Southwest Gas 10-Q 8 Aug 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Jul. 31, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
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		49,133,829

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Utility plant:
Gas plant	$ 6,810,737	$ 6,629,644
Less: accumulated depreciation	(2,260,793)	(2,231,242)
Construction work in progress	167,114	125,248
Net utility plant	4,717,058	4,523,650
Other property and investments	462,236	428,180
Current assets:
Cash and cash equivalents	34,730	43,622
Accounts receivable, net of allowances	357,881	347,375
Accrued utility revenue	34,200	78,200
Income taxes receivable, net	18,256	7,960
Deferred purchased gas costs		14,581
Prepaids and other current assets	250,992	165,294
Total current assets	696,059	657,032
Noncurrent assets:
Goodwill	174,233	179,314
Deferred income taxes	1,252	1,480
Deferred charges and other assets	425,555	447,410
Total noncurrent assets	601,040	628,204
Total assets	6,476,393	6,237,066
Capitalization:
Common stock	50,756	49,720
Additional paid-in capital	1,021,508	955,332
Accumulated other comprehensive income (loss), net	(55,520)	(47,682)
Retained earnings	916,275	857,398
Total Southwest Gas Holdings, Inc./Total Southwest Gas Corporation equity	1,933,019	1,814,768
Noncontrolling interest	(452)	(2,365)
Total equity	1,932,567	1,812,403
Long-term debt, less current maturities	2,037,743	1,798,576
Total capitalization	3,970,310	3,610,979
Current liabilities:
Current maturities of long-term debt	31,928	25,346
Short-term debt	22,500	214,500
Accounts payable	188,156	228,315
Customer deposits	69,247	69,781
Income taxes payable		5,946
Accrued general taxes	42,826	43,879
Accrued interest	20,512	17,870
Deferred purchased gas costs	26,365	6,841
Other current liabilities	262,113	203,403
Total current liabilities	663,647	815,881
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	510,536	476,960
Accumulated removal costs	319,000	315,000
Other deferred credits and other long-term liabilities	1,012,900	1,018,246
Total deferred income taxes and other credits	1,842,436	1,810,206
Total capitalization and liabilities	6,476,393	6,237,066
Southwest Gas Corporation [Member]
Utility plant:
Gas plant	6,810,737	6,629,644
Less: accumulated depreciation	(2,260,793)	(2,231,242)
Construction work in progress	167,114	125,248
Net utility plant	4,717,058	4,523,650
Other property and investments	120,476	119,114
Current assets:
Cash and cash equivalents	20,900	37,946
Accounts receivable, net of allowances	83,645	119,748
Accrued utility revenue	34,200	78,200
Income taxes receivable, net	6,752
Deferred purchased gas costs		14,581
Prepaids and other current assets	233,441	153,771
Total current assets	378,938	404,246
Noncurrent assets:
Goodwill	10,095	10,095
Deferred charges and other assets	410,553	425,564
Total noncurrent assets	420,648	435,659
Total assets	5,637,120	5,482,669
Capitalization:
Common stock	49,112	49,112
Additional paid-in capital	1,006,065	948,767
Accumulated other comprehensive income (loss), net	(53,310)	(47,073)
Retained earnings	717,126	659,193
Total Southwest Gas Holdings, Inc./Total Southwest Gas Corporation equity	1,718,993	1,609,999
Total equity	1,718,993
Long-term debt, less current maturities	1,716,307	1,521,031
Total capitalization	3,435,300	3,131,030
Current liabilities:
Short-term debt		191,000
Accounts payable	102,579	158,474
Customer deposits	69,247	69,781
Income taxes payable		4,971
Accrued general taxes	42,826	43,879
Accrued interest	20,441	17,171
Deferred purchased gas costs	26,365	6,841
Payable to parent	285	194
Other current liabilities	162,207	108,785
Total current liabilities	423,950	601,096
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	473,581	445,243
Accumulated removal costs	319,000	315,000
Other deferred credits and other long-term liabilities	985,289	990,300
Total deferred income taxes and other credits	1,777,870	1,750,543
Total capitalization and liabilities	$ 5,637,120	$ 5,482,669

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Common stock, par	$ 1	$ 1
Common stock, authorized	60,000,000	60,000,000
Common stock, issued	49,126,254	48,090,470
Common stock, outstanding	49,126,254	48,090,470

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating revenues:
Gas operating revenues	$ 275,679	$ 260,162	$ 769,992	$ 722,764	$ 1,349,536	$ 1,263,428
Construction revenues	395,204	300,307	655,221	492,442	1,409,263	1,133,272
Total operating revenues	670,883	560,469	1,425,213	1,215,206	2,758,799	2,396,700
Gas operating revenues	275,679	260,162	769,992	722,764	1,349,536	1,263,428
Operating expenses:
Net cost of gas sold	83,466	69,421	269,198	216,300	407,943	328,405
Operations and maintenance	105,435	98,203	207,786	202,498	398,051	394,802
Depreciation and amortization	61,307	58,082	123,785	130,560	244,176	271,773
Taxes other than income taxes	14,666	14,497	29,923	29,279	58,590	54,655
Construction expenses	352,671	272,001	611,623	463,957	1,296,629	1,031,072
Total operating expenses	617,545	512,204	1,242,315	1,042,594	2,405,389	2,080,707
Operating income	53,338	48,265	182,898	172,612	353,410	315,993
Other income and (expenses):
Net interest deductions	(23,652)	(18,655)	(46,283)	(37,369)	(86,978)	(75,087)
Other income (deductions)	(2,706)	(2,810)	(7,040)	(3,800)	(9,270)	(8,401)
Total other income and (expenses)	(26,358)	(21,465)	(53,323)	(41,169)	(96,248)	(83,488)
Income before income taxes	26,980	26,800	129,575	131,443	257,162	232,505
Income tax expense	5,429	8,679	29,730	44,317	50,501	76,778
Net income (loss)	21,551	18,121	99,845	87,126	206,661	155,727
Net income (loss) attributable to noncontrolling interest		257	(797)	(46)	(650)	903
Net income (loss)	$ 21,551	$ 17,864	$ 100,642	$ 87,172	$ 207,311	$ 154,824
Basic earnings per share	$ 0.44	$ 0.38	$ 2.07	$ 1.83	$ 4.29	$ 3.26
Diluted earnings per share	0.44	0.37	2.07	1.82	4.28	3.24
Dividends declared per share	$ 0.520	$ 0.495	$ 1.040	$ 0.990	$ 2.030	$ 1.890
Average number of common shares	48,826	47,571	48,622	47,550	48,338	47,516
Average shares (assuming dilution)	48,880	47,884	48,671	47,874	48,387	47,857
Southwest Gas Corporation [Member]
Operating revenues:
Gas operating revenues	$ 275,679	$ 260,162	$ 769,992	$ 722,764	$ 1,349,536	$ 1,263,428
Gas operating revenues	275,679	260,162	769,992	722,764	1,349,536	1,263,428
Operating expenses:
Net cost of gas sold	83,466	69,421	269,198	216,300	407,943	328,405
Operations and maintenance	105,208	97,644	207,398	201,468	397,251	393,772
Depreciation and amortization	47,664	46,254	97,625	107,449	192,098	222,935
Taxes other than income taxes	14,666	14,497	29,923	29,279	58,590	54,655
Total operating expenses	251,004	227,816	604,144	554,496	1,055,882	999,767
Operating income	24,675	32,346	165,848	168,268	293,654	263,661
Other income and (expenses):
Net interest deductions	(20,149)	(16,991)	(39,404)	(34,201)	(74,936)	(68,407)
Other income (deductions)	(2,094)	(2,805)	(6,697)	(4,049)	(9,036)	(9,843)
Total other income and (expenses)	(22,243)	(19,796)	(46,101)	(38,250)	(83,972)	(78,250)
Income before income taxes	2,432	12,550	119,747	130,018	209,682	185,411
Income tax expense	(190)	3,028	26,776	43,558	46,353	59,469
Net income (loss)			92,971	86,460	163,329	155,061
Income from continuing operations	2,622	9,522	92,971	86,460	163,329	125,942
Discontinued operations-construction services, income before income taxes						45,669
Discontinued operations-construction services, Income tax expense						16,550
Discontinued operations-construction services, Income						29,119
Discontinued operations-construction services - Noncontrolling interests						949
Income - discontinued operations						28,170
Net income (loss)	$ 2,622	$ 9,522	$ 92,971	$ 86,460	$ 163,329	$ 154,112

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net income	$ 21,551	$ 18,121	$ 99,845	$ 87,126	$ 206,661	$ 155,727
Defined benefit pension plans:
Net actuarial gain (loss)					(32,701)	(14,118)
Amortization of prior service cost	254	207	508	414	922	828
Amortization of net actuarial loss	6,387	3,944	12,774	7,888	20,662	16,279
Regulatory adjustment	(5,744)	(3,556)	(11,490)	(7,112)	8,212	(2,982)
Net defined benefit pension plans	897	595	1,792	1,190	(2,905)	7
Forward-starting interest rate swaps:
Amounts reclassified into net income	636	518	1,271	1,036	2,308	2,073
Net forward-starting interest rate swaps	636	518	1,271	1,036	2,308	2,073
Foreign currency translation adjustments	(690)	629	(1,601)	849	(679)	158
Net other comprehensive income	843	1,742	1,462	3,075	(1,276)	2,238
Comprehensive income	22,394	19,863	101,307	90,201	205,385	157,965
Comprehensive income (loss) attributable to noncontrolling interests		279	(797)	(17)	(668)	908
Comprehensive income	22,394	19,584	102,104	90,218	206,053	157,057
Southwest Gas Corporation [Member]
Net income from continuing operations	2,622	9,522	92,971	86,460	163,329	125,942
Net income			92,971	86,460	163,329	155,061
Defined benefit pension plans:
Net actuarial gain (loss)					(32,701)	(14,118)
Amortization of prior service cost	254	207	508	414	922	828
Amortization of net actuarial loss	6,387	3,944	12,774	7,888	20,662	16,279
Regulatory adjustment	(5,744)	(3,556)	(11,490)	(7,112)	8,212	(2,982)
Net defined benefit pension plans	897	595	1,792	1,190	(2,905)	7
Forward-starting interest rate swaps:
Amounts reclassified into net income	636	518	1,271	1,036	2,308	2,073
Net forward-starting interest rate swaps	636	518	1,271	1,036	2,308	2,073
Net other comprehensive income	1,533	1,113	3,063	2,226	(597)	2,080
Comprehensive income	4,155	10,635	96,034	88,686	162,732	128,022
Discontinued operations-construction services, net income						28,170
Discontinued operations-construction services, foreign currency translation adjustments						(691)
Discontinued operations-construction services, comprehensive income						27,479
Discontinued operations-construction services, comprehensive income (loss) attributable to noncontrolling interests						(24)
Comprehensive income attributable to discontinued operations - construction services						27,503
Comprehensive income	$ 4,155	$ 10,635	$ 96,034	$ 88,686	$ 162,732	$ 155,525

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$ 99,845	$ 87,126	$ 206,661	$ 155,727
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	123,785	130,560	244,176	271,773
Deferred income taxes	33,318	47,836	48,871	70,652
Changes in current assets and liabilities:
Accounts receivable, net of allowances	(12,704)	3,545	(57,196)	(19,266)
Accrued utility revenue	44,000	42,600	(600)	(1,100)
Deferred purchased gas costs	34,105	(73,196)	11,693	(111,627)
Accounts payable	(44,465)	(50,447)	25,943	4,606
Accrued taxes	(17,350)	(6,100)	(9,138)	32,361
Other current assets and liabilities	(16,806)	20,390	(45,399)	(14,803)
Gains on sale	(250)	(1,427)	(3,019)	(5,833)
Changes in undistributed stock compensation	3,300	7,731	6,457	9,673
AFUDC	(586)	(1,109)	(1,773)	(2,116)
Changes in other assets and deferred charges	(5,122)	(11,521)	(15,870)	5,216
Changes in other liabilities and deferred credits	5,952	2,204	7,979	(13,741)
Net cash provided by operating activities	247,022	198,192	418,785	381,522
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(339,011)	(262,234)	(700,426)	(526,893)
Acquisition of businesses, net of cash acquired	(4,209)		(98,413)
Changes in customer advances	8,158	(1,430)	9,911	4,318
Miscellaneous inflows	2,564	6,905	12,304	15,818
Net cash used in investing activities	(332,498)	(256,759)	(776,624)	(506,757)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	69,139	(96)	110,390	(111)
Dividends paid	(48,985)	(44,949)	(96,166)	(87,683)
Centuri distribution to redeemable noncontrolling interest		(204)		(544)
Issuance of long-term debt, net	455,398	80,579	781,882	408,397
Retirement of long-term debt	(100,776)	(60,041)	(379,704)	(262,348)
Change in credit facility and commercial paper	(102,000)	87,000	(44,000)	89,500
Change in short-term debt	(192,000)	2,500	20,000	2,500
Principal payments on capital lease obligations	(316)	(573)	(723)	(1,092)
Redemption of Centuri shares from noncontrolling parties			(23,000)
Withholding remittance - share-based compensation	(2,854)	(3,120)	(2,910)	(3,271)
Other	(898)	(1,036)	(2,936)	(2,481)
Net cash provided by financing activities	76,708	60,060	362,833	142,867
Effects of currency translation on cash and cash equivalents	(124)	143	34	(56)
Change in cash and cash equivalents	(8,892)	1,636	5,028	17,576
Cash and cash equivalents at beginning of period	43,622	28,066	29,702	12,126
Cash and cash equivalents at end of period	34,730	29,702	34,730	29,702
Supplemental information:
Interest paid, net of amounts capitalized	40,082	35,182	76,843	69,398
Income taxes paid (received)	16,507	3,043	19,137	(20,726)
Southwest Gas Corporation [Member]
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	92,971	86,460	163,329	155,061
Income from discontinued operations				29,119
Income from continuing operations	92,971	86,460	163,329	125,942
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	97,625	107,449	192,098	222,935
Deferred income taxes	27,371	46,874	47,666	74,241
Changes in current assets and liabilities:
Accounts receivable, net of allowances	36,104	37,484	(9,282)	(973)
Accrued utility revenue	44,000	42,600	(600)	(1,100)
Deferred purchased gas costs	34,105	(73,196)	11,693	(111,627)
Accounts payable	(52,095)	(55,707)	8,157	3,157
Accrued taxes	(12,776)	(2,823)	430	23,024
Other current assets and liabilities	(24,366)	24,265	(62,357)	(14,022)
Changes in undistributed stock compensation	3,220	6,931	5,577	8,873
AFUDC	(586)	(1,109)	(1,773)	(2,116)
Changes in other assets and deferred charges	(5,490)	(11,801)	(16,607)	4,761
Changes in other liabilities and deferred credits	5,477	1,695	7,323	(14,250)
Net cash provided by operating activities	245,560	209,122	345,654	318,845
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(285,999)	(224,085)	(622,362)	(466,780)
Changes in customer advances	8,158	(1,430)	9,911	4,318
Miscellaneous inflows	778	1,354	2,165	2,546
Dividends received				9,660
Net cash used in investing activities	(277,063)	(224,161)	(610,286)	(450,256)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net				(15)
Contributions from parent	56,596		97,955
Dividends paid	(43,000)	(39,896)	(84,601)	(82,630)
Issuance of long-term debt, net	297,495		297,495	296,469
Retirement of long-term debt		(25,000)		(149,855)
Change in credit facility and commercial paper	(102,000)	87,000	(44,000)	89,500
Change in short-term debt	(191,000)
Withholding remittance - share-based compensation	(2,855)	(3,120)	(2,911)	(3,271)
Other	(779)	(525)	(850)	(1,970)
Net cash provided by financing activities	14,457	18,459	263,088	148,228
Net cash provided by discontinued operating activities				73,607
Net cash used in discontinued investing activities				(23,903)
Net cash used in discontinued financing activities				(46,962)
Effects of currency translation on cash and cash equivalents				(199)
Change in cash and cash equivalents	(17,046)	3,420	(1,544)	19,360
Change in cash and cash equivalents of discontinued operations included in discontinued operations construction services assets				(2,543)
Change in cash and cash equivalents of continuing operations	(17,046)	3,420	(1,544)	16,817
Cash and cash equivalents at beginning of period	37,946	19,024	22,444	5,627
Cash and cash equivalents at end of period	20,900	22,444	20,900	22,444
Supplemental information:
Interest paid, net of amounts capitalized	33,452	32,205	66,037	63,221
Income taxes paid (received)	$ 10,886	$ 19	$ 3,013	$ (32,885)

