Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$ 78,294	$ 69,005	$ 203,231	$ 146,705
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	62,478	72,478	240,951	286,250
Deferred income taxes	23,228	37,245	49,372	87,046
Changes in current assets and liabilities:
Accounts receivable, net of allowances	8,858	36,889	(68,978)	21,460
Accrued utility revenue	30,900	30,300	(1,400)	(800)
Deferred purchased gas costs	(10,628)	(69,906)	(36,330)	(83,025)
Accounts payable	(48,497)	(55,298)	26,762	1,523
Accrued taxes	18,776	20,397	491	10,660
Other current assets and liabilities	(643)	21,099	(29,945)	(15,228)
Gains on sale	(230)	(339)	(4,087)	(6,154)
Changes in undistributed stock compensation	1,861	6,111	6,638	10,173
AFUDC	(229)	(475)	(2,050)	(2,232)
Changes in other assets and deferred charges	(1,845)	(7,173)	(16,941)	10,078
Changes in other liabilities and deferred credits	18,244	1,510	20,965	(18,656)
Net cash provided by operating activities	180,567	161,843	388,679	447,800
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(154,542)	(115,790)	(662,401)	(532,760)
Acquisition of businesses, net of cash acquired	(4,209)		(98,413)	(17,000)
Changes in customer advances	3,038	1,057	2,304	5,296
Miscellaneous inflows	1,505	4,721	13,429	16,634
Net cash used in investing activities	(154,208)	(110,012)	(745,081)	(527,830)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	11,220		52,375	71
Dividends paid	(23,839)	(21,397)	(94,572)	(85,494)
Centuri distribution to redeemable noncontrolling interest	(102)	(102)	(204)	(442)
Issuance of long-term debt, net	335,382	26,280	716,165	400,851
Retirement of long-term debt	(21,102)	(47,763)	(312,308)	(260,724)
Change in credit facility and commercial paper	(111,000)	10,000	24,000	15,000
Change in short-term debt	(192,000)		22,500
Principal payments on capital lease obligations	(165)	(199)	(946)	(1,240)
Redemption of Centuri shares from noncontrolling parties			(23,000)
Withholding remittance - share-based compensation	(2,852)	(2,518)	(3,510)	(2,739)
Other	(337)	(913)	(2,498)	(2,624)
Net cash provided by (used in) financing activities	(4,795)	(36,612)	378,002	62,659
Effects of currency translation on cash and cash equivalents	(71)	116	114	(37)
Change in cash and cash equivalents	21,493	15,335	21,714	(17,408)
Cash and cash equivalents at beginning of period	43,622	28,066	43,401	60,809
Cash and cash equivalents at end of period	65,115	43,401	65,115	43,401
Supplemental information:
Interest paid, net of amounts capitalized	13,294	10,288	74,949	67,135
Income taxes paid (received)	4,418	1,827	8,264	(19,341)
Southwest Gas Corporation [Member]
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	90,349	76,938	170,229	154,638
Income (loss) from discontinued operations				35,860
Income from continuing operations	90,349	76,938	170,229	118,778
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	49,961	61,195	190,688	233,913
Deferred income taxes	18,676	39,223	46,622	87,729
Changes in current assets and liabilities:
Accounts receivable, net of allowances	(28,201)	(7,072)	(29,031)	5,750
Accrued utility revenue	30,900	30,300	(1,400)	(800)
Deferred purchased gas costs	(10,628)	(69,906)	(36,330)	(83,025)
Accounts payable	(34,564)	(44,736)	14,717	3,771
Accrued taxes	27,476	25,176	12,683	10,637
Other current assets and liabilities	3,163	37,342	(47,905)	(6,677)
Changes in undistributed stock compensation	2,118	5,711	5,695	9,773
AFUDC	(229)	(475)	(2,050)	(2,232)
Changes in other assets and deferred charges	(1,998)	(7,261)	(17,655)	9,728
Changes in other liabilities and deferred credits	17,887	1,198	20,230	(18,968)
Net cash provided by operating activities	164,910	147,633	326,493	368,377
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(131,743)	(101,007)	(591,184)	(468,413)
Changes in customer advances	3,038	1,057	2,304	5,296
Miscellaneous inflows	293	784	2,250	3,055
Dividends received				9,660
Net cash used in investing activities	(128,412)	(99,166)	(586,630)	(450,402)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net				71
Contributions from parent			41,359
Dividends paid	(21,000)	(18,500)	(83,997)	(82,597)
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	(111,000)	10,000	24,000	15,000
Change in short-term debt	(191,000)
Withholding remittance - share-based compensation	(2,852)	(2,518)	(3,510)	(2,739)
Other	(298)	(523)	(371)	(2,234)
Net cash provided by (used in) financing activities	(28,655)	(36,541)	274,976	74,115
Net cash provided by discontinued operating activities				65,213
Net cash used in discontinued investing activities				(66,582)
Net cash provided by (used in) discontinued financing activities				(11,385)
Effects of currency translation on cash and cash equivalents				(153)
Change in cash and cash equivalents	7,843	11,926	14,839	(20,817)
Change in cash and cash equivalents of discontinued operations included in discontinued operations construction services assets				12,907
Change in cash and cash equivalents of continuing operations	7,843	11,926	14,839	(7,910)
Cash and cash equivalents at beginning of period	37,946	19,024	30,950	38,860
Cash and cash equivalents at end of period	45,789	30,950	45,789	30,950
Supplemental information:
Interest paid, net of amounts capitalized	$ 10,296	8,989	66,097	61,212
Income taxes paid (received)		$ (38)	$ (7,816)	$ (36,767)