	614	1,408	233
Translation adjustments, Tax (Expense) or Benefit	0	0	0	0
Amortization of net actuarial (gain)/loss, Before-Tax Amount	6,362	6,767	19,084	20,300	25,850	28,895
Foreign currency other comprehensive income (loss), Before-Tax Amount	1,012	(238)	1,861	614	1,408	233
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0	0	0	0
Net actuarial gain/(loss), Before-Tax Amount					(22,770)	(30,519)
Total other comprehensive income (loss), Before-Tax Amount	2,808	1,575	7,249	6,055	4,747	2,635
Total other comprehensive income (loss), Tax (Expense) or Benefit	(682)	(688)	(2,048)	(2,066)	(1,270)	(912)
Regulatory adjustment, Before-Tax Amount	(5,734)	(6,122)	(17,204)	(18,368)	(4,420)	(653)
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	961	978	2,881	2,933	(5)	(942)
Amounts reclassified into net income, Before-Tax Amount	835	835	2,507	2,508	3,344	3,344
FSIRS other comprehensive income (loss), Before-Tax Amount	835	835	2,507	2,508	3,344	3,344
Net actuarial gain/(loss), Tax (Expense) or Benefit					8,652	11,597
Amortization of prior service cost, Tax (Expense) or Benefit	(126)	(126)	(380)	(380)	(507)	(507)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(2,418)	(2,571)	(7,252)	(7,714)	(9,823)	(10,980)
Regulatory adjustment, Tax (Expense) or Benefit	2,179	2,326	6,537	6,980	1,679	249
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(365)	(371)	(1,095)	(1,114)	1	359
Amounts reclassified into net income, Tax (Expense) or Benefit	(317)	(317)	(953)	(952)	(1,271)	(1,271)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(317)	(317)	(953)	(952)	(1,271)	(1,271)
Net actuarial gain/(loss), Net-of-Tax Amount					(14,118)	(18,922)
Amortization of prior service cost, Net-of-Tax Amount	207	207	621	621	828	828
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	3,944	4,196	11,832	12,586	16,027	17,915
Regulatory adjustment, Net-of-Tax Amount	(3,555)	(3,796)	(10,667)	(11,388)	(2,741)	(404)
Pension plans other comprehensive income (loss), Net-of-Tax Amount	596	607	1,786	1,819	(4)	(583)
Amounts reclassified into net income	518	518	1,554	1,556	2,073	2,073
FSIRS other comprehensive income (loss), Net-of-Tax Amount	518	518	1,554	1,556	2,073	2,073
Translation adjustments, Net-of-Tax Amount	1,012	(238)	1,861	614	1,408	233
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	1,012	(238)	1,861	614	1,408	233
Total other comprehensive income (loss), net of tax	2,126	887	5,201	3,989	3,477	1,723
Southwest Gas Corporation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service cost, Before-Tax Amount			1,001
Amortization of net actuarial (gain)/loss, Before-Tax Amount			19,084
Total other comprehensive income (loss), Before-Tax Amount	1,796	1,813	5,388	5,441	3,339	2,402
Total other comprehensive income (loss), Tax (Expense) or Benefit	(682)	(688)	(2,048)	(2,066)	(1,270)	(912)
Regulatory adjustment, Before-Tax Amount			(17,204)
Amounts reclassified into net income, Before-Tax Amount			2,507
Amortization of prior service cost, Tax (Expense) or Benefit			(380)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit			(7,252)
Regulatory adjustment, Tax (Expense) or Benefit			6,537
Amounts reclassified into net income, Tax (Expense) or Benefit			(953)
Net actuarial gain/(loss), Net-of-Tax Amount					(14,118)	(18,922)
Amortization of prior service cost, Net-of-Tax Amount	207	207	621	621	828	828
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	3,944	4,196	11,832	12,586	16,027	17,915
Regulatory adjustment, Net-of-Tax Amount	(3,555)	(3,796)	(10,667)	(11,388)	(2,741)	(404)
Pension plans other comprehensive income (loss), Net-of-Tax Amount	596	607	1,786	1,819	(4)	(583)
Amounts reclassified into net income	518	518	1,554	1,556	2,073	2,073
FSIRS other comprehensive income (loss), Net-of-Tax Amount	518	518	1,554	1,556	2,073	2,073
Total other comprehensive income (loss), net of tax	$ 1,114	$ 1,125	$ 3,340	$ 3,375	$ 2,069	$ 1,490