Nature of Operations and Basis

Nature of Operations and Basis of Presentation	6 Months Ended
	Jun. 30, 2018
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. 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”), Canyon Pipeline Construction, Inc. (“Canyon”), NPL Canada Ltd. (“NPL Canada”), W.S. Nicholls Construction, Inc. (“W.S. Nicholls”), and Canyon Special Projects, Inc. (“Special Projects,” formerly Brigadier Pipelines Inc.). 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. Centuri acquired New England Utility Constructors, Inc. (“Neuco”) in November 2017, thereby expanding its core services in the Northeast region of the United States. See Note 11 – Acquisition of Construction Services Business Basis of Presentation. Form 10-Q, 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, or due to the acquisition of Neuco. Following the 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 2017 Annual Report to Shareholders, which is incorporated by reference into the 2017 Form 10-K. Early Adoption of Accounting Standards Update (“ASU”) No. 2018-02. No. 2018-02 Note 9 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income Prepaids and other current assets. Income Taxes. gross-up. In the first quarter of 2018, management recorded a regulatory liability and reduced utility revenues by approximately $14 million for potential regulatory rate reductions to customers resulting from the reduced cost-of-service levels during the period. Based on regulatory activity in the second quarter of 2018, management has updated its estimated reserve to approximately $12.5 million. In July 2018, the Arizona Corporation Commission (“ACC”) staff issued a recommended opinion and order that would require Southwest to return to customers amounts related to excess cost-of-service Other current liabilities Cash and Cash Equivalents. Significant non-cash non-cash Goodwill. st Goodwill: (Thousands of dollars) Natural Construction Consolidated December 31, 2017 $ 10,095 $ 169,219 $ 179,314 Additional goodwill from Neuco acquisition — 182 182 Foreign currency translation adjustment — (5,263 ) (5,263 ) June 30, 2018 $ 10,095 $ 164,138 $ 174,233 Intercompany Transactions Note 4 - Segment Information June 30, 2018 December 31, 2017 Centuri accounts receivable for services provided to Southwest $ 13,899 $ 12,987 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. June 30, 2018 December 31, 2017 Southwest Gas Corporation: Net cash surrender value of COLI policies $ 118,733 $ 117,341 Other property 1,743 1,773 Total Southwest Gas Corporation 120,476 119,114 Centuri property, equipment, and intangibles 606,750 554,730 Centuri accumulated depreciation/amortization (278,209 ) (258,906 ) Other property 13,219 13,242 Total Southwest Gas Holdings, Inc. $ 462,236 $ 428,180 Other Income (Deductions). Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2018 2017 2018 2017 2018 2017 Southwest Gas Corporation - natural gas operations segment: Change in COLI policies $ 2,000 $ 1,900 $ 1,300 $ 4,700 $ 6,900 $ 9,000 Interest income 1,377 614 2,795 1,178 4,401 2,269 Equity AFUDC 357 633 586 1,109 1,773 2,116 Other components of net periodic benefit cost (5,264 ) (4,857 ) (10,529 ) (9,712 ) (20,241 ) (19,591 ) Miscellaneous income and (expense) (564 ) (1,095 ) (849 ) (1,324 ) (1,869 ) (3,637 ) Southwest Gas Corporation - total other income (deductions) (2,094 ) (2,805 ) (6,697 ) (4,049 ) (9,036 ) (9,843 ) Construction services segment: Interest income 1 1 2 1 4 1 Foreign transaction gain (loss) 202 (197 ) 349 (198 ) (207 ) (201 ) Miscellaneous income and (expense) (835 ) 190 (720 ) 445 (69 ) 1,641 Centuri - total other income (deductions) (632 ) (6 ) (369 ) 248 (272 ) 1,441 Corporate and administrative 20 1 26 1 38 1 Consolidated Southwest Gas Holdings, Inc. - total other income (deductions) $ (2,706 ) $ (2,810 ) $ (7,040 ) $ (3,800 ) $ (9,270 ) $ (8,401 ) 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 Note 2 – Components of Net Periodic Benefit Cost Recently Issued Accounting Standards Updates. 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

Components of Net Periodic Bene

Components of Net Periodic Benefit Cost	6 Months Ended
	Jun. 30, 2018
Text Block [Abstract]
Components of Net Periodic Benefit Cost	Note 2 – Components of Net Periodic Benefit Cost As of January 1, 2018, the Company and Southwest adopted “Compensation – Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost.” 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 six-, 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. During the first quarter of 2018, qualifying term-vested participants were offered a lump-sum one-quarter The service cost component of net periodic benefit costs included in the table below are components of an overhead loading process associated with the cost of labor (refer to discussion above related to the update to Topic 715). The overhead process ultimately results in allocation of that portion of overall net periodic benefit costs to the same accounts to which productive labor is charged. As a result, service costs become components of various accounts, primarily operations and maintenance expense, net utility plant, and deferred charges and other assets for both the Company and Southwest. Refer also to the practical expedient elected related to amounts capitalized as part of assets prior to the adoption date. Qualified Retirement Plan Period Ended June 30, Three Months Six Months Twelve Months 2018 2017 2018 2017 2018 2017 (Thousands of dollars) Service cost $ 7,139 $ 5,848 $ 14,278 $ 11,696 $ 25,974 $ 23,112 Interest cost 11,044 11,521 22,087 23,041 45,129 46,055 Expected return on plan assets (14,688 ) (13,799 ) (29,377 ) (27,598 ) (56,975 ) (55,877 ) Amortization of net actuarial loss 8,028 6,001 16,057 12,002 28,059 24,635 Net periodic benefit cost $ 11,523 $ 9,571 $ 23,045 $ 19,141 $ 42,187 $ 37,925 SERP Period Ended June 30, Three Months Six Months Twelve Months 2018 2017 2018 2017 2018 2017 (Thousands of dollars) Service cost $ 61 $ 77 $ 122 $ 155 $ 276 $ 321 Interest cost 414 471 829 942 1,770 1,871 Amortization of net actuarial loss 376 360 751 720 1,472 1,411 Net periodic benefit cost $ 851 $ 908 $ 1,702 $ 1,817 $ 3,518 $ 3,603 PBOP Period Ended June 30, Three Months Six Months Twelve Months 2018 2017 2018 2017 2018 2017 (Thousands of dollars) Service cost $ 369 $ 367 $ 737 $ 734 $ 1,471 $ 1,484 Interest cost 687 808 1,374 1,616 2,990 3,205 Expected return on plan assets (930 ) (839 ) (1,860 ) (1,679 ) (3,539 ) (3,253 ) Amortization of prior service costs 334 334 668 668 1,335 1,335 Amortization of net actuarial loss — — — — — 209 Net periodic benefit cost $ 460 $ 670 $ 919 $ 1,339 $ 2,257 $ 2,980

Revenue

Revenue	6 Months Ended
	Jun. 30, 2018
Revenue from Contract with Customer [Abstract]
Revenue	Note 3 – Revenue Effective January 2018, the Company and Southwest adopted the FASB Accounting Standards Codification update, Topic 606, “Revenue from Contracts with Customers”, using the modified retrospective transition method. Under the modified retrospective approach, the information for periods prior to the adoption date has not been restated and continues to be reported under the accounting standards in effect for those periods. As permitted under the standard, the Company and Southwest have elected to apply the guidance retrospectively only to those contracts that were not completed at January 1, 2018. Management assessed the effects the new guidance has on the Company’s (and Southwest’s, in the case of utility operations) financial position, results of operations, and cash flows. Based on these assessments, the adoption of Topic 606 had no material impact on any of the financial statements of Southwest or the Company. The following information about the Company’s revenues is presented by segment. Southwest comprises one segment – natural gas operations. Natural Gas Operations Segment Southwest is engaged in the business of purchasing, distributing, and transporting natural gas for customers in portions of Arizona, Nevada, and California. Public utility rates, practices, facilities, and service territories of Southwest are subject to regulatory oversight. Southwest recognizes revenue when it satisfies its performance by transferring gas to the customer. Revenues also include the net impacts of margin tracker/decoupling accruals based on criteria in U.S. GAAP for rate-regulated entities associated with alternative revenue programs. Revenues from customer arrangements and from alternative revenue programs are described below. Southwest acts as an agent for state and local taxing authorities in the collection and remission of a variety of taxes, including sales and use taxes and surcharges. These taxes are not included in gas operating revenues. Management uses the net classification method to report taxes collected from customers to be remitted to governmental authorities. Southwest generally has two types of services to its customers: tariff sales and transportation–only service. Tariff sales encompass sales to many types of customers (primarily residential) under various rate schedules, subject to cost-of-service ratemaking, Transportation-only service is also governed by tariff rate provisions. Transportation-only service is generally only available to very large customers under requirements of Southwest’s various tariffs. With this service, customers secure their own gas supply and Southwest provides transportation services to move the customer-supplied gas to the intended location. Southwest concluded that transportation/transmission service is suitable to an “over time” model. Rate structures under Southwest’s regulation for transportation customers include a combination of volumetric charges and monthly “fixed” charges (including charges commonly referred to as capacity charges, demand charges, or reservation charges) as part of the rate design of regulated jurisdictions. These types of fixed charges represent a separate performance obligation associated with standing ready over the period of the month to deliver quantities of gas, regardless of whether the customer takes delivery of any quantity of gas. The performance obligations under these circumstances are satisfied over the course of the month under an output measure of progress based on time, which correlates to the period for which the charges are eligible to be invoiced. Under its regulation, Southwest enters into negotiated rate contracts for those customers located in proximity to another pipeline, which pose a threat of bypassing its distribution system. Southwest may also enter into similar contracts for customers otherwise able to satisfy their energy needs by means of alternative fuel to natural gas. Less than two dozen customers are party to contracts with rate components subject to negotiation. Many rate provisions and terms of service for these less common types of contracts are also subject to regulatory oversight and tariff provisions. The performance obligations for these customers are satisfied similar to those for other customers by means of transporting/delivering natural gas to the customer. Many or most of the rate components, and structures, for these types of customers are the same as those for similar customers without negotiated rate components; and the negotiated rates are within the parameters of the tariff guidelines. Management determined that these arrangements qualify for the invoice practical expedient for recognizing revenue. Furthermore, while some of these contracts include contract periods extending over time, including multiple years, as amounts billable under the contract are based on rates in effect for the customer for service provided to date, no significant financing component is deemed to exist. As indicated above, revenues also include the net impacts of margin tracker/decoupling accruals. All of Southwest’s service territories have decoupled rate structures (also referred to as alternative revenue programs) that are designed to eliminate the direct link between volumetric sales and revenue, thereby mitigating the impacts of unusual weather variability and conservation on margin. The primary alternative revenue programs involve permissible adjustments for differences between stated tariff benchmarks and amounts billable through revenue from contracts with customers via existing rates. Such adjustments are recognized monthly in revenue and in the associated regulatory asset/liability in advance of rate adjustments intended to collect or return amounts recognized. Revenues recognized for the adjustment to the benchmarks noted are required to be presented separately from revenues from contracts from customers, and as such, are provided below and identified as alternative revenue program revenue. Gas operating revenues on the Condensed Consolidated Statements of Income of both the Company and Southwest include revenue from contracts with customers, which is shown below disaggregated by customer type, and various categories of revenue: Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, (Thousands of dollars) 2018 2017 2018 2017 2018 2017 Residential $ 166,702 $ 158,442 $ 511,313 $ 501,179 $ 867,338 $ 850,344 Small commercial 55,653 51,018 143,596 132,143 254,966 235,684 Large commercial 13,134 12,781 28,574 26,376 54,577 49,561 Industrial/other 5,491 5,043 12,001 10,494 23,533 20,157 Transportation 20,719 20,958 44,773 43,690 88,842 86,457 Revenue from contracts with customers 261,699 248,242 740,257 713,882 1,289,256 1,242,203 Alternative revenue program revenues (deferrals) 10,393 10,135 37,602 5,903 67,046 17,280 Other revenues (a) 3,587 1,785 (7,867 ) 2,979 (6,766 ) 3,945 Total Gas operating revenues $ 275,679 $ 260,162 $ 769,992 $ 722,764 $ 1,349,536 $ 1,263,428 (a) Includes various other revenues, including $1.6 million during the three months, and an offset of $12.5 million in both the six months and twelve months ending June 30, 2018 related to tax reform savings adjustments. Refer to Income Taxes Note 1 – Nature of Operations and Basis of Presentation Construction Services Segment The majority of Centuri contracts are performed under unit-price contracts. Generally, these contracts state prices per unit of installation. Typical installations are accomplished in a few weeks or less. Revenues are recorded as installations are completed. Revenues are recorded for long-term fixed-price contracts in a pattern that reflects the transfer of control of promised goods and services to the customer over time. The amount of revenue recognized on fixed-price contracts is based on costs expended to date relative to anticipated final contract costs. Some unit-price contracts contain caps that if encroached, trigger revenue and loss recognition similar to a fixed-price contract model. Centuri is required to collect taxes imposed by various governmental agencies on the work performed by Centuri for its customers. These taxes are not included in construction revenues. Management uses the net classification method to report taxes collected from customers to be remitted to governmental authorities. Centuri derives revenue from the installation, replacement, repair, and maintenance of energy distribution systems, and in developing industrial construction solutions. Centuri has operations in the U.S. and Canada. The majority of Centuri’s revenues are related to construction contracts for natural gas pipeline replacement and installation work for natural gas utilities. In addition, Centuri performs certain industrial construction activities for various customers and industries. Centuri has two types of agreements with its customers: master services agreements (“MSAs”) and bid contracts. Most of Centuri’s customers supply many of their own materials in order for Centuri to complete its work under the contracts. An MSA identifies most of the terms describing each party’s rights and obligations that will govern future work authorizations. An MSA is often effective for multiple years. A work authorization is issued by the customer to describe the location, timing, and any additional information necessary to complete the work for the customer. The combination of the MSA and the work authorization determines when a contract exists and revenue recognition may begin. Each work authorization is generally a single performance obligation as Centuri is performing a significant integration service. Centuri has elected to use the portfolio method practical expedient at the customer level as the terms and conditions of the work performed under MSAs are similar in nature with each customer but vary significantly between customers. A bid contract is typically a one-time agreement Centuri’s MSA and bid contracts are characterized as either fixed-price contracts or unit-price contracts for revenue recognition purposes. The cost-to-cost Actual revenues and project costs can vary, sometimes substantially, from previous estimates due to changes in a variety of factors including unforeseen circumstances not originally contemplated. These factors, along with other risks inherent in performing fixed-price contracts may cause actual revenues and gross profit for a project to differ from previous estimates and could result in reduced profitability or losses on projects. Changes in these factors may result in revisions to costs and earnings, the impacts for which are recognized in the period in which the changes are identified. Once identified, these types of conditions continue to be evaluated for each project throughout the project term and ongoing revisions in management’s estimates of contract value, contract cost, and contract profit are recognized as necessary in the period determined. Centuri categorizes work performed under MSAs and bid contracts into three primary service types: replacement gas construction, new gas construction, and other construction. Replacement gas construction includes work involving previously existing gas pipelines. New gas construction involves the installation of new pipelines or service lines to areas that do not already have gas services. Other construction includes all other work and can include industrial construction, water infrastructure construction, electric infrastructure construction, etc. Contracts can have compensation/consideration that is variable. For MSAs, variable consideration is evaluated at the customer level as the terms creating variability in pricing are included within the MSA and are not specific to a work authorization. For multi-year MSAs, variable consideration items are typically determined for each year of the contract and not for the full contract term. For bid contracts, variable consideration is evaluated at the individual contract level. The expected value method or most likely amount method is used based on the nature of the variable consideration. Types of variable consideration include liquidated damages, delay penalties, performance incentives, safety bonuses, payment discounts, and volume rebates. Centuri will typically estimate variable consideration and adjust financial information, as necessary. Change orders involve the modification in scope, price, or both to the current contract, requiring approval by both parties. The existing terms of the contract continue to be accounted for under the current contract until such time as a change order is approved. Once approved, the change order is either treated as a separate contract or as part of the existing contract, as appropriate, under the circumstances. When the scope is agreed upon in the change order but not the price, Centuri estimates the change to the transaction price. The following tables display Centuri’s revenue from contracts with customers disaggregated by service type and contract type: (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2018 2017 2018 2017 2018 2017 Service Types: Replacement gas construction $256,070 $181,512 $413,421 $301,950 $899,537 $733,927 New gas construction 44,276 40,226 80,473 73,305 172,545 197,100 Other construction 94,858 78,569 161,327 117,187 337,181 202,245 Total Construction revenues $395,204 $300,307 $655,221 $492,442 $1,409,263 $1,133,272 (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2018 2017 2018 2017 2018 2017 Contract Types: Master services agreement $290,075 $208,361 $484,539 $355,553 $1,014,517 $825,980 Bid contract 105,129 91,946 170,682 136,889 394,746 307,292 Total Construction revenues $395,204 $300,307 $655,221 $492,442 $1,409,263 $1,133,272 Unit priced contracts $345,390 $272,427 $579,675 $450,098 $1,249,602 $974,107 Fixed priced contracts 49,814 27,880 75,546 42,344 159,661 159,165 Total Construction revenues $395,204 $300,307 $655,221 $492,442 $1,409,263 $1,133,272 The following table provides information about receivables, revenue earned on contracts in progress in excess of billings, which are included within accounts receivable, net of allowances, and amounts billed in excess of revenue earned on contracts, which is included in other current liabilities as of June 30, 2018 and December 31, 2017 on the Company’s Condensed Consolidated Balance Sheets: (Thousands of dollars) June 30, 2018 December 31, 2017 Contracts receivable, net $ 187,485 $ 221,859 Revenue earned on contracts in progress in excess of billings 86,750 5,768 Amounts billed in excess of revenue earned on contracts 8,592 9,602 The revenue earned on contracts in progress in excess of billings primarily relates to Centuri’s rights to consideration for work completed but not billed and/or approved at the reporting date. The revenue earned on contracts in progress in excess of billings are transferred to contracts receivable when the rights become unconditional. The amounts billed in excess of revenue earned on contracts primarily relates to the advance consideration received from customers for which work has not yet been completed. The amount of revenue recognized in 2018 from performance obligations satisfied (or partially satisfied) in previous periods under these contracts is $34.7 million for the three months ended June 30, 2018 and $88.7 million for the six months ended June 30, 2018. For Centuri’s contracts that have an original duration of one year or less, Centuri uses the practical expedient applicable to such contracts and does not consider the time value of money. Further, because of the short duration of these contracts, Centuri has not disclosed the transaction price for the remaining performance obligations as of the end of each reporting period or when the Company expects to recognize the revenue. Centuri has sixteen contracts that had an original duration of more than one year. The aggregate amount of the transaction price allocated to the unsatisfied performance obligations of these contracts as of June 30, 2018 is $86 million. Centuri expects to recognize the remaining performance obligations over the next four years, however, the timing of that recognition is largely within the control of the customer, including when the necessary equipment and materials required to complete the work are provided by the customer. Construction services contracts receivable consists of the following: (Thousands of dollars) June 30, 2018 Billed on completed contracts and contracts in progress $ 185,911 Other receivables 1,660 Contracts receivable, gross 187,571 Allowance for doubtful accounts (86 ) Contracts receivable, net $ 187,485 The balance of contracts receivable above is included in Accounts Receivable, net of allowances in the Southwest Gas Holdings, Inc. Condensed Consolidated Balance Sheet at June 30, 2018. Management recognizes revenue on contracts in progress in excess of billings (a contract asset) within Accounts receivable, net of allowances in the Company’s Condensed Consolidated Balance Sheets, and amounts billed in excess of revenue earned (a contract liability) in Other current liabilities. However, the following shows the significant changes in these asset and liability balances associated with Centuri since January 1, 2018: (Thousands of dollars) June 30, 2018 Revenue earned on Amounts billed in Revenue recognized that was included in the amounts billed in excess of revenue earned on contracts balance at the beginning of the period $ — $ (9,602 ) Increases due to amounts billed to customers in excess of revenue earned during the period — 8,592 Transferred to contracts receivable from revenue earned on contracts in progress in excess of billings recognized at the beginning of the period (5,768 ) — Increases from the reclassification of contract assets due to the adoption of topic 606 51,744 — Increases from contract assets, contingent on a future event occurring 35,006 — In regards to the table above, prior to the adoption of ASC Topic 606, revenue earned on contracts in progress in excess of billings was only used to recognize contract assets related to fixed-price contracts under previous accounting guidance. This balance now includes any conditional contract assets for both fixed-price contracts and unit-price contracts. Centuri considers retention and unbilled amounts to customers to be conditional contract assets, as payment is contingent on the occurrence of a future event. Contracts receivable, net, included in Accounts receivable, net of allowances, includes only amounts that are unconditional in nature, which means only the passage of time remains and Centuri has invoiced the customer. Similarly, amounts billed in excess of revenue earned on contracts, which is included in the Other current liabilities line item on the Company’s Condensed Consolidated Balance Sheets, was only used to recognize contract liabilities related to fixed-price contracts under previous accounting guidance. This line item now includes contract liabilities related to both fixed-price contracts and unit-price contracts. In the event a contract asset or contract liability is expected to be recognized for greater than one year from the financial statement date, Centuri classifies those amounts as long-term contract assets or contract liabilities, included in Other deferred credits and other long-term liabilities on the Company’s Condensed Consolidated Balance Sheets.

