Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) - USD ($)		3 Months Ended		12 Months Ended
	Dec. 22, 2018	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance AOCI December 31, 2017, Defined Benefit Plans Before-Tax		$ (61,520,000)
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		334,000	$ 334,000	$ 1,335,000		$ 1,335,000
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax		8,404,000	6,361,000	27,488,000		26,660,000
Regulatory adjustment, , Defined Benefit Plans Before-Tax		(7,560,000)	(5,735,000)	10,515,000		(5,196,000)
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.Defined Benefit Plans Before-Tax		1,178,000
Reclass of stranded deferred taxes due to TCJA, , Defined Benefit Plans Before-Tax		0
Beginning Balance AOCI December 31, 2017		22,293,000
Ending Balance AOCI March 31, 2018, Defined Benefit Plans Before-Tax		(60,342,000)		(60,342,000)	$ (61,520,000)
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		80,000	127,000	460,000		507,000
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit		(2,017,000)	(2,417,000)	(9,269,000)		(10,131,000)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit		(1,814,000)	(2,179,000)	115,000		(1,974,000)
Net current period other comprehensive income (loss)attributable to southwest gas holdings, Inc Defined Benefit Plans Tax (Expense) Benefit		(283,000)
Reclass of stranded deferred taxes due to TCJA, Defined Benefit Plans Tax (Expense) Benefit		0
Ending Balance AOCI March 31, 2018, Defined Benefit Plans Tax (Expense) Benefit		22,010,000		22,010,000	22,293,000
Translation adjustments, Defined Benefit Plans After-Tax		0
Beginning Balance AOCI December 31, 2017, Defined Benefit Plans After-Tax		(39,227,000)
Other comprehensive income before reclassifications, Defined Benefit Plans After-Tax		0
Amortization of prior service cost, Defined Benefit Plans After-Tax		254,000	207,000	875,000		828,000
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax		6,387,000	3,944,000	18,219,000		16,529,000
Regulatory adjustment, Defined Benefit Plans After-Tax		(5,746,000)
Net current period other comprehensive income (loss)attributable to southwest gas holdings, Inc Defined Benefit Plans Tax (Expense) Benefit		895,000
Reclass of stranded deferred taxes due to TCJA, Defined Benefit Plans After-Tax		0
Beginning Balance AOCI December 31, 2017, FSIRS - Before-Tax		(12,655,000)
Ending Balance AOCI March 31, 2018, Defined Benefit Plans After-Tax		(38,332,000)		(38,332,000)	(39,227,000)
Translation adjustments, FSIRS - Before-Tax		0
Net other comprehensive income (loss)		619,000	1,333,000	(377,000)		1,690,000
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax		837,000	836,000	(3,345,000)		(3,345,000)
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. FSIRS - Before-Tax		837,000		3,345,000		3,345,000
Reclass of stranded deferred taxes due to TCJA, FSIRS - Before-Tax		0
Ending Balance AOCI March 31, 2018, FSIRS - Before-Tax		(11,818,000)		(11,818,000)	(12,655,000)
Translation adjustments, FSIRS - Tax (Expense) Benefit		0
Beginning Balance AOCI December 31, 2017, FSIRS - Tax (Expense) Benefit		4,809,000
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc. FSIRS - Tax (Expense) Benifit		(202,000)
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit		(202,000)	(318,000)	(1,155,000)		(1,271,000)
Reclass of stranded deferred taxes due to TCJA ,FSIRS - Tax (Expense) Benefit		0
Ending Balance AOCI March 31, 2018, FSIRS - Tax (Expense) Benefit		4,607,000		4,607,000	4,809,000
Translation adjustments		0
Beginning Balance AOCI December 31, 2017		(7,846,000)
Other comprehensive income before reclassifications		0
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.		635,000	518,000	2,190,000		2,074,000
Amounts reclassified into net income		635,000	518,000	2,190,000		2,074,000
Beginning Balance AOCI December 31, 2017		(609,000)
Reclass of stranded deferred taxes due to TCJA (4)		0
Translation adjustments		(911,000)	220,000	640,000		(401,000)
Ending Balance AOCI March 31, 2018		(7,211,000)		(7,211,000)	(7,846,000)
Other comprehensive income before reclassifications		(911,000)	220,000	640,000		(401,000)
FSIRS amounts reclassified from AOCI (1)		0
Amortization of prior service cost (2)		0
Amortization of net actuarial loss (2)		0
Regulatory adjustment (3)		0
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.		(911,000)
Ending Balance AOCI March 31, 2018		(1,520,000)		(1,520,000)	(609,000)
Beginning Balance AOCI December 31, 2017		0
Translation adjustments		0
Other comprehensive income before reclassifications		0
FSIRS amounts reclassified from AOCI (1)		0
Amortization of prior service cost (2)		0
Amortization of net actuarial loss (2)		0
Regulatory adjustment (3)		0
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.		0	0	0		0
Reclass of stranded deferred taxes due to TCJA (4)		0
Ending Balance AOCI March 31, 2018		0		0	0
Beginning Balance AOCI December 31, 2017		(609,000)