Equity, Other Comprehensive I49

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Parenthetical) (Detail)	9 Months Ended
	Sep. 30, 2017USD ($)
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%
Tax expense or benefit for currency translation adjustment	$ 0

Equity, Other Comprehensive I50

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Additional Information (Detail) $ in Millions	Sep. 30, 2017USD ($)
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$ 2.1

Equity, Other Comprehensive I51

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance AOCI December 31, 2016, Defined Benefit Plans Before-Tax			$ (57,613)
Translation adjustments, Defined Benefit Plans Before-Tax			0
Other comprehensive income before reclassifications, Defined Benefit Plans Before-Tax			0
Amortization of prior service cost, Defined Benefit Plans Before-Tax	$ 333	$ 333	1,001	$ 1,001	$ 1,335	$ 1,335
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	6,362	6,767	19,084	20,300	25,850	28,895
Regulatory adjustment, Defined Benefit Plans Before-Tax	(5,734)	(6,122)	(17,204)	(18,368)	(4,420)	(653)
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	961	978	2,881	2,933	(5)	(942)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans Before-Tax			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans Before-Tax			2,881
Ending Balance AOCI September 30, 2017, Defined Benefit Plans Before-Tax	(54,732)		(54,732)		(54,732)
Beginning Balance AOCI December 31, 2016, Defined Benefit Plans Tax (Expense) Benefit			21,893
Translation adjustments, Defined Benefit Plans Tax (Expense) Benefit			0
Other comprehensive income before reclassifications, Defined Benefit Plans Tax (Expense) Benefit			0
Amortization of prior service cost, Defined Benefit Plans Tax (Expense) Benefit	(126)	(126)	(380)	(380)	(507)	(507)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	(2,418)	(2,571)	(7,252)	(7,714)	(9,823)	(10,980)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,179	2,326	6,537	6,980	1,679	249
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(365)	(371)	(1,095)	(1,114)	1	359
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans Tax (Expense) Benefit			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans Tax (Expense) Benefit			(1,095)
Ending Balance AOCI September 30, 2017, Defined Benefit Plans Tax (Expense) Benefit	20,798		20,798		20,798
Beginning Balance AOCI December 31, 2016, Defined Benefit Plans After-Tax			(35,720)
Translation adjustments, Defined Benefit Plans After-Tax			0
Other comprehensive income before reclassifications, Defined Benefit Plans After-Tax			0
Amortization of prior service cost, Defined Benefit Plans After-Tax	207	207	621	621	828	828
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	3,944	4,196	11,832	12,586	16,027	17,915
Regulatory adjustment, Defined Benefit Plans After-Tax			(10,667)
Net defined benefit pension plans	596	607	1,786	1,819	(4)	(583)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans After-Tax			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans After-Tax			1,786
Ending Balance AOCI September 30, 2017, Defined Benefit Plans After-Tax	(33,934)		(33,934)		(33,934)
Beginning Balance AOCI December 31, 2016, FSIRS - Before-Tax			(15,999)
Translation adjustments, FSIRS - Before-Tax			0
Other comprehensive income before reclassifications, FSIRS - Before-Tax			0
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	835	835	2,507	2,508	3,344	3,344
Net current period other comprehensive income (loss), FSIRS - Before-Tax	835	835	2,507	2,508	3,344	3,344
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - Before-Tax			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Before-Tax			2,507
Ending Balance AOCI September 30, 2017, FSIRS - Before-Tax	(13,492)		(13,492)		(13,492)
Beginning Balance AOCI December 31, 2016, FSIRS - Tax (Expense) Benefit			6,080
Translation adjustments, FSIRS - Tax (Expense) Benefit			0