Segment Information

Segment Information	6 Months Ended
	Jun. 30, 2018
Segment Reporting [Abstract]
Segment Information	Note 4 – 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 June 30, 2018 Revenues from external customers $ 275,679 $ 362,132 $ — $ 637,811 Intersegment revenues — 33,072 — 33,072 Total $ 275,679 $ 395,204 $ — $ 670,883 Segment net income (loss) $ 2,622 $ 19,236 $ (307 ) $ 21,551 Three months ended June 30, 2017 Revenues from external customers $ 260,162 $ 277,384 $ — $ 537,546 Intersegment revenues — 22,923 — 22,923 Total $ 260,162 $ 300,307 $ — $ 560,469 Segment net income (loss) $ 9,522 $ 8,716 $ (374 ) $ 17,864 Natural Gas Construction Other Total Six months ended June 30, 2018 Revenues from external customers $ 769,992 $ 594,991 $ — $ 1,364,983 Intersegment revenues — 60,230 — 60,230 Total $ 769,992 $ 655,221 $ — $ 1,425,213 Segment net income (loss) $ 92,971 $ 8,235 $ (564 ) $ 100,642 Six months ended June 30, 2017 Revenues from external customers $ 722,764 $ 448,223 $ — $ 1,170,987 Intersegment revenues — 44,219 — 44,219 Total $ 722,764 $ 492,442 $ — $ 1,215,206 Segment net income (loss) $ 86,460 $ 1,382 $ (670 ) $ 87,172 Natural Gas Construction Other Total Twelve months ended June 30, 2018 Revenues from external customers $ 1,349,536 $ 1,296,093 $ — $ 2,645,629 Intersegment revenues — 113,170 — 113,170 Total $ 1,349,536 $ 1,409,263 $ — $ 2,758,799 Segment net income (loss) $ 163,329 $ 45,213 $ (1,231 ) $ 207,311 Twelve months ended June 30, 2017 Revenues from external customers $ 1,263,428 $ 1,038,876 $ — $ 2,302,304 Intersegment revenues — 94,396 — 94,396 Total $ 1,263,428 $ 1,133,272 $ — $ 2,396,700 Segment net income (loss) $ 125,942 $ 29,552 $ (670 ) $ 154,824

Derivatives and Fair Value Meas

Derivatives and Fair Value Measurements	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Derivatives and Fair Value Measurements	Note 5 – 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 July 2018 through October 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): June 30, 2018 December 31, 2017 Contract notional amounts 13,035 10,929 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-, six-, Gains (losses) recognized in income for derivatives not designated as hedging instruments: (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended Location of Gain or (Loss) Recognized in Income on Derivative June 30 June 30 June 30 Instrument 2018 2017 2018 2017 2018 2017 Swaps Net cost of gas sold $ 870 $ (1,168 ) $ (4,326 ) $(6,305) $(9,593) $ (5,624 ) Swaps Net cost of gas sold (870 )* 1,168 * 4,326 * 6,305 * 9,593 * 5,624 * 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: June 30, 2018 Instrument Balance Sheet Location Asset Liability Net Swaps Other current liabilities $ 654 $ (5,901 ) $ (5,247 ) Swaps Other deferred credits 83 (1,016 ) (933 ) Total $ 737 $ (6,917 ) $ (6,180 ) December 31, 2017 Instrument Balance Sheet Location Asset Liability Net Swaps Other current liabilities $ 11 $ (4,468 ) $ (4,457 ) Swaps Other deferred credits 19 (1,342 ) (1,323 ) Total $ 30 $ (5,810 ) $ (5,780 ) 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 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 Six Months Ended Twelve Months Ended (Thousands of dollars) June 30, 2018 June 30, 2018 June 30, 2018 Paid to counterparties $ 1,216 $ 3,931 $ 5,620 Received from counterparties $ 6 $ 6 $ 6 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). June 30, 2018 Instrument Balance Sheet Location Net Total Swaps Prepaids and other current assets $ 5,247 Swaps Deferred charges and other assets 933 December 31, 2017 Instrument Balance Sheet Location Net Total Swaps Prepaids and other current assets $ 4,457 Swaps Deferred charges and other assets 1,323 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) June 30, 2018 December 31, 2017 Liabilities at fair value: Other current liabilities - Swaps $ (5,247 ) $ (4,457 ) Other deferred credits - Swaps (933 ) (1,323 ) Net Assets (Liabilities) $ (6,180 ) $ (5,780 ) 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 2017. Refer to Note 11 – Pension and Other Post Retirement Benefits 10-K, 10-K.

Common Stock

Common Stock	6 Months Ended
	Jun. 30, 2018
Text Block [Abstract]
Common Stock	Note 6 – Common Stock On March 29, 2017, the Company filed with the SEC an automatic shelf registration statement on Form S-3 No. 333-217018), at-the-market During the six months ended June 30, 2018, the Company issued approximately 77,000 shares of common stock through the Restricted Stock/Unit Plan and Management Incentive Plan. Also during the six months ended June 30, 2018, the Company issued 73,000 shares of common stock through the Dividend Reinvestment and Stock Purchase Plan (“DRSPP”), raising approximately $5 million.

Long-Term Debt

Long-Term Debt	6 Months Ended
	Jun. 30, 2018
Debt Disclosure [Abstract]
Long-Term Debt	Note 7 – Long-Term Debt Carrying amounts of long-term debt and related estimated fair values as of June 30, 2018 and December 31, 2017 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. June 30, 2018 December 31, 2017 Carrying Fair Carrying Fair Amount Value Amount Value (Thousands of dollars) Southwest Gas Corporation: Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 127,258 $ 125,000 $ 129,273 Notes, 6.1%, due 2041 125,000 150,816 125,000 158,304 Notes, 3.875%, due 2022 250,000 252,108 250,000 256,163 Notes, 4.875%, due 2043 250,000 267,985 250,000 283,243 Notes, 3.8%, due 2046 300,000 281,028 300,000 302,970 Notes, 3.7%, due 2028 300,000 298,068 — — 8% Series, due 2026 75,000 95,789 75,000 96,063 Medium-term notes, 7.78% series, due 2022 25,000 27,984 25,000 28,714 Medium-term notes, 7.92% series, due 2027 25,000 30,228 25,000 31,542 Medium-term notes, 6.76% series, due 2027 7,500 8,740 7,500 8,882 Unamortized discount and debt issuance costs (12,259 ) (9,350 ) 1,470,241 1,173,150 Revolving credit facility and commercial paper 48,000 48,000 150,000 150,000 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 (1,934 ) (2,119 ) 198,066 197,881 Less: current maturities — — Long-term debt, less current maturities - Southwest Gas Corporation $ 1,716,307 $ 1,521,031 Centuri: Centuri term loan facility $ 189,876 $ 190,524 $ 199,578 $ 207,588 Unamortized debt issuance costs (999 ) (1,111 ) 188,877 198,467 Centuri secured revolving credit facility 87,952 87,998 56,472 56,525 Centuri other debt obligations 76,535 76,419 47,952 48,183 Less: current maturities (31,928 ) (25,346 ) Long-term debt, less current maturities - Centuri $ 321,436 $ 277,545 Consolidated Southwest Gas Holdings, Inc.: Southwest Gas Corporation long-term debt $ 1,716,307 $ 1,521,031 Centuri long-term debt 353,364 302,891 Less: current maturities (31,928 ) (25,346 ) Long-term debt, less current maturities - Southwest Gas Holdings, Inc. $ 2,037,743 $ 1,798,576 Southwest has a $400 million credit facility that is scheduled to expire in March 2022. Southwest designates $150 million of capacity related to the facility as long-term debt and has designated the remaining $250 million for working capital purposes. Interest rates for the credit facility are calculated at either the London Interbank Offered Rate (“LIBOR”) or an “alternate base rate,” plus in each case an applicable margin that is determined based on Southwest’s senior unsecured debt rating. At June 30, 2018, the applicable margin is 1% for loans bearing interest with reference to LIBOR and 0% for loans bearing interest with reference to the alternative base rate. At June 30, 2018, $48 million was outstanding on the long-term portion (not including the commercial paper program, discussed below) and no borrowings were outstanding on the short-term portion of this credit facility. Southwest has a $50 million commercial paper program. Any issuance under the commercial paper program is supported by Southwest’s current revolving credit facility and, therefore, does not represent additional borrowing capacity. Borrowings under the commercial paper program are designated as long-term debt. Interest rates for the program are calculated at the then current commercial paper rate. At June 30, 2018, no borrowings were outstanding under the commercial paper program. In March 2018, Southwest issued $300 million in 3.7% Senior Notes at a discount of 0.185%. The notes will mature in April 2028. A portion of the proceeds were used to repay amounts then outstanding under the revolving portion of the credit facility and the remainder to repay amounts then outstanding under the commercial paper program. Centuri has a $450 million senior secured revolving credit and term loan facility that is scheduled to expire in November 2022. This facility includes a revolving credit facility and a term loan facility. The line of credit portion of the facility is $250 million; amounts borrowed and repaid under the revolving credit facility are available to be re-borrowed.

Short-Term Debt

Short-Term Debt	6 Months Ended
	Jun. 30, 2018
Debt Disclosure [Abstract]
Short-Term Debt	Note 8 – Short-Term Debt The Company has a $100 million credit facility that is scheduled to expire in March 2022. The Company had $22.5 million in short-term borrowings outstanding at June 30, 2018 under this facility.

