CONDENSED CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Thousands	Total	Southwest Gas Corporation	Common Stock	Common Stock Southwest Gas Corporation	Additional paid-in capital	Additional paid-in capital Southwest Gas Corporation	Accumulated other comprehensive income (loss)	Accumulated other comprehensive income (loss) Southwest Gas Corporation	Retained Earnings	Retained Earnings Southwest Gas Corporation	Southwest Gas Holdings, Inc.	Non-controlling Interest
Beginning balance (in shares) at Dec. 31, 2017			48,090,000	47,482,000
Beginning balance at Dec. 31, 2017			$ 49,720	$ 49,112	$ 955,332	$ 948,767	$ (47,682)	$ (47,073)	$ 857,398	$ 659,193		$ (2,365)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			1,333,000
Common stock issuances			$ 1,333		93,218
Net income (loss)									112,973	79,301		(797)
Change in ownership of noncontrolling interest					(2,710)							2,710
Foreign currency exchange translation adjustment							(1,002)
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax							2,687	2,687
FSIRS amounts reclassified to net income, net of tax	$ 1,907	$ 1,907					1,907	1,907
Reclassification of excess deferred taxes							(9,300)	(9,300)	9,300	9,300
Share-based compensation						1,899				(510)
Dividends declared									(76,941)	(66,000)
Contributions from Southwest Gas Holdings, Inc.						90,644
Ending balance (in shares) at Sep. 30, 2018			49,423,000	47,482,000
Ending balance at Sep. 30, 2018	$ 1,945,781	1,719,927	$ 51,053	$ 49,112	1,045,840	1,041,310	(53,390)	(51,779)	902,730	681,284	$ 1,946,233	(452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 1.56
Beginning balance (in shares) at Jun. 30, 2018			49,126,000	47,482,000
Beginning balance at Jun. 30, 2018			$ 50,756	$ 49,112	1,021,508	1,006,065	(55,520)	(53,310)	916,275	717,126		(452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			297,000
Common stock issuances			$ 297		24,332
Net income (loss)									12,331	(13,670)		0
Change in ownership of noncontrolling interest					0							0
Foreign currency exchange translation adjustment							599
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax							895	895
FSIRS amounts reclassified to net income, net of tax	$ 636	636					636	636
Reclassification of excess deferred taxes							0	0	0	0
Share-based compensation						1,197				(172)
Dividends declared									(25,876)	(22,000)
Contributions from Southwest Gas Holdings, Inc.						34,048
Ending balance (in shares) at Sep. 30, 2018			49,423,000	47,482,000
Ending balance at Sep. 30, 2018	$ 1,945,781	1,719,927	$ 51,053	$ 49,112	1,045,840	1,041,310	(53,390)	(51,779)	902,730	681,284	1,946,233	(452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 0.52
FSIRS amounts reclassified to net income, net of tax	$ 2,540	2,540
Ending balance (in shares) at Sep. 30, 2019	54,624,090		54,624,000	47,482,000
Ending balance at Sep. 30, 2019	$ 2,423,710	1,929,606	$ 56,254	$ 49,112	1,437,733	1,194,745	(47,775)	(45,287)	977,498	731,036	2,423,710	0
Beginning balance (in shares) at Dec. 31, 2018	53,026,848		53,026,000	47,482,000
Beginning balance at Dec. 31, 2018	$ 2,251,590		$ 54,656	$ 49,112	1,305,769	1,065,242	(52,668)	(49,049)	944,285	717,155		(452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			1,598,000
Common stock issuances			$ 1,598		132,416
Net income (loss)									122,218	86,746		0
Change in ownership of noncontrolling interest					(452)							452
Foreign currency exchange translation adjustment							1,131
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax							1,856	1,856
FSIRS amounts reclassified to net income, net of tax	$ 1,906	1,906					1,906	1,906
Reclassification of excess deferred taxes							0	0	0	0
Share-based compensation						3,317				(465)
Dividends declared									(89,005)	(72,400)
Contributions from Southwest Gas Holdings, Inc.						126,186
Ending balance (in shares) at Sep. 30, 2019	54,624,090		54,624,000	47,482,000
Ending balance at Sep. 30, 2019	$ 2,423,710	1,929,606	$ 56,254	$ 49,112	1,437,733	1,194,745	(47,775)	(45,287)	977,498	731,036	2,423,710	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 1.635
Beginning balance (in shares) at Jun. 30, 2019			54,321,000	47,482,000
Beginning balance at Jun. 30, 2019			$ 55,951	$ 49,112	1,409,923	1,169,549	(48,581)	(46,540)	1,002,070	776,101		0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			303,000
Common stock issuances			$ 303		27,810
Net income (loss)									5,353	(20,012)		0
Change in ownership of noncontrolling interest					0							0
Foreign currency exchange translation adjustment							(447)
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax							618	618
FSIRS amounts reclassified to net income, net of tax	$ 635	635					635	635
Reclassification of excess deferred taxes							0	0	0	0
Share-based compensation						1,102				(153)
Dividends declared									(29,925)	(24,900)
Contributions from Southwest Gas Holdings, Inc.						24,094
Ending balance (in shares) at Sep. 30, 2019	54,624,090		54,624,000	47,482,000
Ending balance at Sep. 30, 2019	$ 2,423,710	$ 1,929,606	$ 56,254	$ 49,112	$ 1,437,733	$ 1,194,745	$ (47,775)	$ (45,287)	$ 977,498	$ 731,036	$ 2,423,710	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 0.545