Other comprehensive income before reclassifications, FSIRS - Tax (Expense) Benefit			0
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	(317)	(317)	(953)	(952)	(1,271)	(1,271)
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	(317)	(317)	(953)	(952)	(1,271)	(1,271)
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - Tax (Expense) Benefit			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Tax (Expense) Benefit			(953)
Ending Balance AOCI September 30, 2017, FSIRS - Tax (Expense) Benefit	5,127		5,127		5,127
Beginning Balance AOCI December 31, 2016, FSIRS - After-Tax			(9,919)
Translation adjustments, FSIRS - After-Tax			0
Other comprehensive income before reclassifications			0
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	518	518	1,554	1,556	2,073	2,073
Net current period other comprehensive income (loss)	518	518	1,554	1,556	2,073	2,073
Less: Translation adjustment attributable to redeemable noncontrolling interest			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - After-Tax			1,554
Ending Balance AOCI September 30, 2017, FSIRS - After-Tax	(8,365)		(8,365)		(8,365)
Beginning Balance AOCI December 31, 2016, Foreign Currency Items Before-Tax			(2,369)
Translation adjustments, Foreign Currency Items Before-Tax	1,012	(238)	1,861	614	1,408	233
Other comprehensive income before reclassifications	1,012	(238)	1,861	614	1,408	233
FSIRS amounts reclassified from AOCI, Foreign Currency Items Before-Tax			0
Amortization of prior service cost, Foreign Currency Items Before-Tax			0
Amortization of net actuarial loss, Foreign Currency Items Before-Tax			0
Regulatory adjustment, Foreign Currency Items Before-Tax			0
Net current period other comprehensive income (loss)			1,861
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency Items Before-Tax			11
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings,Inc., Foreign Currency Items Before-Tax			1,850
Ending Balance AOCI September 30, 2017	(519)		(519)		(519)
Beginning Balance AOCI December 31, 2016, Foreign Currency items - Tax (Expense) Benefit			0
Translation adjustments, Foreign Currency items - Tax (Expense) Benefit	0	0	0	0
Other comprehensive income before reclassifications, Foreign Currency items - Tax (Expense) Benefit			0
FSIRS amounts reclassified from AOCI, Foreign Currency items - Tax (Expense) Benefit			0
Amortization of prior service cost ,Foreign Currency items - Tax (Expense) Benefit			0
Amortization of net actuarial loss, Foreign Currency items - Tax (Expense) Benefit			0
Regulatory adjustment, Foreign Currency items - Tax (Expense) Benefit			0
Net current period other comprehensive income (loss), Foreign Currency items - Tax (Expense) Benefit	0	0	0	0
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - Tax (Expense) Benefit			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings,Inc., Foreign Currency items - Tax (Expense) Benefit	0	0	0	0	0	0
Ending Balance AOCI September 30, 2017, Foreign Currency items - Tax (Expense) Benefit	0		0		0
Beginning Balance AOCI December 31, 2016, Foreign Currency items - After-Tax			(2,369)
Translation adjustments, Foreign Currency items - After-Tax	1,012	(238)	1,861	614	1,408	233
Other comprehensive income before reclassifications, Foreign Currency items - After-Tax	1,012	(238)	1,861	614	1,408	233
FSIRS amounts reclassified from AOCI, Foreign Currency items - After-Tax			0
Amortization of prior service cost, Foreign Currency items - After-Tax			0
Amortization of net actuarial loss, Foreign Currency items - After-Tax			0
Regulatory adjustment, Foreign Currency items - After-Tax			0
Net current period other comprehensive income (loss), Foreign Currency items - After-Tax			1,861
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - After-Tax			11
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings,Inc., Foreign Currency Items - After-Tax			1,850
Ending Balance AOCI September 30, 2017, Foreign Currency items - After-Tax	(519)		(519)		(519)