Equity, Other Comprehensive Inc

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	6 Months Ended
	Jun. 30, 2018
Equity [Abstract]
Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	Note 9 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income The table below provides details of activity in equity and the noncontrolling interest for Southwest Gas Holdings, Inc. on a consolidated basis during the six months ended June 30, 2018. Southwest Gas Holdings, Inc. Equity Accumulated Additional Other Non- Common Stock Paid-in Comprehensive Retained controlling (In thousands, except per share amounts) Shares Amount Capital Income (Loss) Earnings Interest Total DECEMBER 31, 2017 48,090 $ 49,720 $ 955,332 $ (47,682 ) $ 857,398 $ (2,365 ) $ 1,812,403 Common stock issuances 1,036 1,036 68,886 69,922 Net income (loss) 100,642 (797 ) 99,845 Foreign currency exchange translation adj. (1,601 ) (1,601 ) Other comprehensive income (loss): Net actuarial gain arising during period, less amortization of unamortized benefit plan cost, net of tax 1,792 1,792 Amounts reclassified to net income, net of tax (FSIRS) 1,271 1,271 Reclassification of excess deferred taxes (a) (9,300 ) 9,300 — Elimination of shares from noncontrolling interest (b) (2,710 ) 2,710 — Dividends declared Common: $0.52 per share (51,065 ) (51,065 ) JUNE 30, 2018 49,126 $ 50,756 $ 1,021,508 $ (55,520 ) $ 916,275 $ (452 ) $ 1,932,567 (a) Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02, (b) Centuri, through its subsidiary, NPL, has historically held a 65% ownership interest in IntelliChoice Energy, LLC (“ICE”). A residual interest of 35% has been held by a third party. During the second quarter of 2018, an additional $1 million of capital was contributed by NPL, thereby increasing NPL’s ownership interest to 95%. The carrying amount of the noncontrolling interest has been adjusted with a corresponding charge to Additional paid-in The table below provides details of activity in equity for Southwest Gas Corporation during the six months ended June 30, 2018. Only equity shares of the Company are publicly traded, under the ticker symbol “SWX.” Southwest Gas Corporation Equity Accumulated Additional Other Common Stock Paid-in Comprehensive Retained (In thousands) Shares Amount Capital Income (Loss) Earnings Total DECEMBER 31, 2017 47,482 $ 49,112 $ 948,767 $ (47,073 ) $ 659,193 $ 1,609,999 Net income 92,971 92,971 Other comprehensive income (loss): Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax 1,792 1,792 Amounts reclassified to net income, net of tax (FSIRS) 1,271 1,271 Reclassification of excess deferred taxes (a) (9,300 ) 9,300 — Stock-based compensation (b) 702 (338 ) 364 Dividends declared to Southwest Gas Holdings, Inc. (44,000 ) (44,000 ) Contributions from Southwest Gas Holdings, Inc. 56,596 56,596 JUNE 30, 2018 47,482 $ 49,112 $ 1,006,065 $ (53,310 ) $ 717,126 $ 1,718,993 (a) Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02, (b) Stock-based compensation is based on stock awards of Southwest Gas Corporation to be issued in shares of Southwest Gas Holdings, Inc. The following information provides insight into amounts impacting the Company’s Other Comprehensive Income (Loss), both before and after-tax Note 5 – Derivatives and Fair Value Measurements Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Thousands of dollars) Three Months Ended Three Months Ended June 30, 2018 June 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 $ 334 $ (80 ) $ 254 $ 334 $ (127 ) $ 207 Amortization of net actuarial (gain)/loss 8,404 (2,017 ) 6,387 6,361 (2,417 ) 3,944 Regulatory adjustment (7,559 ) 1,815 (5,744 ) (5,735 ) 2,179 (3,556 ) Pension plans other comprehensive income 1,179 (282 ) 897 960 (365 ) 595 FSIRS (designated hedging activities): Amounts reclassifed into net income 836 (200 ) 636 836 (318 ) 518 FSIRS other comprehensive income 836 (200 ) 636 836 (318 ) 518 Total other comprehensive income - Southwest Gas Corporation 2,015 (482 ) 1,533 1,796 (683 ) 1,113 Foreign currency translation adjustments: Translation adjustments (690 ) — (690 ) 629 — 629 Foreign currency other comprehensive income (loss) (690 ) — (690 ) 629 — 629 Total other comprehensive income - Southwest Gas Holdings, Inc. $ 1,325 $ (482 ) $ 843 $ 2,425 $ (683 ) $ 1,742 Six Months Ended Six Months Ended June 30, 2018 June 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 $ 668 $ (160 ) $ 508 $ 668 $ (254 ) $ 414 Amortization of net actuarial (gain)/loss 16,808 (4,034 ) 12,774 12,722 (4,834 ) 7,888 Regulatory adjustment (15,119 ) 3,629 (11,490 ) (11,470 ) 4,358 (7,112 ) Pension plans other comprehensive income 2,357 (565 ) 1,792 1,920 (730 ) 1,190 FSIRS (designated hedging activities): Amounts reclassifed into net income 1,673 (402 ) 1,271 1,672 (636 ) 1,036 FSIRS other comprehensive income 1,673 (402 ) 1,271 1,672 (636 ) 1,036 Total other comprehensive income - Southwest Gas Corporation 4,030 (967 ) 3,063 3,592 (1,366 ) 2,226 Foreign currency translation adjustments: Translation adjustments (1,601 ) — (1,601 ) 849 — 849 Foreign currency other comprehensive income (loss) (1,601 ) — (1,601 ) 849 — 849 Total other comprehensive income - Southwest Gas Holdings, Inc. $ 2,429 $ (967 ) $ 1,462 $ 4,441 $ (1,366 ) $ 3,075 Twelve Months Ended Twelve Months Ended June 30, 2018 June 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: Net actuarial gain/(loss) $ (43,027 ) $ 10,326 $ (32,701 ) $ (22,770 ) $ 8,652 $ (14,118 ) Amortization of prior service cost 1,335 (413 ) 922 1,335 (507 ) 828 Amortization of net actuarial (gain)/loss 29,531 (8,869 ) 20,662 26,255 (9,976 ) 16,279 Regulatory adjustment 8,691 (479 ) 8,212 (4,808 ) 1,826 (2,982 ) Pension plans other comprehensive income (loss) (3,470 ) 565 (2,905 ) 12 (5 ) 7 FSIRS (designated hedging activities): Amounts reclassifed into net income 3,345 (1,037 ) 2,308 3,344 (1,271 ) 2,073 FSIRS other comprehensive income 3,345 (1,037 ) 2,308 3,344 (1,271 ) 2,073 Total other comprehensive income (loss) - Southwest Gas Corporation (125 ) (472 ) (597 ) 3,356 (1,276 ) 2,080 Foreign currency translation adjustments: Translation adjustments (679 ) — (679 ) 158 — 158 Foreign currency other comprehensive income (loss) (679 ) — (679 ) 158 — 158 Total other comprehensive income (loss) - Southwest Gas Holdings, Inc. $ (804 ) $ (472 ) $ (1,276 ) $ 3,514 $ (1,276 ) $ 2,238 (1) Tax amounts are calculated using a 24% rate following the December 22, 2017 enactment date of the TCJA. For periods prior to the enactment date (and included in specific line items of the tables for the twelve months ended June 30, 2018 and 2017), tax amounts were calculated using a 38% rate. The tax effect of before-tax Approximately $2.5 million of realized losses (net of tax) related to the FSIRS, reported in Accumulated other comprehensive income at June 30, 2018, 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 (Expense) Benefit (5) After-Tax Before-Tax Tax (Expense) Benefit (5) After-Tax Before-Tax Tax (Expense) Benefit After-Tax Other AOCI Beginning Balance AOCI $ (61,520 ) $ 22,293 $ (39,227 ) $ (12,655 ) $ 4,809 $ (7,846 ) $ (609 ) $ — $ (609 ) $ — $ (47,682 ) Translation adjustments — — — — — — (1,601 ) — (1,601 ) — (1,601 ) Other comprehensive income before reclassifications — — — — — — (1,601 ) — (1,601 ) — (1,601 ) FSIRS amounts reclassified from AOCI (1) — — — 1,673 (402 ) 1,271 — — — — 1,271 Amortization of prior service cost (2) 668 (160 ) 508 — — — — — — — 508 Amortization of net actuarial loss (2) 16,808 (4,034 ) 12,774 — — — — — — — 12,774 Regulatory adjustment (3) (15,119 ) 3,629 (11,490 ) — — — — — — — (11,490 ) Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. 2,357 (565 ) 1,792 1,673 (402 ) 1,271 (1,601 ) — (1,601 ) — 1,462 Reclassification of excess deferred taxes (4) — — — — — — — — — (9,300 ) (9,300 ) Ending Balance AOCI June 30, 2018 $ (59,163 ) $ 21,728 $ (37,435 ) $ (10,982 ) $ 4,407 $ (6,575 ) $ (2,210 ) $ — $ (2,210 ) $ (9,300 ) $ (55,520 ) (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) Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02, (5) Tax amounts related to the before-tax 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 (Expense) Benefit (10) After-Tax Before-Tax Tax (Expense) Benefit (10) After-Tax Other AOCI Beginning Balance AOCI $ (61,520 ) $ 22,293 $ (39,227 ) $ (12,655 ) $ 4,809 $ (7,846 ) $ — $ (47,073 ) FSIRS amounts reclassified from AOCI (6) — — — 1,673 (402 ) 1,271 — 1,271 Amortization of prior service cost (7) 668 (160 ) 508 — — — — 508 Amortization of net actuarial loss (7) 16,808 (4,034 ) 12,774 — — — — 12,774 Regulatory adjustment (8) (15,119 ) 3,629 (11,490 ) — — — — (11,490 ) Net current period other comprehensive income attributable to Southwest Gas Corporation 2,357 (565 ) 1,792 1,673 (402 ) 1,271 — 3,063 Reclassification of excess deferred taxes (9) — — — — — — (9,300 ) (9,300 ) Ending Balance AOCI June 30, 2018 $ (59,163 ) $ 21,728 $ (37,435 ) $ (10,982 ) $ 4,407 $ (6,575 ) $ (9,300 ) $ (53,310 ) (6) The FSIRS reclassification amounts are included in the Net interest deductions line item on Southwest’s Condensed Consolidated Statements of Income. (7) These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost (8) The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on Southwest’s Condensed Consolidated Balance Sheets). (9) Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02, (10) Tax amounts related to the before-tax 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) June 30, 2018 December 31, 2017 Net actuarial (loss) gain $ (431,747 ) $ (448,555 ) Prior service cost (3,700 ) (4,368 ) Less: amount recognized in regulatory assets 376,284 391,403 Recognized in AOCI $ (59,163 ) $ (61,520 )

Reorganization Impacts - Discon

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation	6 Months Ended
	Jun. 30, 2018
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 As a result of a holding company structure in January 2017, no substantive change 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 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 income statement components of discontinued operations – construction services reported in the Condensed Consolidated Statements of Income Southwest Gas Corporation: Results of Construction Services (Thousands of dollars) Twelve Months Ended June 30, 2017 Construction revenues $ 640,830 Operating expenses: Construction expenses 567,115 Depreciation and amortization 25,727 Operating income 47,988 Other income (deductions) 1,193 Net interest deductions 3,512 Income before income taxes 45,669 Income tax expense 16,550 Net income 29,119 Net income attributable to noncontrolling interests 949 Discontinued operations - construction services - net income $ 28,170

Acquisition of Construction Ser

Acquisition of Construction Services Business	6 Months Ended
	Jun. 30, 2018
Business Combinations [Abstract]
Acquisition of Construction Services Business	Note 11 – Acquisition of Construction Services Business In November 2017, the Company, through its subsidiaries, led principally by Centuri, completed the acquisition of a privately held construction business, New England Utility Constructors, Inc. (“Neuco”) for approximately $99 million, less assumed debt. See the Company’s 2017 Form 10-K

Nature of Operations and Basi18

Nature of Operations and Basis of Presentation (Policies)	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations. 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”), Canyon Pipeline Construction, Inc. (“Canyon”), NPL Canada Ltd. (“NPL Canada”), W.S. Nicholls Construction, Inc. (“W.S. Nicholls”), and Canyon Special Projects, Inc. (“Special Projects,” formerly Brigadier Pipelines Inc.). 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. Centuri acquired New England Utility Constructors, Inc. (“Neuco”) in November 2017, thereby expanding its core services in the Northeast region of the United States. See Note 11 – Acquisition of Construction Services Business
Basis of Presentation	Basis of Presentation. Form 10-Q, 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, or due to the acquisition of Neuco. Following the 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 2017 Annual Report to Shareholders, which is incorporated by reference into the 2017 Form 10-K.
Early Adoption of Accounting Standards Update	Early Adoption of Accounting Standards Update (“ASU”) No. 2018-02. No. 2018-02 Note 9 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income
Prepaids and Other Current Assets	Prepaids and other current assets.
Income Taxes	Income Taxes. gross-up. In the first quarter of 2018, management recorded a regulatory liability and reduced utility revenues by approximately $14 million for potential regulatory rate reductions to customers resulting from the reduced cost-of-service levels during the period. Based on regulatory activity in the second quarter of 2018, management has updated its estimated reserve to approximately $12.5 million. In July 2018, the Arizona Corporation Commission (“ACC”) staff issued a recommended opinion and order that would require Southwest to return to customers amounts related to excess cost-of-service
Other current liabilities	Other current liabilities
Cash and Cash Equivalents	Cash and Cash Equivalents. Significant non-cash non-cash
Goodwill	Goodwill. st Goodwill: (Thousands of dollars) Natural Construction Consolidated December 31, 2017 $ 10,095 $ 169,219 $ 179,314 Additional goodwill from Neuco acquisition — 182 182 Foreign currency translation adjustment — (5,263 ) (5,263 ) June 30, 2018 $ 10,095 $ 164,138 $ 174,233
Intercompany Transactions	Intercompany Transactions Note 4 - Segment Information June 30, 2018 December 31, 2017 Centuri accounts receivable for services provided to Southwest $ 13,899 $ 12,987 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. June 30, 2018 December 31, 2017 Southwest Gas Corporation: Net cash surrender value of COLI policies $ 118,733 $ 117,341 Other property 1,743 1,773 Total Southwest Gas Corporation 120,476 119,114 Centuri property, equipment, and intangibles 606,750 554,730 Centuri accumulated depreciation/amortization (278,209 ) (258,906 ) Other property 13,219 13,242 Total Southwest Gas Holdings, Inc. $ 462,236 $ 428,180
Other Income (Deductions)	Other Income (Deductions). Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2018 2017 2018 2017 2018 2017 Southwest Gas Corporation - natural gas operations segment: Change in COLI policies $ 2,000 $ 1,900 $ 1,300 $ 4,700 $ 6,900 $ 9,000 Interest income 1,377 614 2,795 1,178 4,401 2,269 Equity AFUDC 357 633 586 1,109 1,773 2,116 Other components of net periodic benefit cost (5,264 ) (4,857 ) (10,529 ) (9,712 ) (20,241 ) (19,591 ) Miscellaneous income and (expense) (564 ) (1,095 ) (849 ) (1,324 ) (1,869 ) (3,637 ) Southwest Gas Corporation - total other income (deductions) (2,094 ) (2,805 ) (6,697 ) (4,049 ) (9,036 ) (9,843 ) Construction services segment: Interest income 1 1 2 1 4 1 Foreign transaction gain (loss) 202 (197 ) 349 (198 ) (207 ) (201 ) Miscellaneous income and (expense) (835 ) 190 (720 ) 445 (69 ) 1,641 Centuri - total other income (deductions) (632 ) (6 ) (369 ) 248 (272 ) 1,441 Corporate and administrative 20 1 26 1 38 1 Consolidated Southwest Gas Holdings, Inc. - total other income (deductions) $ (2,706 ) $ (2,810 ) $ (7,040 ) $ (3,800 ) $ (9,270 ) $ (8,401 ) 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 Note 2 – Components of Net Periodic Benefit Cost
Recently Issued Accounting Standards Updates	Recently Issued Accounting Standards Updates. 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

Nature of Operations and Basi19

Nature of Operations and Basis of Presentation (Tables)	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Goodwill	(Thousands of dollars) Natural Construction Consolidated December 31, 2017 $ 10,095 $ 169,219 $ 179,314 Additional goodwill from Neuco acquisition — 182 182 Foreign currency translation adjustment — (5,263 ) (5,263 ) June 30, 2018 $ 10,095 $ 164,138 $ 174,233
Accounts Receivable for Services	Centuri’s accounts receivable for these services are presented in the table below (thousands of dollars): June 30, 2018 December 31, 2017 Centuri accounts receivable for services provided to Southwest $ 13,899 $ 12,987
Schedule of Other Property and Investments	Other Property and Investments. June 30, 2018 December 31, 2017 Southwest Gas Corporation: Net cash surrender value of COLI policies $ 118,733 $ 117,341 Other property 1,743 1,773 Total Southwest Gas Corporation 120,476 119,114 Centuri property, equipment, and intangibles 606,750 554,730 Centuri accumulated depreciation/amortization (278,209 ) (258,906 ) Other property 13,219 13,242 Total Southwest Gas Holdings, Inc. $ 462,236 $ 428,180
Other Income (Deductions)	The following table provides the composition of significant items included in Other income (deductions) in the Condensed Consolidated Statements of Income (thousands of dollars): Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2018 2017 2018 2017 2018 2017 Southwest Gas Corporation - natural gas operations segment: Change in COLI policies $ 2,000 $ 1,900 $ 1,300 $ 4,700 $ 6,900 $ 9,000 Interest income 1,377 614 2,795 1,178 4,401 2,269 Equity AFUDC 357 633 586 1,109 1,773 2,116 Other components of net periodic benefit cost (5,264 ) (4,857 ) (10,529 ) (9,712 ) (20,241 ) (19,591 ) Miscellaneous income and (expense) (564 ) (1,095 ) (849 ) (1,324 ) (1,869 ) (3,637 ) Southwest Gas Corporation - total other income (deductions) (2,094 ) (2,805 ) (6,697 ) (4,049 ) (9,036 ) (9,843 ) Construction services segment: Interest income 1 1 2 1 4 1 Foreign transaction gain (loss) 202 (197 ) 349 (198 ) (207 ) (201 ) Miscellaneous income and (expense) (835 ) 190 (720 ) 445 (69 ) 1,641 Centuri - total other income (deductions) (632 ) (6 ) (369 ) 248 (272 ) 1,441 Corporate and administrative 20 1 26 1 38 1 Consolidated Southwest Gas Holdings, Inc. - total other income (deductions) $ (2,706 ) $ (2,810 ) $ (7,040 ) $ (3,800 ) $ (9,270 ) $ (8,401 )

