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)