Beginning Balance AOCI December 31, 2016, AOCI			(48,008)
Translation adjustments, AOCI			1,861
Other comprehensive income before reclassifications, AOCI			1,861
Amounts reclassified into net income	518	518	1,554	1,556	2,073	2,073
Amortization of prior service cost, AOCI	207	207	621	621	828	828
Amortization of net actuarial loss	3,944	4,196	11,832	12,586	16,027	17,915
Regulatory adjustment, AOCI			(10,667)
Total other comprehensive income (loss), net of tax	2,126	887	5,201	3,989	3,477	1,723
Less: Translation adjustment attributable to redeemable noncontrolling interest, AOCI			11
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, AOCI			5,190
Ending Balance AOCI September 30, 2017	(42,818)		(42,818)		(42,818)
Southwest Gas Corporation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance AOCI December 31, 2016, Defined Benefit Plans Before-Tax			(57,613)
Amortization of prior service cost, Defined Benefit Plans Before-Tax			1,001
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax			19,084
Regulatory adjustment, Defined Benefit Plans Before-Tax			(17,204)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans Before-Tax			2,881
Ending Balance AOCI September 30, 2017, Defined Benefit Plans Before-Tax	(54,732)		(54,732)		(54,732)
Beginning Balance AOCI December 31, 2016, Defined Benefit Plans Tax (Expense) Benefit			21,893
Amortization of prior service cost, Defined Benefit Plans Tax (Expense) Benefit			(380)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit			(7,252)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit			6,537
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans Tax (Expense) Benefit			(1,095)
Ending Balance AOCI September 30, 2017, Defined Benefit Plans Tax (Expense) Benefit	20,798		20,798		20,798
Beginning Balance AOCI December 31, 2016, Defined Benefit Plans After-Tax			(35,720)
Amortization of prior service cost, Defined Benefit Plans After-Tax	207	207	621	621	828	828
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	3,944	4,196	11,832	12,586	16,027	17,915
Regulatory adjustment, Defined Benefit Plans After-Tax			(10,667)
Net defined benefit pension plans	596	607	1,786	1,819	(4)	(583)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans After-Tax			1,786
Ending Balance AOCI September 30, 2017, Defined Benefit Plans After-Tax	(33,934)		(33,934)		(33,934)
Beginning Balance AOCI December 31, 2016, FSIRS - Before-Tax			(15,999)
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax			2,507
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Before-Tax			2,507
Ending Balance AOCI September 30, 2017, FSIRS - Before-Tax	(13,492)		(13,492)		(13,492)
Beginning Balance AOCI December 31, 2016, FSIRS - Tax (Expense) Benefit			6,080
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit			(953)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Tax (Expense) Benefit			(953)
Ending Balance AOCI September 30, 2017, FSIRS - Tax (Expense) Benefit	5,127		5,127		5,127
Beginning Balance AOCI December 31, 2016, FSIRS - After-Tax			(9,919)
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	518	518	1,554	1,556	2,073	2,073
Net current period other comprehensive income (loss)	518	518	1,554	1,556	2,073	2,073
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - After-Tax			1,554
Ending Balance AOCI September 30, 2017, FSIRS - After-Tax	(8,365)		(8,365)		(8,365)
Beginning Balance AOCI December 31, 2016, AOCI			(45,639)
Amounts reclassified into net income	518	518	1,554	1,556	2,073	2,073
Amortization of prior service cost, AOCI	207	207	621	621	828	828
Amortization of net actuarial loss	3,944	4,196	11,832	12,586	16,027	17,915
Regulatory adjustment, AOCI			(10,667)
Total other comprehensive income (loss), net of tax	1,114	$ 1,125	3,340	$ 3,375	2,069	$ 1,490
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, AOCI			3,340
Ending Balance AOCI September 30, 2017	$ (42,299)		$ (42,299)		$ (42,299)