Components of Net Periodic Be20

Components of Net Periodic Benefit Cost (Tables)	6 Months Ended
	Jun. 30, 2018
Text Block [Abstract]
Schedule of Net Periodic Benefit Costs	The service cost component of net periodic benefit costs included in the table below are components of an overhead loading process associated with the cost of labor (refer to discussion above related to the update to Topic 715). The overhead process ultimately results in allocation of that portion of overall net periodic benefit costs to the same accounts to which productive labor is charged. As a result, service costs become components of various accounts, primarily operations and maintenance expense, net utility plant, and deferred charges and other assets for both the Company and Southwest. Refer also to the practical expedient elected related to amounts capitalized as part of assets prior to the adoption date. Qualified Retirement Plan Period Ended June 30, Three Months Six Months Twelve Months 2018 2017 2018 2017 2018 2017 (Thousands of dollars) Service cost $ 7,139 $ 5,848 $ 14,278 $ 11,696 $ 25,974 $ 23,112 Interest cost 11,044 11,521 22,087 23,041 45,129 46,055 Expected return on plan assets (14,688 ) (13,799 ) (29,377 ) (27,598 ) (56,975 ) (55,877 ) Amortization of net actuarial loss 8,028 6,001 16,057 12,002 28,059 24,635 Net periodic benefit cost $ 11,523 $ 9,571 $ 23,045 $ 19,141 $ 42,187 $ 37,925 SERP Period Ended June 30, Three Months Six Months Twelve Months 2018 2017 2018 2017 2018 2017 (Thousands of dollars) Service cost $ 61 $ 77 $ 122 $ 155 $ 276 $ 321 Interest cost 414 471 829 942 1,770 1,871 Amortization of net actuarial loss 376 360 751 720 1,472 1,411 Net periodic benefit cost $ 851 $ 908 $ 1,702 $ 1,817 $ 3,518 $ 3,603 PBOP Period Ended June 30, Three Months Six Months Twelve Months 2018 2017 2018 2017 2018 2017 (Thousands of dollars) Service cost $ 369 $ 367 $ 737 $ 734 $ 1,471 $ 1,484 Interest cost 687 808 1,374 1,616 2,990 3,205 Expected return on plan assets (930 ) (839 ) (1,860 ) (1,679 ) (3,539 ) (3,253 ) Amortization of prior service costs 334 334 668 668 1,335 1,335 Amortization of net actuarial loss — — — — — 209 Net periodic benefit cost $ 460 $ 670 $ 919 $ 1,339 $ 2,257 $ 2,980

Revenue (Tables)

Revenue (Tables)	6 Months Ended
	Jun. 30, 2018
Summary of Revenue Disaggregated by Service Type and Contract Type	The following tables display Centuri’s revenue from contracts with customers disaggregated by service type and contract type: (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2018 2017 2018 2017 2018 2017 Service Types: Replacement gas construction $256,070 $181,512 $413,421 $301,950 $899,537 $733,927 New gas construction 44,276 40,226 80,473 73,305 172,545 197,100 Other construction 94,858 78,569 161,327 117,187 337,181 202,245 Total Construction revenues $395,204 $300,307 $655,221 $492,442 $1,409,263 $1,133,272 (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, 2018 2017 2018 2017 2018 2017 Contract Types: Master services agreement $290,075 $208,361 $484,539 $355,553 $1,014,517 $825,980 Bid contract 105,129 91,946 170,682 136,889 394,746 307,292 Total Construction revenues $395,204 $300,307 $655,221 $492,442 $1,409,263 $1,133,272 Unit priced contracts $345,390 $272,427 $579,675 $450,098 $1,249,602 $974,107 Fixed priced contracts 49,814 27,880 75,546 42,344 159,661 159,165 Total Construction revenues $395,204 $300,307 $655,221 $492,442 $1,409,263 $1,133,272
Summary of Information about Receivables, Revenue Earned on Contracts in Progress in Excess of Billings, Which are Included Within Accounts Receivable, Net of Allowances, and Amounts Billed in Excess of Revenue Earned on Contracts	The following table provides information about receivables, revenue earned on contracts in progress in excess of billings, which are included within accounts receivable, net of allowances and amounts billed in excess of revenue earned on contracts, which is included in other current liabilities as of June 30, 2018 and December 31, 2017 on the Company’s Condensed Consolidated Balance Sheets: (Thousands of dollars) June 30, 2018 December 31, 2017 Contracts receivable, net $ 187,485 $ 221,859 Revenue earned on contracts in progress in excess of billings 86,750 5,768 Amounts billed in excess of revenue earned on contracts 8,592 9,602
Schedule of Construction Services Contracts Receivable	Construction services contracts receivable consists of the following: (Thousands of dollars) June 30, 2018 Billed on completed contracts and contracts in progress $ 185,911 Other receivables 1,660 Contracts receivable, gross 187,571 Allowance for doubtful accounts (86 ) Contracts receivable, net $ 187,485
Summary of Revenue on Contracts in Progress in Excess of Billings within Accounts Receivable Net of Allowances and Amounts Billed in Excess of Revenue Earned	Management recognizes revenue on contracts in progress in excess of billings (a contract asset) within Accounts receivable, net of allowances in the Company’s Condensed Consolidated Balance Sheets, and amounts billed in excess of revenue earned (a contract liability) in Other current liabilities. However, the following shows the significant changes in these asset and liability balances associated with Centuri since January 1, 2018: (Thousands of dollars) June 30, 2018 Revenue earned on Amounts billed in Revenue recognized that was included in the amounts billed in excess of revenue earned on contracts balance at the beginning of the period $ — $ (9,602 ) Increases due to amounts billed to customers in excess of revenue earned during the period — 8,592 Transferred to contracts receivable from revenue earned on contracts in progress in excess of billings recognized at the beginning of the period (5,768 ) — Increases from the reclassification of contract assets due to the adoption of topic 606 51,744 — Increases from contract assets, contingent on a future event occurring 35,006 —
Accounting Standards Update 2014-09 [Member]
Summary of Revenue Disaggregated by Service Type and Contract Type	Gas operating revenues on the Condensed Consolidated Statements of Income of both the Company and Southwest include revenue from contracts with customers, which is shown below disaggregated by customer type, and various categories of revenue: Three Months Ended Six Months Ended Twelve Months Ended June 30, June 30, June 30, (Thousands of dollars) 2018 2017 2018 2017 2018 2017 Residential $ 166,702 $ 158,442 $ 511,313 $ 501,179 $ 867,338 $ 850,344 Small commercial 55,653 51,018 143,596 132,143 254,966 235,684 Large commercial 13,134 12,781 28,574 26,376 54,577 49,561 Industrial/other 5,491 5,043 12,001 10,494 23,533 20,157 Transportation 20,719 20,958 44,773 43,690 88,842 86,457 Revenue from contracts with customers 261,699 248,242 740,257 713,882 1,289,256 1,242,203 Alternative revenue program revenues (deferrals) 10,393 10,135 37,602 5,903 67,046 17,280 Other revenues (a) 3,587 1,785 (7,867 ) 2,979 (6,766 ) 3,945 Total Gas operating revenues $ 275,679 $ 260,162 $ 769,992 $ 722,764 $ 1,349,536 $ 1,263,428 (a) Includes various other revenues, including $1.6 million during the three months, and an offset of $12.5 million in both the six months and twelve months ending June 30, 2018 related to tax reform savings adjustments. Refer to Income Taxes Note 1 – Nature of Operations and Basis of Presentation

Segment Information (Tables)

Segment Information (Tables)	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 Revenues from external customers $ 275,679 $ 362,132 $ — $ 637,811 Intersegment revenues — 33,072 — 33,072 Total $ 275,679 $ 395,204 $ — $ 670,883 Segment net income (loss) $ 2,622 $ 19,236 $ (307 ) $ 21,551 Three months ended June 30, 2017 Revenues from external customers $ 260,162 $ 277,384 $ — $ 537,546 Intersegment revenues — 22,923 — 22,923 Total $ 260,162 $ 300,307 $ — $ 560,469 Segment net income (loss) $ 9,522 $ 8,716 $ (374 ) $ 17,864 Natural Gas Construction Other Total Six months ended June 30, 2018 Revenues from external customers $ 769,992 $ 594,991 $ — $ 1,364,983 Intersegment revenues — 60,230 — 60,230 Total $ 769,992 $ 655,221 $ — $ 1,425,213 Segment net income (loss) $ 92,971 $ 8,235 $ (564 ) $ 100,642 Six months ended June 30, 2017 Revenues from external customers $ 722,764 $ 448,223 $ — $ 1,170,987 Intersegment revenues — 44,219 — 44,219 Total $ 722,764 $ 492,442 $ — $ 1,215,206 Segment net income (loss) $ 86,460 $ 1,382 $ (670 ) $ 87,172 Natural Gas Construction Other Total Twelve months ended June 30, 2018 Revenues from external customers $ 1,349,536 $ 1,296,093 $ — $ 2,645,629 Intersegment revenues — 113,170 — 113,170 Total $ 1,349,536 $ 1,409,263 $ — $ 2,758,799 Segment net income (loss) $ 163,329 $ 45,213 $ (1,231 ) $ 207,311 Twelve months ended June 30, 2017 Revenues from external customers $ 1,263,428 $ 1,038,876 $ — $ 2,302,304 Intersegment revenues — 94,396 — 94,396 Total $ 1,263,428 $ 1,133,272 $ — $ 2,396,700 Segment net income (loss) $ 125,942 $ 29,552 $ (670 ) $ 154,824

Derivatives and Fair Value Me23

Derivatives and Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2018
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): June 30, 2018 December 31, 2017 Contract notional amounts 13,035 10,929
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-, six-, Gains (losses) recognized in income for derivatives not designated as hedging instruments: (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended Location of Gain or (Loss) Recognized in Income on Derivative June 30 June 30 June 30 Instrument 2018 2017 2018 2017 2018 2017 Swaps Net cost of gas sold $ 870 $ (1,168 ) $ (4,326 ) $(6,305) $(9,593) $ (5,624 ) Swaps Net cost of gas sold (870 )* 1,168 * 4,326 * 6,305 * 9,593 * 5,624 * 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: June 30, 2018 Instrument Balance Sheet Location Asset Liability Net Swaps Other current liabilities $ 654 $ (5,901 ) $ (5,247 ) Swaps Other deferred credits 83 (1,016 ) (933 ) Total $ 737 $ (6,917 ) $ (6,180 ) December 31, 2017 Instrument Balance Sheet Location Asset Liability Net Swaps Other current liabilities $ 11 $ (4,468 ) $ (4,457 ) Swaps Other deferred credits 19 (1,342 ) (1,323 ) Total $ 30 $ (5,810 ) $ (5,780 )
Paid to and Received from Counterparties for Settlements of Matured Swaps	The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps. Three Months Ended Six Months Ended Twelve Months Ended (Thousands of dollars) June 30, 2018 June 30, 2018 June 30, 2018 Paid to counterparties $ 1,216 $ 3,931 $ 5,620 Received from counterparties $ 6 $ 6 $ 6
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). June 30, 2018 Instrument Balance Sheet Location Net Total Swaps Prepaids and other current assets $ 5,247 Swaps Deferred charges and other assets 933 December 31, 2017 Instrument Balance Sheet Location Net Total Swaps Prepaids and other current assets $ 4,457 Swaps Deferred charges and other assets 1,323
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) June 30, 2018 December 31, 2017 Liabilities at fair value: Other current liabilities - Swaps $ (5,247 ) $ (4,457 ) Other deferred credits - Swaps (933 ) (1,323 ) Net Assets (Liabilities) $ (6,180 ) $ (5,780 )

Long-Term Debt (Tables)

Long-Term Debt (Tables)	6 Months Ended
	Jun. 30, 2018
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. June 30, 2018 December 31, 2017 Carrying Fair Carrying Fair Amount Value Amount Value (Thousands of dollars) Southwest Gas Corporation: Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 127,258 $ 125,000 $ 129,273 Notes, 6.1%, due 2041 125,000 150,816 125,000 158,304 Notes, 3.875%, due 2022 250,000 252,108 250,000 256,163 Notes, 4.875%, due 2043 250,000 267,985 250,000 283,243 Notes, 3.8%, due 2046 300,000 281,028 300,000 302,970 Notes, 3.7%, due 2028 300,000 298,068 — — 8% Series, due 2026 75,000 95,789 75,000 96,063 Medium-term notes, 7.78% series, due 2022 25,000 27,984 25,000 28,714 Medium-term notes, 7.92% series, due 2027 25,000 30,228 25,000 31,542 Medium-term notes, 6.76% series, due 2027 7,500 8,740 7,500 8,882 Unamortized discount and debt issuance costs (12,259 ) (9,350 ) 1,470,241 1,173,150 Revolving credit facility and commercial paper 48,000 48,000 150,000 150,000 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 (1,934 ) (2,119 ) 198,066 197,881 Less: current maturities — — Long-term debt, less current maturities - Southwest Gas Corporation $ 1,716,307 $ 1,521,031 Centuri: Centuri term loan facility $ 189,876 $ 190,524 $ 199,578 $ 207,588 Unamortized debt issuance costs (999 ) (1,111 ) 188,877 198,467 Centuri secured revolving credit facility 87,952 87,998 56,472 56,525 Centuri other debt obligations 76,535 76,419 47,952 48,183 Less: current maturities (31,928 ) (25,346 ) Long-term debt, less current maturities - Centuri $ 321,436 $ 277,545 Consolidated Southwest Gas Holdings, Inc.: Southwest Gas Corporation long-term debt $ 1,716,307 $ 1,521,031 Centuri long-term debt 353,364 302,891 Less: current maturities (31,928 ) (25,346 ) Long-term debt, less current maturities - Southwest Gas Holdings, Inc. $ 2,037,743 $ 1,798,576

