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 loss	Accumulated other comprehensive loss Southwest Gas Corporation	Retained earnings	Retained earnings Southwest Gas Corporation	Southwest Gas Holdings, Inc.	Noncontrolling interest
Beginning balance (in shares) at Dec. 31, 2018			53,026,000
Beginning balance at Dec. 31, 2018			$ 54,656		$ 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
Change in ownership of noncontrolling interest					(452)							452
Foreign currency exchange translation adjustment							1,131
Net actuarial gain 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
Net income									122,218	86,746
Share-based compensation						3,317				(465)
Contributions from Southwest Gas Holdings, Inc.						126,186
Dividends declared									(89,005)	(72,400)
Redemption value adjustments									0
Ending balance (in shares) at Sep. 30, 2019			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
Beginning balance at Jun. 30, 2019			$ 55,951		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
Change in ownership of noncontrolling interest					0							0
Foreign currency exchange translation adjustment							(447)
Net actuarial gain 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
Net income									5,353	(20,012)
Share-based compensation						1,102				(153)
Contributions from Southwest Gas Holdings, Inc.						24,094
Dividends declared									(29,925)	(24,900)
Redemption value adjustments									0
Ending balance (in shares) at Sep. 30, 2019			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
FSIRS amounts reclassified to net income, net of tax	$ 2,689	2,689
Ending balance (in shares) at Sep. 30, 2020	56,459,346		56,459,000	47,482,000
Ending balance at Sep. 30, 2020	$ 2,619,238	2,144,273	$ 58,089	$ 49,112	1,559,814	1,363,441	(52,783)	(50,015)	1,054,118	781,735	2,619,238	0
Beginning balance (in shares) at Dec. 31, 2019	55,007,433		55,007,000
Beginning balance at Dec. 31, 2019			$ 56,637		1,466,937	1,229,083	(56,732)	(55,151)	1,039,072	782,108		0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			1,452,000
Common stock issuances			$ 1,452		92,877
Change in ownership of noncontrolling interest					0							0
Foreign currency exchange translation adjustment							(1,187)
Net actuarial gain arising during period, less amortization of unamortized benefit plan cost, net of tax							3,082	3,082
FSIRS amounts reclassified to net income, net of tax	$ 2,054	2,054					2,054	2,054
Net income									128,780	79,568
Share-based compensation						2,397				(641)
Contributions from Southwest Gas Holdings, Inc.						131,961
Dividends declared									(96,161)	(79,300)
Redemption value adjustments	$ 17,573								(17,573)
Ending balance (in shares) at Sep. 30, 2020	56,459,346		56,459,000	47,482,000
Ending balance at Sep. 30, 2020	$ 2,619,238	2,144,273	$ 58,089	$ 49,112	1,559,814	1,363,441	(52,783)	(50,015)	1,054,118	781,735	2,619,238	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 1.71
Beginning balance (in shares) at Jun. 30, 2020			55,910,000
Beginning balance at Jun. 30, 2020			$ 57,540		1,523,630	1,329,843	(55,617)	(51,825)	1,085,742	824,847		0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issuances (in shares)			549,000
Common stock issuances			$ 549		36,184
Change in ownership of noncontrolling interest					0							0
Foreign currency exchange translation adjustment							1,024
Net actuarial gain arising during period, less amortization of unamortized benefit plan cost, net of tax							1,027	1,027
FSIRS amounts reclassified to net income, net of tax	$ 783	783					783	783
Net income									18,273	(15,973)
Share-based compensation						1,137				(139)
Contributions from Southwest Gas Holdings, Inc.						32,461
Dividends declared									(32,324)	(27,000)
Redemption value adjustments	$ 17,600								(17,573)
Ending balance (in shares) at Sep. 30, 2020	56,459,346		56,459,000	47,482,000
Ending balance at Sep. 30, 2020	$ 2,619,238	$ 2,144,273	$ 58,089	$ 49,112	$ 1,559,814	$ 1,363,441	$ (52,783)	$ (50,015)	$ 1,054,118	$ 781,735	$ 2,619,238	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share (in USD per share)	$ 0.57