Equity, Other Comprehensive I52

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Parenthetical) (Detail)	9 Months Ended
	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income loss, effective income tax rate	38.00%
Southwest Gas Corporation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income loss, effective income tax rate	38.00%

Equity, Other Comprehensive I53

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Amount Recognized Before Income Tax Associated with Defined Benefit Plans in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognized in AOCI	$ (54,732)	$ (57,613)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	(411,889)	(430,973)
Prior service cost	(4,702)	(5,703)
Less: amount recognized in regulatory assets	361,859	379,063
Recognized in AOCI	$ (54,732)	$ (57,613)

Construction Services Redeema54

Construction Services Redeemable Noncontrolling Interest - Additional Information (Detail) - Previous Owners [Member] - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Aug. 31, 2017	Sep. 30, 2017
Noncontrolling Interest [Line Items]
Purchase price of the previous owners' equity interest in Centuri	$ 23
Link-Line and WS Nicholls [Member]
Noncontrolling Interest [Line Items]
Description of equity interest		In conjunction with the acquisition of the Canadian construction businesses in October 2014, the previous owners of the acquired companies retained a 3.4% equity interest in Centuri, which, subject to an eligibility timeline, would have been redeemable at the election of the noncontrolling parties (in its entirety) beginning in July 2022. In August 2017, in advance of when otherwise eligible, the parties agreed to a current redemption. Southwest Gas Holdings, Inc. paid $23 million to the previous owners, thereby acquiring the remaining 3.4% equity interest in Centuri in accordance with an early redemption agreement. Accordingly, Centuri is now a wholly owned subsidiary of the Company.
Link-Line and WS Nicholls [Member] | Construction Services [Member]
Noncontrolling Interest [Line Items]
Percentage of earnings attribution by Centuri to previous owners	3.40%

Construction Services Redeema55

Construction Services Redeemable Noncontrolling Interest - Summary of Redeemable Noncontrolling Interest (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2017USD ($)
Redeemable Noncontrolling Interest [Line Items]
Balance	$ 22,590
Adjustment to redemption value	(355)
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Redeemable Noncontrolling Interest [Line Items]
Balance	22,590
Net income attributable to redeemable noncontrolling interest	248
Foreign currency exchange translation adjustment	11
Centuri dividend to redeemable noncontrolling interest	(204)
Adjustment to redemption value	355
Redemption of Centuri shares from noncontrolling parties	(23,000)
Balance	$ 0

Reorganization Impacts - Disc56

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation - Summary of Assets and Liabilities within Amounts Reported as Discontinued Operations - Construction Services in Condensed Consolidated Balance Sheet (Detail) - Construction Services [Member] - Southwest Gas Corporation [Member] $ in Thousands	Dec. 31, 2016USD ($)
Assets:
Other property and investments	$ 233,774
Cash and cash equivalents	9,042
Accounts receivable, net of allowances	173,300
Prepaids and other current assets	10,470
Goodwill	129,888
Other noncurrent assets	22,897
Discontinued operations - construction services - assets	579,371
Liabilities:
Current maturities of long-term debt	25,101
Accounts payable	46,440
Other current liabilities	74,518
Long-term debt, less current maturities	174,903
Deferred income taxes and other deferred credits	59,653
Discontinued operations-construction services-liabilities	$ 380,615

Reorganization Impacts - Disc57

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation - Components of Discontinued Operations - Construction Services Non-Owner Equity Amount in Condensed Consolidated Balance Sheet (Detail) - Southwest Gas Corporation [Member] $ in Thousands	Dec. 31, 2016USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued operations - construction services non-owner equity	$ 15,983
Construction Services [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Construction services equity	(4,390)
Construction services noncontrolling interest	(2,217)
Construction services redeemable noncontrolling interest	22,590
Discontinued operations - construction services non-owner equity	$ 15,983

Reorganization Impacts - Disc58

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation - Summary of Income Statement Components of Discontinued Operations - Construction Services in Condensed Consolidated Income Statements (Detail) - Southwest Gas Corporation [Member] - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating expenses:
Income before income taxes	$ 24,020	$ 31,867	$ 21,649	$ 54,934
Income tax expense	8,708	12,042	7,842	20,711
Net income	15,312	19,825	13,807	34,223
Net income attributable to noncontrolling interests	435	500	514	1,079
Discontinued operations - construction services - net income	14,877	19,325	13,293	33,144
Construction Services [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Construction revenues	339,790	838,038	301,040	1,127,982
Operating expenses:
Construction expenses	300,611	757,919	266,504	1,009,188
Depreciation and amortization	13,409	43,351	12,318	58,368
Operating income	25,770	36,768	22,218	60,426
Other income (deductions)	44	44	1,149	1,246
Net interest deductions	1,794	4,945	1,718	6,738
Income before income taxes	24,020	31,867	21,649	54,934
Income tax expense	8,708	12,042	7,842	20,711
Net income	15,312	19,825	13,807	34,223
Net income attributable to noncontrolling interests	435	500	514	1,079
Discontinued operations - construction services - net income	$ 14,877	$ 19,325	$ 13,293	$ 33,144