Equity, Other Comprehensive I25

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income (Tables)	6 Months Ended
	Jun. 30, 2018
Detailed Activities of Equity and Redeemable Noncontrolling Interest	The table below provides details of activity in equity and the noncontrolling interest for Southwest Gas Holdings, Inc. on a consolidated basis during the six months ended June 30, 2018. Southwest Gas Holdings, Inc. Equity Accumulated Additional Other Non- Common Stock Paid-in Comprehensive Retained controlling (In thousands, except per share amounts) Shares Amount Capital Income (Loss) Earnings Interest Total DECEMBER 31, 2017 48,090 $ 49,720 $ 955,332 $ (47,682 ) $ 857,398 $ (2,365 ) $ 1,812,403 Common stock issuances 1,036 1,036 68,886 69,922 Net income (loss) 100,642 (797 ) 99,845 Foreign currency exchange translation adj. (1,601 ) (1,601 ) Other comprehensive income (loss): Net actuarial gain arising during period, less amortization of unamortized benefit plan cost, net of tax 1,792 1,792 Amounts reclassified to net income, net of tax (FSIRS) 1,271 1,271 Reclassification of excess deferred taxes (a) (9,300 ) 9,300 — Elimination of shares from noncontrolling interest (b) (2,710 ) 2,710 — Dividends declared Common: $0.52 per share (51,065 ) (51,065 ) JUNE 30, 2018 49,126 $ 50,756 $ 1,021,508 $ (55,520 ) $ 916,275 $ (452 ) $ 1,932,567 (a) Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02, (b) Centuri, through its subsidiary, NPL, has historically held a 65% ownership interest in IntelliChoice Energy, LLC (“ICE”). A residual interest of 35% has been held by a third party. During the second quarter of 2018, an additional $1 million of capital was contributed by NPL, thereby increasing NPL’s ownership interest to 95%. The carrying amount of the noncontrolling interest has been adjusted with a corresponding charge to Additional paid-in
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 Three Months Ended June 30, 2018 June 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 $ 334 $ (80 ) $ 254 $ 334 $ (127 ) $ 207 Amortization of net actuarial (gain)/loss 8,404 (2,017 ) 6,387 6,361 (2,417 ) 3,944 Regulatory adjustment (7,559 ) 1,815 (5,744 ) (5,735 ) 2,179 (3,556 ) Pension plans other comprehensive income 1,179 (282 ) 897 960 (365 ) 595 FSIRS (designated hedging activities): Amounts reclassifed into net income 836 (200 ) 636 836 (318 ) 518 FSIRS other comprehensive income 836 (200 ) 636 836 (318 ) 518 Total other comprehensive income - Southwest Gas Corporation 2,015 (482 ) 1,533 1,796 (683 ) 1,113 Foreign currency translation adjustments: Translation adjustments (690 ) — (690 ) 629 — 629 Foreign currency other comprehensive income (loss) (690 ) — (690 ) 629 — 629 Total other comprehensive income - Southwest Gas Holdings, Inc. $ 1,325 $ (482 ) $ 843 $ 2,425 $ (683 ) $ 1,742 Six Months Ended Six Months Ended June 30, 2018 June 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 $ 668 $ (160 ) $ 508 $ 668 $ (254 ) $ 414 Amortization of net actuarial (gain)/loss 16,808 (4,034 ) 12,774 12,722 (4,834 ) 7,888 Regulatory adjustment (15,119 ) 3,629 (11,490 ) (11,470 ) 4,358 (7,112 ) Pension plans other comprehensive income 2,357 (565 ) 1,792 1,920 (730 ) 1,190 FSIRS (designated hedging activities): Amounts reclassifed into net income 1,673 (402 ) 1,271 1,672 (636 ) 1,036 FSIRS other comprehensive income 1,673 (402 ) 1,271 1,672 (636 ) 1,036 Total other comprehensive income - Southwest Gas Corporation 4,030 (967 ) 3,063 3,592 (1,366 ) 2,226 Foreign currency translation adjustments: Translation adjustments (1,601 ) — (1,601 ) 849 — 849 Foreign currency other comprehensive income (loss) (1,601 ) — (1,601 ) 849 — 849 Total other comprehensive income - Southwest Gas Holdings, Inc. $ 2,429 $ (967 ) $ 1,462 $ 4,441 $ (1,366 ) $ 3,075 Twelve Months Ended Twelve Months Ended June 30, 2018 June 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: Net actuarial gain/(loss) $ (43,027 ) $ 10,326 $ (32,701 ) $ (22,770 ) $ 8,652 $ (14,118 ) Amortization of prior service cost 1,335 (413 ) 922 1,335 (507 ) 828 Amortization of net actuarial (gain)/loss 29,531 (8,869 ) 20,662 26,255 (9,976 ) 16,279 Regulatory adjustment 8,691 (479 ) 8,212 (4,808 ) 1,826 (2,982 ) Pension plans other comprehensive income (loss) (3,470 ) 565 (2,905 ) 12 (5 ) 7 FSIRS (designated hedging activities): Amounts reclassifed into net income 3,345 (1,037 ) 2,308 3,344 (1,271 ) 2,073 FSIRS other comprehensive income 3,345 (1,037 ) 2,308 3,344 (1,271 ) 2,073 Total other comprehensive income (loss) - Southwest Gas Corporation (125 ) (472 ) (597 ) 3,356 (1,276 ) 2,080 Foreign currency translation adjustments: Translation adjustments (679 ) — (679 ) 158 — 158 Foreign currency other comprehensive income (loss) (679 ) — (679 ) 158 — 158 Total other comprehensive income (loss) - Southwest Gas Holdings, Inc. $ (804 ) $ (472 ) $ (1,276 ) $ 3,514 $ (1,276 ) $ 2,238 (1) Tax amounts are calculated using a 24% rate following the December 22, 2017 enactment date of the TCJA. For periods prior to the enactment date (and included in specific line items of the tables for the twelve months ended June 30, 2018 and 2017), tax amounts were calculated using a 38% rate. The tax effect of before-tax
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 (Expense) Benefit (10) After-Tax Before-Tax Tax (Expense) Benefit (10) After-Tax Other AOCI Beginning Balance AOCI $ (61,520 ) $ 22,293 $ (39,227 ) $ (12,655 ) $ 4,809 $ (7,846 ) $ — $ (47,073 ) FSIRS amounts reclassified from AOCI (6) — — — 1,673 (402 ) 1,271 — 1,271 Amortization of prior service cost (7) 668 (160 ) 508 — — — — 508 Amortization of net actuarial loss (7) 16,808 (4,034 ) 12,774 — — — — 12,774 Regulatory adjustment (8) (15,119 ) 3,629 (11,490 ) — — — — (11,490 ) Net current period other comprehensive income attributable to Southwest Gas Corporation 2,357 (565 ) 1,792 1,673 (402 ) 1,271 — 3,063 Reclassification of excess deferred taxes (9) — — — — — — (9,300 ) (9,300 ) Ending Balance AOCI June 30, 2018 $ (59,163 ) $ 21,728 $ (37,435 ) $ (10,982 ) $ 4,407 $ (6,575 ) $ (9,300 ) $ (53,310 ) (6) The FSIRS reclassification amounts are included in the Net interest deductions line item on Southwest’s Condensed Consolidated Statements of Income. (7) These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost (8) The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on Southwest’s Condensed Consolidated Balance Sheets). (9) Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02, (10) Tax amounts related to the before-tax
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) June 30, 2018 December 31, 2017 Net actuarial (loss) gain $ (431,747 ) $ (448,555 ) Prior service cost (3,700 ) (4,368 ) Less: amount recognized in regulatory assets 376,284 391,403 Recognized in AOCI $ (59,163 ) $ (61,520 )
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 six months ended June 30, 2018. Only equity shares of the Company are publicly traded, under the ticker symbol “SWX.” Southwest Gas Corporation Equity Accumulated Additional Other Common Stock Paid-in Comprehensive Retained (In thousands) Shares Amount Capital Income (Loss) Earnings Total DECEMBER 31, 2017 47,482 $ 49,112 $ 948,767 $ (47,073 ) $ 659,193 $ 1,609,999 Net income 92,971 92,971 Other comprehensive income (loss): Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax 1,792 1,792 Amounts reclassified to net income, net of tax (FSIRS) 1,271 1,271 Reclassification of excess deferred taxes (a) (9,300 ) 9,300 — Stock-based compensation (b) 702 (338 ) 364 Dividends declared to Southwest Gas Holdings, Inc. (44,000 ) (44,000 ) Contributions from Southwest Gas Holdings, Inc. 56,596 56,596 JUNE 30, 2018 47,482 $ 49,112 $ 1,006,065 $ (53,310 ) $ 717,126 $ 1,718,993 (a) Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02, (b) Stock-based compensation is based on stock awards of Southwest Gas Corporation to be issued in shares of Southwest Gas Holdings, Inc.
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 (Expense) Benefit (5) After-Tax Before-Tax Tax (Expense) Benefit (5) After-Tax Before-Tax Tax (Expense) Benefit After-Tax Other AOCI Beginning Balance AOCI $ (61,520 ) $ 22,293 $ (39,227 ) $ (12,655 ) $ 4,809 $ (7,846 ) $ (609 ) $ — $ (609 ) $ — $ (47,682 ) Translation adjustments — — — — — — (1,601 ) — (1,601 ) — (1,601 ) Other comprehensive income before reclassifications — — — — — — (1,601 ) — (1,601 ) — (1,601 ) FSIRS amounts reclassified from AOCI (1) — — — 1,673 (402 ) 1,271 — — — — 1,271 Amortization of prior service cost (2) 668 (160 ) 508 — — — — — — — 508 Amortization of net actuarial loss (2) 16,808 (4,034 ) 12,774 — — — — — — — 12,774 Regulatory adjustment (3) (15,119 ) 3,629 (11,490 ) — — — — — — — (11,490 ) Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. 2,357 (565 ) 1,792 1,673 (402 ) 1,271 (1,601 ) — (1,601 ) — 1,462 Reclassification of excess deferred taxes (4) — — — — — — — — — (9,300 ) (9,300 ) Ending Balance AOCI June 30, 2018 $ (59,163 ) $ 21,728 $ (37,435 ) $ (10,982 ) $ 4,407 $ (6,575 ) $ (2,210 ) $ — $ (2,210 ) $ (9,300 ) $ (55,520 ) (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) Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02, (5) Tax amounts related to the before-tax

Reorganization Impacts - Disc26

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation (Tables)	6 Months Ended
	Jun. 30, 2018
Restructuring and Related Activities [Abstract]
Summary of Discontinued Operations - Construction Services in Condensed Consolidated Balance Sheet and Condensed Consolidated Statements of Income	The following table presents the major income statement components of discontinued operations – construction services reported in the Condensed Consolidated Statements of Income Southwest Gas Corporation: Results of Construction Services (Thousands of dollars) Twelve Months Ended June 30, 2017 Construction revenues $ 640,830 Operating expenses: Construction expenses 567,115 Depreciation and amortization 25,727 Operating income 47,988 Other income (deductions) 1,193 Net interest deductions 3,512 Income before income taxes 45,669 Income tax expense 16,550 Net income 29,119 Net income attributable to noncontrolling interests 949 Discontinued operations - construction services - net income $ 28,170

Nature of Operations and Basi27

Nature of Operations and Basis of Presentation - Additional Information (Detail) - USD ($)	Jul. 30, 2017	May 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Aug. 31, 2017
Significant Accounting Policies [Line Items]
Previous Tax effects from TCJA reclassified to retained earnings from AOCI					$ 9,300,000
Corporate federal income tax rate					21.00%		35.00%
Reductions in accumulated deferred income tax			$ 12,500,000	$ 14,000,000
Dividends declared but not yet paid			22,000,000		$ 22,000,000
Non-cash construction advances - non-cash investing item					512,000	$ 1,600,000
Gas Pipe Materials and Supplies [Member]
Significant Accounting Policies [Line Items]
Refund from El Paso Natural Gas L.L.C		$ 49,000,000
Gas Pipe Materials and Supplies [Member] | Prepaids and Other Current Assets [Member]
Significant Accounting Policies [Line Items]
Gas pipe materials and operating supplies			55,000,000		55,000,000		$ 33,000,000
Prepaid and other current assets			69,000,000		$ 69,000,000		$ 40,000,000
Centuri Construction Group Inc [Member]
Significant Accounting Policies [Line Items]
Ownership percentage in subsidiary	96.60%				100.00%
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%
Pooled Funds and Mutual Funds [Member] | Level 2 - Significant Other Observable Inputs [Member]
Significant Accounting Policies [Line Items]
Money market fund investments			$ 1,900,000		$ 1,900,000	$ 13,200,000

Nature of Operations and Basi28

Nature of Operations and Basis of Presentation - Schedule of Goodwill (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018USD ($)
Goodwill [Line Items]
Goodwill, Beginning balance	$ 179,314
Foreign currency translation adjustment	(5,263)
Goodwill, Ending balance	174,233
Neuco [Member]
Goodwill [Line Items]
Additional goodwill from Neuco acquisition	182
Southwest Gas [Member]
Goodwill [Line Items]
Goodwill, Beginning balance	10,095
Goodwill, Ending balance	10,095
Construction Services [Member]
Goodwill [Line Items]
Goodwill, Beginning balance	169,219
Foreign currency translation adjustment	(5,263)
Goodwill, Ending balance	164,138
Construction Services [Member] | Neuco [Member]
Goodwill [Line Items]
Additional goodwill from Neuco acquisition	$ 182

Nature of Operations and Basi29

Nature of Operations and Basis of Presentation - Accounts Receivable for Services (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Centuri Construction Group Inc [Member]
Segment Reporting Information [Line Items]
Centuri accounts receivable for services provided to Southwest	$ 13,899	$ 12,987

Nature of Operations and Basi30

Nature of Operations and Basis of Presentation - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Other property	$ 13,219	$ 13,242
Total	462,236	428,180
Southwest Gas Corporation [Member]
Investment [Line Items]
Net cash surrender value of COLI policies	118,733	117,341
Other property	1,743	1,773
Total	120,476	119,114
Centuri Construction Group Inc [Member]
Investment [Line Items]
Centuri property, equipment, and intangibles	606,750	554,730
Centuri accumulated depreciation/amortization	$ (278,209)	$ (258,906)

Nature of Operations and Basi31

Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other income (deductions)
Total other income (deductions)	$ (2,706)	$ (2,810)	$ (7,040)	$ (3,800)	$ (9,270)	$ (8,401)
Southwest Gas Corporation [Member]
Other income (deductions)
Change in COLI policies	2,000	1,900	1,300	4,700	6,900	9,000
Interest income	1,377	614	2,795	1,178	4,401	2,269
Equity AFUDC	357	633	586	1,109	1,773	2,116
Other components of net periodic benefit cost	(5,264)	(4,857)	(10,529)	(9,712)	(20,241)	(19,591)
Miscellaneous income and (expense)	(564)	(1,095)	(849)	(1,324)	(1,869)	(3,637)
Total other income (deductions)	(2,094)	(2,805)	(6,697)	(4,049)	(9,036)	(9,843)
Centuri Construction Group Inc [Member]
Other income (deductions)
Interest income	1	1	2	1	4	1
Foreign transaction gain (loss)	202	(197)	349	(198)	(207)	(201)
Miscellaneous income and (expense)	(835)	190	(720)	445	(69)	1,641
Total other income (deductions)	(632)	(6)	(369)	248	(272)	1,441
Corporate and administrative	$ 20	$ 1	$ 26	$ 1	$ 38	$ 1

Components of Net Periodic Be32

Components of Net Periodic Benefit Cost - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2018USD ($)Participant	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		$ 105,435	$ 98,203	$ 207,786	$ 202,498	$ 398,051	$ 394,802
Other income (deductions)		(2,706)	(2,810)	(7,040)	(3,800)	(9,270)	(8,401)
Subsequent Event [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of eligible participants for pension payout offer | Participant	385
Payment from pension assets	$ 6,800
Southwest Gas Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		105,208	97,644	207,398	201,468	397,251	393,772
Other income (deductions)		$ (2,094)	(2,805)	$ (6,697)	(4,049)	$ (9,036)	(9,843)
Operating Expense [Member]
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance			103,100		212,200		414,400
Operating Expense [Member] | Southwest Gas Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance			102,500		211,200		413,400
Other Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)			2,100		5,900		11,200
Other Income [Member] | Southwest Gas Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)			2,100		5,700		9,800
Revised [Member] | Operating Expense [Member]
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance			98,200		202,500		394,800
Revised [Member] | Operating Expense [Member] | Southwest Gas Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance			97,600		201,500		393,800
Revised [Member] | Other Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)			2,800		3,800		8,400
Revised [Member] | Other Income [Member] | Southwest Gas Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)			$ 2,800		$ 4,000		$ 9,800

Components of Net Periodic Be33

Components of Net Periodic Benefit Cost - Schedule of Net Periodic Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,139	$ 5,848	$ 14,278	$ 11,696	$ 25,974	$ 23,112
Interest cost	11,044	11,521	22,087	23,041	45,129	46,055
Expected return on plan assets	(14,688)	(13,799)	(29,377)	(27,598)	(56,975)	(55,877)
Amortization of net actuarial loss	8,028	6,001	16,057	12,002	28,059	24,635
Net periodic benefit cost	11,523	9,571	23,045	19,141	42,187	37,925
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	61	77	122	155	276	321
Interest cost	414	471	829	942	1,770	1,871
Amortization of net actuarial loss	376	360	751	720	1,472	1,411
Net periodic benefit cost	851	908	1,702	1,817	3,518	3,603
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	369	367	737	734	1,471	1,484
Interest cost	687	808	1,374	1,616	2,990	3,205
Expected return on plan assets	(930)	(839)	(1,860)	(1,679)	(3,539)	(3,253)
Amortization of prior service costs	334	334	668	668	1,335	1,335
Amortization of net actuarial loss						209
Net periodic benefit cost	$ 460	$ 670	$ 919	$ 1,339	$ 2,257	$ 2,980

Revenue - Additional Informatio

Revenue - Additional Information (Detail) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018USD ($)	Jun. 30, 2018USD ($)Segment
Southwest Gas Corporation [Member]
Segment Reporting Information [Line Items]
Number of operating segment | Segment		1
Centuri Construction Group Inc [Member]
Segment Reporting Information [Line Items]
Revenue recognized from performance obligations satisfied under contracts	$ 34.7	$ 88.7
Transaction price allocated to unsatisfied performance obligations of contracts	$ 86	$ 86

Revenue - Schedule of Gas Opera

Revenue - Schedule of Gas Operating Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 261,699	$ 248,242	$ 740,257	$ 713,882	$ 1,289,256	$ 1,242,203
Alternative revenue program revenues (deferrals)	10,393	10,135	37,602	5,903	67,046	17,280
Other revenues	3,587	1,785	(7,867)	2,979	(6,766)	3,945
Total Gas operating revenues	275,679	260,162	769,992	722,764	1,349,536	1,263,428
Residential [Member]
Disaggregation of Revenue [Line Items]
Revenues	166,702	158,442	511,313	501,179	867,338	850,344
Small Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues	55,653	51,018	143,596	132,143	254,966	235,684
Large Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenues	13,134	12,781	28,574	26,376	54,577	49,561
Industrial/other [Member]
Disaggregation of Revenue [Line Items]
Revenues	5,491	5,043	12,001	10,494	23,533	20,157
Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 20,719	$ 20,958	$ 44,773	$ 43,690	$ 88,842	$ 86,457

Revenue - Schedule of Gas Ope36

Revenue - Schedule of Gas Operating Revenue (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Other revenues	$ 3,587	$ 1,785	$ (7,867)	$ 2,979	$ (6,766)	$ 3,945
Tax Reform Savings Revenue Adjustments [Member] | Southwest Gas Holdings, Inc. [Member]
Disaggregation of Revenue [Line Items]
Other revenues	$ 1,600		$ 12,500		$ 12,500

Revenue - Summary of Revenue fr

Revenue - Summary of Revenue from Contracts With Customers Disaggregated by Service Type, and Contract Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 261,699	$ 248,242	$ 740,257	$ 713,882	$ 1,289,256	$ 1,242,203
Centuri Construction Group Inc [Member]
Disaggregation of Revenue [Line Items]
Revenues	395,204	300,307	655,221	492,442	1,409,263	1,133,272
Master Services Agreement [Member] | Centuri Construction Group Inc [Member]
Disaggregation of Revenue [Line Items]
Revenues	290,075	208,361	484,539	355,553	1,014,517	825,980
Bid Contract [Member] | Centuri Construction Group Inc [Member]
Disaggregation of Revenue [Line Items]
Revenues	105,129	91,946	170,682	136,889	394,746	307,292
Unit Priced Contracts [Member] | Centuri Construction Group Inc [Member]
Disaggregation of Revenue [Line Items]
Revenues	345,390	272,427	579,675	450,098	1,249,602	974,107
Fixed-price Contract [Member] | Centuri Construction Group Inc [Member]
Disaggregation of Revenue [Line Items]
Revenues	49,814	27,880	75,546	42,344	159,661	159,165
Replacement Gas Construction [Member] | Centuri Construction Group Inc [Member]
Disaggregation of Revenue [Line Items]
Revenues	256,070	181,512	413,421	301,950	899,537	733,927
New Gas Construction [Member] | Centuri Construction Group Inc [Member]
Disaggregation of Revenue [Line Items]
Revenues	44,276	40,226	80,473	73,305	172,545	197,100
Other Construction [Member] | Centuri Construction Group Inc [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 94,858	$ 78,569	$ 161,327	$ 117,187	$ 337,181	$ 202,245

Revenue - Summary of Informatio

Revenue - Summary of Information about Receivables, Revenue Earned on Contracts in Progress in Excess of Billings, Which are Included Within Accounts Receivable, Net of Allowances, and Amounts Billed in Excess of Revenue Earned on Contracts (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Contracts receivable, net	$ 187,485
Centuri Construction Group Inc [Member]
Disaggregation of Revenue [Line Items]
Contracts receivable, net	187,485	$ 221,859
Revenue earned on contracts in progress in excess of billings	86,750	5,768
Amounts billed in excess of revenue earned on contracts	$ 8,592	$ 9,602

Revenue - Schedule of Construct

Revenue - Schedule of Construction Services Contracts Receivable (Detail) $ in Thousands	Jun. 30, 2018USD ($)
Receivables [Abstract]
Billed on completed contracts and contracts in progress	$ 185,911
Other receivables	1,660
Contracts receivable, gross	187,571
Allowance for doubtful accounts	(86)
Contracts receivable, net	$ 187,485

Revenue - Summary of Significan

Revenue - Summary of Significant Changes in Asset and Liability Balances (Detail) - Centuri Construction Group Inc [Member] $ in Thousands	6 Months Ended
	Jun. 30, 2018USD ($)
Amounts Billed in Excess of Revenue Earned on Contracts [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue recognized that was included in the amounts billed in excess of revenue earned on contracts balance at the beginning of the period	$ (9,602)
Increases due to amounts billed to customers in excess of revenue earned during the period	8,592
Revenue Earned on Contracts in Progress in Excess of Billings [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Transferred to contracts receivable from revenue earned on contracts in progress in excess of billings recognized at the beginning of the period	(5,768)
Increases from the reclassification of contract assets due to the adoption of topic 606	51,744
Increases from contract assets, contingent on a future event occurring	$ 35,006

Segment Information - Additiona

Segment Information - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018Segment
Segment Reporting Information [Line Items]
Number of segments	2
Southwest Gas Holdings, Inc. [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		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 637,811	$ 537,546	$ 1,364,983	$ 1,170,987	$ 2,645,629	$ 2,302,304
Segment net income (loss)	21,551	17,864	100,642	87,172	207,311	154,824
Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	33,072	22,923	60,230	44,219	113,170	94,396
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	670,883	560,469	1,425,213	1,215,206	2,758,799	2,396,700
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Revenues	275,679	260,162	769,992	722,764	1,349,536	1,263,428
Segment net income (loss)	2,622	9,522	92,971	86,460	163,329	125,942
Natural Gas Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	275,679	260,162	769,992	722,764	1,349,536	1,263,428
Construction Services [Member]
Segment Reporting Information [Line Items]
Revenues	362,132	277,384	594,991	448,223	1,296,093	1,038,876
Segment net income (loss)	19,236	8,716	8,235	1,382	45,213	29,552
Construction Services [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	33,072	22,923	60,230	44,219	113,170	94,396
Construction Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	395,204	300,307	655,221	492,442	1,409,263	1,133,272
Other [Member]
Segment Reporting Information [Line Items]
Segment net income (loss)	$ (307)	$ (374)	$ (564)	$ (670)	$ (1,231)	$ (670)

Derivatives and Fair Value Me43

Derivatives and Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$ 636,000	$ 518,000	$ 1,271,000	$ 1,036,000	$ 2,308,000	$ 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			Jul. 31, 2018
Maximum [Member]
Derivative [Line Items]
Maturities of natural gas swaps			Oct. 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 Me44

Derivatives and Fair Value Measurements - Notional Amounts under Swaps Contracts (Detail) - MMBTU	Jun. 30, 2018	Dec. 31, 2017
Offsetting [Abstract]
Contract notional amounts	13,035,000	10,929,000

Derivatives and Fair Value Me45

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		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 870	$ (1,168)	$ (4,326)	$ (6,305)	$ (9,593)	$ (5,624)
Regulatory Deferral Accounting Treatment [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ (870)	$ 1,168	$ 4,326	$ 6,305	$ 9,593	$ 5,624

Derivatives and Fair Value Me46

Derivatives and Fair Value Measurements - Fair Values of Swaps in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	$ 737	$ 30
Liability derivatives not designated as hedging instruments	(6,917)	(5,810)
Net total not designated as hedging instruments	(6,180)	(5,780)
Swaps [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	654	11
Liability derivatives not designated as hedging instruments	(5,901)	(4,468)
Net total not designated as hedging instruments	(5,247)	(4,457)
Swaps [Member] | Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	83	19
Liability derivatives not designated as hedging instruments	(1,016)	(1,342)
Net total not designated as hedging instruments	$ (933)	$ (1,323)

Derivatives and Fair Value Me47

Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 1,216	$ 3,931	$ 5,620
Received from counterparties	$ 6	$ 6	$ 6

Derivatives and Fair Value Me48

Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) - Swaps [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 5,247	$ 4,457
Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 933	$ 1,323

Derivatives and Fair Value Me49

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	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Net Assets (Liabilities)	$ (6,180)	$ (5,780)
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	(5,247)	(4,457)
Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	$ (933)	$ (1,323)

Common Stock - Additional Infor

Common Stock - Additional Information (Detail) - USD ($)	Mar. 29, 2017	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Value of common stock	$ 150,000,000
Net proceeds to company			$ 69,139,000	$ (96,000)	$ 110,390,000	$ (111,000)
Common stock issued through Restricted Stock/Unit Plan, and Management Incentive Plan			77,000
Common stock issued, amount			$ 69,922,000
Dividend Reinvestment And Stock Purchase Plan [Member]
Common stock issued through Restricted Stock/Unit Plan, and Management Incentive Plan			73,000
Common stock issued, amount			$ 5,000,000
BNY Mellon Capital Markets, LLC [Member]
Common stock sold through agent		748,932	886,232		1,391,939
Open market weighted average price per share		$ 74.10	$ 73		$ 76.49
Net proceeds to company		$ 54,940,503	$ 64,048,167		$ 105,407,194
Agent commissions		554,954	646,951		1,064,719
Common stock available for sale under the program		$ 43,528,087	$ 43,528,087		$ 43,528,087

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	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Less: current maturities	$ (31,928)		$ (25,346)
Long-term debt, less current maturities	2,037,743		1,798,576
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(31,928)		(25,346)
Long-term debt, less current maturities	2,037,743		1,798,576
Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Long-term debt, less current maturities	1,716,307		1,521,031
Long-term Debt, Current and Noncurrent Abstract
Long-term debt, less current maturities	1,716,307		1,521,031
Notes 3.7% due 2028 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		$ 300,000
Carrying Amount [Member]
Debt Instrument [Line Items]
Less: current maturities	(31,928)		(25,346)
Long-term debt, less current maturities	2,037,743		1,798,576
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(31,928)		(25,346)
Long-term debt, less current maturities	2,037,743		1,798,576
Carrying Amount [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Long-term debt, less current maturities	1,716,307		1,521,031
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	1,716,307		1,521,031
Long-term debt, less current maturities	1,716,307		1,521,031
Carrying Amount [Member] | Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Less: current maturities	(31,928)		(25,346)
Long-term debt, less current maturities	321,436		277,545
Unamortized debt issuance costs	(999)		(1,111)
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	353,364		302,891
Less: current maturities	(31,928)		(25,346)
Long-term debt, less current maturities	321,436		277,545
Centuri secured revolving credit facility	188,877		198,467
Carrying Amount [Member] | Centuri Secured Revolving Credit Facility [Member] | Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	87,952		56,472
Carrying Amount [Member] | Centuri Term Loan Facility [Member] | Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	189,876		199,578
Carrying Amount [Member] | Centuri Other Debt Obligations [Member] | Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	76,535		47,952
Carrying Amount [Member] | Debentures [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(12,259)		(9,350)
Notes payable	1,470,241		1,173,150
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] | Notes 3.7% due 2028 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	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, 7.78% 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	48,000		150,000
Carrying Amount [Member] | IDRBs [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(1,934)		(2,119)
Unsecured debt	198,066		197,881
Carrying Amount [Member] | IDRBs [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] | IDRBs [Member] | 2003 Series A, due 2038 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount [Member] | IDRBs [Member] | 2008 Series A, due 2038 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount [Member] | IDRBs [Member] | 2009 Series A, due 2039 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Fair Value [Member] | Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	190,524		207,588
Centuri other debt obligations	76,419		48,183
Fair Value [Member] | Centuri Secured Revolving Credit Facility [Member] | Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	87,998		56,525
Fair Value [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	127,258		129,273
Fair Value [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	150,816		158,304
Fair Value [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	252,108		256,163
Fair Value [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	267,985		283,243
Fair Value [Member] | Debentures [Member] | Notes, 3.8%, due 2046 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	281,028		302,970
Fair Value [Member] | Debentures [Member] | Notes 3.7% due 2028 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	298,068
Fair Value [Member] | Debentures [Member] | 8% Series, due 2026 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	95,789		96,063
Fair 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	27,984		28,714
Fair 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	30,228		31,542
Fair 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	8,740		8,882
Fair 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	48,000		150,000
Fair Value [Member] | IDRBs [Member] | Tax-exempt Series A, due 2028 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Fair Value [Member] | IDRBs [Member] | 2003 Series A, due 2038 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Fair Value [Member] | IDRBs [Member] | 2008 Series A, due 2038 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Fair Value [Member] | IDRBs [Member] | 2009 Series A, due 2039 [Member] | Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 50,000		$ 50,000

Long-Term Debt - Schedule of 52

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Parenthetical) (Detail)	1 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Notes 3.7% due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.70%
Debt instrument due date	Apr. 30, 2028
Debentures [Member] | Southwest Gas Corporation [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] | Southwest Gas Corporation [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] | Southwest Gas Corporation [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] | Southwest Gas Corporation [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] | Southwest Gas Corporation [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] | Southwest Gas Corporation [Member] | Notes 3.7% due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		3.70%	3.70%
Debt instrument due date		Dec. 1, 2028	Dec. 1, 2028
Debentures [Member] | Southwest Gas Corporation [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] | Southwest Gas Corporation [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] | Southwest Gas Corporation [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] | Southwest Gas Corporation [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
IDRBs [Member] | Southwest Gas Corporation [Member] | Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument due date		Dec. 1, 2028	Dec. 1, 2028
IDRBs [Member] | Southwest Gas Corporation [Member] | 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date		Mar. 1, 2038	Mar. 1, 2038
IDRBs [Member] | Southwest Gas Corporation [Member] | 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date		Mar. 1, 2038	Mar. 1, 2038
IDRBs [Member] | Southwest Gas Corporation [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	3 Months Ended	6 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2018
Secured Revolving Credit and Term Loan Facility [Member] | Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 450,000,000	$ 450,000,000
Credit facility expiration date			Nov. 30, 2022
Borrowings outstanding under facility		278,000,000	$ 278,000,000
Debt secured by assets		659,000,000	659,000,000
Notes 3.7% due 2028 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	$ 300,000,000
Debt instrument interest rate	3.70%
Debt instrument discount rate	0.185%
Debt instrument due date	Apr. 30, 2028
Southwest Gas Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		400,000,000	400,000,000
Line of credit designated as long term debt		150,000,000	150,000,000
Line of credit designated for working capital purposes		250,000,000	$ 250,000,000
Credit facility expiration date			Mar. 1, 2022
Southwest Gas Credit Facility [Member] | Long-term Debt [Member]
Debt Instrument [Line Items]
Borrowings outstanding under facility		48,000,000	$ 48,000,000
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%
Commercial Paper Program [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		0	$ 0
Borrowings outstanding under facility		50,000,000	50,000,000
Line of Credit [Member] | Secured Revolving Credit and Term Loan Facility [Member] | Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		250,000,000	250,000,000
Centuri Term Loan Facility [Member] | Secured Revolving Credit and Term Loan Facility [Member] | Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		200,000,000	200,000,000
Equipment Loans [Member] | Secured Revolving Credit and Term Loan Facility [Member] | Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 40,000,000	$ 40,000,000
Credit facility expiration date		May 31, 2023

Short-Term Debt - Additional In

Short-Term Debt - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Short term borrowings outstanding	$ 22,500,000	$ 214,500,000
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
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. 1, 2022
Short-term Debt [Member] | Southwest Gas Credit Facility [Member] | Credit Facility [Member]
Short-term Debt [Line Items]
Short term borrowings outstanding	$ 22,500,000

Equity, Other Comprehensive I55

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule Of Stockholders Equity [Line Items]
Balance			$ 1,812,403
Balance, shares			48,090,470
Common stock issuances			$ 69,922
Net income	$ 21,551	$ 18,121	99,845	$ 87,126	$ 206,661	$ 155,727
Foreign currency exchange translation adj.			(1,601)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			(1,792)
Amounts reclassified to net income, net of tax (FSIRS)	(636)	(518)	(1,271)	(1,036)	(2,308)	(2,073)
Dividends declared Common			(51,065)
Balance	$ 1,932,567		$ 1,932,567		$ 1,932,567
Balance, shares	49,126,254		49,126,254		49,126,254
Net income			$ 99,845
Balance			1,814,768
Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Net income			92,971	86,460	$ 163,329	155,061
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			(1,792)
Amounts reclassified to net income, net of tax (FSIRS)	$ (636)	$ (518)	(1,271)	$ (1,036)	(2,308)	$ (2,073)
Dividends declared Common			(44,000)
Balance	1,718,993		1,718,993		1,718,993
Net income			92,971
Stock-based compensation			364
Contributions from Southwest Gas Holdings, Inc.			56,596
Balance			1,609,999
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 49,720
Balance, shares			48,090,000
Common stock issuances			$ 1,036
Common stock issuances, Shares			1,036,000
Other comprehensive income (loss):
Balance	$ 50,756		$ 50,756		$ 50,756
Balance, shares	49,126,000		49,126,000		49,126,000
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			$ 955,332
Common stock issuances			68,886
Other comprehensive income (loss):
Elimination of shares from noncontrolling interest			(2,710)
Balance	$ 1,021,508		1,021,508		$ 1,021,508
Additional Paid-in Capital [Member] | Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			948,767
Other comprehensive income (loss):
Balance	1,006,065		1,006,065		1,006,065
Stock-based compensation			702
Contributions from Southwest Gas Holdings, Inc.			56,596
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(47,682)
Foreign currency exchange translation adj.			(1,601)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			1,792
Amounts reclassified to net income, net of tax (FSIRS)			1,271
Reclassification of excess deferred taxes			(9,300)
Balance	(55,520)		(55,520)		(55,520)
Accumulated Other Comprehensive Income (Loss) [Member] | Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(47,073)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			1,792
Amounts reclassified to net income, net of tax (FSIRS)			1,271
Reclassification of excess deferred taxes			(9,300)
Balance	(53,310)		(53,310)		(53,310)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			857,398
Net income			100,642
Other comprehensive income (loss):
Reclassification of excess deferred taxes			9,300
Dividends declared Common			(51,065)
Balance	916,275		916,275		916,275
Retained Earnings [Member] | Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			659,193
Net income			92,971
Other comprehensive income (loss):
Reclassification of excess deferred taxes			9,300
Dividends declared Common			(44,000)
Balance	717,126		717,126		717,126
Stock-based compensation			(338)
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(2,365)
Other comprehensive income (loss):
Elimination of shares from noncontrolling interest			2,710
Balance	$ (452)		(452)		$ (452)
Net income			$ (797)

Equity, Other Comprehensive I56

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule Of Stockholders Equity [Line Items]
Dividends declared Common, per share	$ 0.520	$ 0.495	$ 1.040	$ 0.990	$ 2.030	$ 1.890
IntelliChoice Energy Llc [Member]
Schedule Of Stockholders Equity [Line Items]
Historical ownership interest percentage	65.00%		65.00%		65.00%
Residual ownership percentage by third party	35.00%		35.00%		35.00%
Increased ownership percentage	95.00%
Additional capital contributed	$ 1

Equity, Other Comprehensive I57

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		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service cost, Before-Tax Amount	$ 334	$ 334	$ 668	$ 668	$ 1,335	$ 1,335
Amortization of net actuarial (gain)/loss, Before-Tax Amount	8,404	6,361	16,808	12,722	29,531	26,255
Regulatory adjustment, Before-Tax Amount	(7,559)	(5,735)	(15,119)	(11,470)	8,691	(4,808)
Pension plans other comprehensive income (loss)	1,179	960	2,357	1,920	(3,470)	12
Amounts reclassified into net income, Before-Tax Amount	836	836	1,673	1,672	3,345	3,344
FS IRS other comprehensive income (loss), Before-Tax Amount	836	836	1,673	1,672	3,345	3,344
Total other comprehensive income (loss), Before-Tax Amount	1,325	2,425	2,429	4,441	(804)	3,514
Translation adjustments, Before-Tax Amount	(690)	629	(1,601)	849	(679)	158
Foreign currency other comprehensive income (loss), Before-Tax Amount	(690)	629	(1,601)	849	(679)	158
Amortization of prior service cost, Tax (Expense) or Benefit	(80)	(127)	(160)	(254)	(413)	(507)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(2,017)	(2,417)	(4,034)	(4,834)	(8,869)	(9,976)
Regulatory adjustment, Tax (Expense) or Benefit	1,815	2,179	3,629	4,358	(479)	1,826
Net pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(282)	(365)	(565)	(730)	565	(5)
Amounts reclassified into net income, Tax (Expense) or Benefit	(200)	(318)	(402)	(636)	(1,037)	(1,271)
FSIRS other comprehensive income, Tax (Expense) or Benefit	(200)	(318)	(402)	(636)	1,037	1,271
Total other comprehensive income (loss), Tax (Expense) or Benefit	(482)	(683)	(967)	(1,366)	(472)	(1,276)
Translation adjustments, Tax (Expense) or Benefit	0	0	0	0	0	0
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0	0	0	0
Amortization of prior service cost, Net-of-Tax Amount	254	207	508	414	922	828
Amortization of net actuarial loss	6,387	3,944	12,774	7,888	20,662	16,279
Regulatory adjustment, Net-of-Tax Amount	(5,744)	(3,556)	(11,490)	(7,112)	8,212	(2,982)
Pension plans other comprehensive income (loss)	897	595	1,792	1,190	(2,905)	7
Amounts reclassified into net income, Net-of-Tax Amount	636	518	1,271	1,036	2,308	2,073
FSIRS other comprehensive income (loss), Net-of-Tax Amount	636	518	1,271	1,036	2,308	2,073
Net other comprehensive income	843	1,742	1,462	3,075	(1,276)	2,238
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	(690)	629	(1,601)	849	(679)	158
Net actuarial gain/(loss), Before-Tax Amount					(43,027)	(22,770)
Net actuarial gain/(loss), Tax (Expense) or Benefit					10,326	8,652
Net actuarial gain/(loss), Net-of-Tax Amount					(32,701)	(14,118)
Southwest Gas Corporation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service cost, Before-Tax Amount			668
Amortization of net actuarial (gain)/loss, Before-Tax Amount			16,808
Regulatory adjustment, Before-Tax Amount			(15,119)
Total other comprehensive income (loss), Before-Tax Amount	2,015	1,796	4,030	3,592	(125)	3,356
Amortization of prior service cost, Tax (Expense) or Benefit			(160)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit			(4,034)
Regulatory adjustment, Tax (Expense) or Benefit			3,629
Amounts reclassified into net income, Tax (Expense) or Benefit			(402)
Total other comprehensive income (loss), Tax (Expense) or Benefit	(482)	(683)	(967)	(1,366)	(472)	(1,276)
Amortization of prior service cost, Net-of-Tax Amount	254	207	508	414	922	828
Amortization of net actuarial loss	6,387	3,944	12,774	7,888	20,662	16,279
Regulatory adjustment, Net-of-Tax Amount	(5,744)	(3,556)	(11,490)	(7,112)	8,212	(2,982)
Amounts reclassified into net income, Net-of-Tax Amount	636	518	1,271	1,036	2,308	2,073
Net other comprehensive income	$ 1,533	$ 1,113	$ 3,063	$ 2,226	(597)	2,080
Net actuarial gain/(loss), Net-of-Tax Amount					$ (32,701)	$ (14,118)

Equity, Other Comprehensive I58

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Parenthetical) (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Effective income tax rate prior period adjustment	0.24
Tax Cuts and Jobs Act [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Effective income tax rate	24.00%	38.00%

Equity, Other Comprehensive I59

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Additional Information (Detail) $ in Millions	Jun. 30, 2018USD ($)
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$ 2.5

Equity, Other Comprehensive I60

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance AOCI December 31, 2017, Defined Benefit Plans Before-Tax			$ (61,520)
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	$ 334	$ 334	668	$ 668	$ 1,335	$ 1,335
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	8,404	6,361	16,808	12,722	29,531	26,255
Regulatory adjustment, , Defined Benefit Plans Before-Tax	(7,559)	(5,735)	(15,119)	(11,470)	8,691	(4,808)
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.Defined Benefit Plans Before-Tax			2,357
Reclassification of excess deferred taxes, Defined Benefit Plans Before-Tax			0
Beginning Balance AOCI December 31, 2017			22,293
Ending Balance AOCI June 30, 2018, Defined Benefit Plans Before-Tax	(59,163)		(59,163)		(59,163)
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	(80)	(127)	(160)	(254)	(413)	(507)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	(2,017)	(2,417)	(4,034)	(4,834)	(8,869)	(9,976)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	1,815	2,179	3,629	4,358	(479)	1,826
Net current period other comprehensive income (loss)attributable to southwest gas holdings, Inc Defined Benefit Plans Tax (Expense) Benefit			(565)
Reclassification of excess deferred taxes, Defined Benefit Plans Tax (Expense) Benefit			0
Ending Balance AOCI June 30, 2018, Defined Benefit Plans Tax (Expense) Benefit	21,728		21,728		21,728
Translation adjustments, Defined Benefit Plans After-Tax			0
Beginning Balance AOCI December 31, 2017, Defined Benefit Plans After-Tax			(39,227)
Other comprehensive income before reclassifications, Defined Benefit Plans After-Tax			0
Amortization of prior service cost, Defined Benefit Plans After-Tax	254	207	508	414	922	828
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	6,387	3,944	12,774	7,888	20,662	16,279
Regulatory adjustment, Defined Benefit Plans After-Tax			(11,490)
Net current period other comprehensive income (loss)attributable to southwest gas holdings, Inc Defined Benefit Plans Tax (Expense) Benefit			(1,792)
Reclassification of excess deferred taxes, Defined Benefit Plans After-Tax			0
Beginning Balance AOCI December 31, 2017, FSIRS - Before-Tax			(12,655)
Ending Balance AOCI June 30, 2018, Defined Benefit Plans After-Tax	(37,435)		(37,435)		(37,435)
Translation adjustments, FSIRS - Before-Tax			0
Net other comprehensive income	843	1,742	1,462	3,075	(1,276)	2,238
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	836	836	1,673	1,672	3,345	3,344
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. FSIRS - Before-Tax			1,673
Reclassification of excess deferred taxes, FSIRS - Before-Tax			0
Ending Balance AOCI June 30, 2018, FSIRS - Before-Tax	(10,982)		(10,982)		(10,982)
Translation adjustments, FSIRS - Tax (Expense) Benefit			0
Beginning Balance AOCI December 31, 2017, FSIRS - Tax (Expense) Benefit			4,809
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. FSIRS - Tax (Expense) Benefit			(402)
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	(200)	(318)	(402)	(636)	(1,037)	(1,271)
Reclassification of excess deferred taxes, FSIRS - Tax (Expense) Benefit			0
Ending Balance AOCI June 30, 2018, FSIRS - Tax (Expense) Benefit	4,407		4,407		4,407
Translation adjustments			0
Beginning Balance AOCI December 31, 2017			(7,846)
Other comprehensive income before reclassifications			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.	636	518	1,271	1,036	2,308	2,073
Amounts reclassified into net income	636	518	1,271	1,036	2,308	2,073
Beginning Balance AOCI December 31, 2017			(609)
Reclassification of excess deferred taxes			0
Translation adjustments	(690)	629	(1,601)	849	(679)	158
Ending Balance AOCI June 30, 2018	(6,575)		(6,575)		(6,575)
Other comprehensive income before reclassifications	(690)	629	(1,601)	849	(679)	158
FSIRS amounts reclassified from AOCI			0
Amortization of prior service cost			0
Amortization of net actuarial loss			0
Regulatory adjustment			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.			(1,601)
Ending Balance AOCI June 30, 2018	(2,210)		(2,210)		(2,210)
Beginning Balance AOCI December 31, 2017			0
Translation adjustments			0
Other comprehensive income before reclassifications			0
FSIRS amounts reclassified from AOCI			0
Amortization of prior service cost			0
Amortization of net actuarial loss			0
Regulatory adjustment			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.	0	0	0	0	0	0
Reclassification of excess deferred taxes			0
Ending Balance AOCI June 30, 2018	0		0		0
Beginning Balance AOCI December 31, 2017			(609)
Translation adjustments			(1,601)
Other comprehensive income before reclassifications	(690)	629	(1,601)	849	(679)	158
FSIRS amounts reclassified from AOCI			0
Amortization of prior service cost			0
Amortization of net actuarial loss			0
Regulatory adjustment			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.			(1,601)
Reclassification of excess deferred taxes			0
Ending Balance AOCI June 30, 2018	(2,210)		(2,210)		(2,210)
Ending Balance AOCI June 30, 2018	(9,300)		(9,300)		(9,300)
Translation adjustments			(1,601)
Other comprehensive income before reclassifications			(1,601)
Beginning Balance AOCI December 31, 2017			(47,682)
Amounts reclassified into net income, Net-of-Tax Amount	636	518	1,271	1,036	2,308	2,073
Amortization of prior service cost	254	207	508	414	922	828
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.			1,462
Amortization of net actuarial loss	6,387	3,944	12,774	7,888	20,662	16,279
Regulatory adjustment			(11,490)
Ending Balance AOCI June 30, 2018	(55,520)		(55,520)		(55,520)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net current period other comprehensive income (loss)attributable to southwest gas holdings, Inc Defined Benefit Plans Tax (Expense) Benefit			1,792
Amounts reclassified into net income			(1,271)
Amounts reclassified into net income, Net-of-Tax Amount			(1,271)
Reclassification of excess deferred taxes			(9,300)
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification of excess deferred taxes			(9,300)
Southwest Gas Corporation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance AOCI December 31, 2017, Defined Benefit Plans Before-Tax			(61,520)
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax			1,673
Amortization of prior service cost, Defined Benefit Plans Before-Tax			668
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax			16,808
Net other comprehensive income (loss), Defined Benefit Plans Tax (Expense) Benefit			(565)
Regulatory adjustment, , Defined Benefit Plans Before-Tax			(15,119)
Reclassification of excess deferred taxes, Defined Benefit Plans Before-Tax			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.Defined Benefit Plans Before-Tax			2,357
Beginning Balance AOCI December 31, 2017			22,293
Ending Balance AOCI June 30, 2018, Defined Benefit Plans Before-Tax	(59,163)		(59,163)		(59,163)
Amortization of prior service cost, Defined Benefit Plans Tax (Expense) Benefit			(160)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit			(4,034)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit			3,629
Reclassification of excess deferred taxes, Defined Benefit Plans Tax (Expense) Benefit			0
Ending Balance AOCI June 30, 2018, Defined Benefit Plans Tax (Expense) Benefit	21,728		21,728		21,728
Beginning Balance AOCI December 31, 2017, Defined Benefit Plans After-Tax			(39,227)
Net other comprehensive income (loss), FSIRS - After-Tax			1,271
Amortization of prior service cost, Defined Benefit Plans After-Tax	254	207	508	414	922	828
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	6,387	3,944	12,774	7,888	20,662	16,279
Regulatory adjustment, Defined Benefit Plans After-Tax			(11,490)
Net current period other comprehensive income (loss)attributable to southwest gas holdings, Inc Defined Benefit Plans Tax (Expense) Benefit			(1,792)
Reclassification of excess deferred taxes, Defined Benefit Plans After-Tax			0
Beginning Balance AOCI December 31, 2017, FSIRS - Before-Tax			(12,655)
Ending Balance AOCI June 30, 2018, Defined Benefit Plans After-Tax	(37,435)		(37,435)		(37,435)
Net other comprehensive income	1,533	1,113	3,063	2,226	(597)	2,080
Net other comprehensive income, FSIRS - Before-Tax			1,673
Reclassification of excess deferred taxes, FSIRS - Before-Tax			0
Ending Balance AOCI June 30, 2018, FSIRS - Before-Tax	(10,982)		(10,982)		(10,982)
Beginning Balance AOCI December 31, 2017, FSIRS - Tax (Expense) Benefit			4,809
Net other comprehensive income, FSIRS - Tax (Expense) Benefit			402
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit			(402)
Reclassification of excess deferred taxes, FSIRS - Tax (Expense) Benefit			0
Ending Balance AOCI June 30, 2018, FSIRS - Tax (Expense) Benefit	4,407		4,407		4,407
Beginning Balance AOCI December 31, 2017			(7,846)
Amounts reclassified into net income	636	518	1,271	1,036	2,308	2,073
Reclassification of excess deferred taxes			0
Ending Balance AOCI June 30, 2018	(6,575)		(6,575)		(6,575)
Ending Balance AOCI June 30, 2018	(9,300)		(9,300)		(9,300)
Beginning Balance AOCI December 31, 2017			(47,073)
Amounts reclassified into net income, Net-of-Tax Amount	636	518	1,271	1,036	2,308	2,073
Amortization of prior service cost	254	207	508	414	922	828
Amortization of net actuarial loss	6,387	$ 3,944	12,774	$ 7,888	20,662	$ 16,279
Regulatory adjustment			(11,490)
Ending Balance AOCI June 30, 2018	$ (53,310)		(53,310)		$ (53,310)
Southwest Gas Corporation [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net current period other comprehensive income (loss)attributable to southwest gas holdings, Inc Defined Benefit Plans Tax (Expense) Benefit			1,792
Amounts reclassified into net income			(1,271)
Reclassification of excess deferred taxes			(9,300)
Amounts reclassified into net income, Net-of-Tax Amount			(1,271)
Reclassification of excess deferred taxes			(9,300)
Southwest Gas Corporation [Member] | Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification of excess deferred taxes			$ (9,300)

Equity, Other Comprehensive I61

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Parenthetical) (Detail) - Tax Cuts and Jobs Act [Member]	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Effective income tax rate	24.00%	38.00%
Southwest Gas Corporation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Effective income tax rate	24.00%	38.00%

Equity, Other Comprehensive I62

Equity, Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Amount Recognized Before Income Tax Associated with Defined Benefit Plans in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognized in AOCI	$ (59,163)	$ (61,520)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	(431,747)	(448,555)
Prior service cost	(3,700)	(4,368)
Less: amount recognized in regulatory assets	376,284	391,403
Recognized in AOCI	$ (59,163)	$ (61,520)

Reorganization Impacts - Disc63

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation - Summary of Income Statement Components of Discontinued Operations - Construction Services in Condensed Consolidated Statements of Income (Detail) - Southwest Gas Corporation [Member] $ in Thousands	12 Months Ended
	Jun. 30, 2017USD ($)
Operating expenses:
Income before income taxes	$ 45,669
Income tax expense	16,550
Net income	29,119
Net income attributable to noncontrolling interests	949
Discontinued operations - construction services - net income	28,170
Construction Services [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Construction revenues	640,830
Operating expenses:
Construction expenses	567,115
Depreciation and amortization	25,727
Operating income	47,988
Other income (deductions)	1,193
Net interest deductions	3,512
Income before income taxes	45,669
Income tax expense	16,550
Net income	29,119
Net income attributable to noncontrolling interests	949
Discontinued operations - construction services - net income	$ 28,170

Acquisition of Construction S64

Acquisition of Construction Services Business - Additional Information (Detail) - New England Utility Constructors Inc [Member] - USD ($)	6 Months Ended
	Jun. 30, 2018	Nov. 30, 2017
Business Acquisition [Line Items]
Net consideration of acquisition less assumed debt		$ 99,000,000
Acquisition costs	$ 0