Translation adjustments		(911,000)
Other comprehensive income before reclassifications		(911,000)	220,000	640,000		(401,000)
FSIRS amounts reclassified from AOCI (1)		0
Amortization of prior service cost (2)		0
Amortization of net actuarial loss (2)		0
Regulatory adjustment (3)		0
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.		(911,000)
Reclass of stranded deferred taxes due to TCJA (4)		0
Ending Balance AOCI March 31, 2018		(1,520,000)		(1,520,000)	(609,000)
Ending Balance AOCI March 31, 2018		(9,300,000)		(9,300,000)
Translation adjustments		(911,000)
Other comprehensive income before reclassifications		(911,000)
Beginning Balance AOCI December 31, 2017		(47,682,000)
Amounts reclassified to net income, net of tax (FSIRS)		635,000	518,000	2,190,000		2,074,000
Amortization of prior service cost		254,000	207,000	875,000		828,000
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.		619,000
Amortization of net actuarial loss		6,387,000	3,944,000	18,219,000		16,529,000
Regulatory adjustment		(5,746,000)
Ending Balance AOCI March 31, 2018		$ (56,363,000)		(56,363,000)	$ (47,682,000)
Tax Cuts and Jobs Act [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Effective income tax rate		38.00%			24.00%
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		$ 895,000
Amounts reclassified into net income		635,000
Amounts reclassified to net income, net of tax (FSIRS)		635,000
Reclass of stranded deferred taxes due to TCJA		(9,300,000)
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclass of stranded deferred taxes due to TCJA (4)		(9,300,000)
Southwest Gas Corporation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance AOCI December 31, 2017, Defined Benefit Plans Before-Tax		(61,520,000)
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax		837,000
Amortization of prior service cost, Defined Benefit Plans Before-Tax		334,000
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax		8,404,000
Net other comprehensive income (loss), Defined Benefit Plans Tax (Expense) Benefit		(283,000)
Regulatory adjustment, , Defined Benefit Plans Before-Tax		(7,560,000)
Reclass of stranded deferred taxes due to TCJA, Defined Benefit Plans Before-Tax		0
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.Defined Benefit Plans Before-Tax		1,178,000
Beginning Balance AOCI December 31, 2017		22,293,000
Ending Balance AOCI March 31, 2018, Defined Benefit Plans Before-Tax		(60,342,000)		(60,342,000)	$ (61,520,000)
Amortization of prior service cost, Defined Benefit Plans Tax (Expense) Benefit		(80,000)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit		(2,017,000)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit		1,814,000
Reclass of stranded deferred taxes due to TCJA, Defined Benefit Plans Tax (Expense) Benefit		0
Ending Balance AOCI March 31, 2018, Defined Benefit Plans Tax (Expense) Benefit		22,010,000		22,010,000	22,293,000
Beginning Balance AOCI December 31, 2017, Defined Benefit Plans After-Tax		(39,227,000)
Net other comprehensive income (loss), FSIRS - After-Tax		635,000
Amortization of prior service cost, Defined Benefit Plans After-Tax		254,000	207,000	875,000		828,000
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax		6,387,000	3,944,000	18,219,000		16,529,000
Regulatory adjustment, Defined Benefit Plans After-Tax		(5,746,000)
Net current period other comprehensive income (loss)attributable to southwest gas holdings, Inc Defined Benefit Plans Tax (Expense) Benefit		895,000
Reclass of stranded deferred taxes due to TCJA, Defined Benefit Plans After-Tax		0
Beginning Balance AOCI December 31, 2017, FSIRS - Before-Tax		(12,655,000)
Ending Balance AOCI March 31, 2018, Defined Benefit Plans After-Tax		(38,332,000)		(38,332,000)	(39,227,000)
Net other comprehensive income (loss)		1,530,000	1,113,000	(1,017,000)		2,091,000
Net other comprehensive income (loss), FSIRS - Before-Tax		837,000
Reclass of stranded deferred taxes due to TCJA, FSIRS - Before-Tax		0
Ending Balance AOCI March 31, 2018, FSIRS - Before-Tax		(11,818,000)		(11,818,000)	(12,655,000)
Beginning Balance AOCI December 31, 2017, FSIRS - Tax (Expense) Benefit		4,809,000
Net other comprehensive income (loss), FSIRS - Tax (Expense) Benefit		202,000
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit		(202,000)
Reclass of stranded deferred taxes due to TCJA ,FSIRS - Tax (Expense) Benefit		0
Ending Balance AOCI March 31, 2018, FSIRS - Tax (Expense) Benefit		4,607,000		4,607,000	4,809,000
Beginning Balance AOCI December 31, 2017		(7,846,000)
Amounts reclassified into net income		635,000	518,000	2,190,000		2,074,000
Reclass of stranded deferred taxes due to TCJA (4)		0
Ending Balance AOCI March 31, 2018		(7,211,000)		(7,211,000)	(7,846,000)
Ending Balance AOCI March 31, 2018		(9,300)		(9,300)
Beginning Balance AOCI December 31, 2017		(47,073,000)
Amounts reclassified to net income, net of tax (FSIRS)		635,000	518,000	2,190,000		2,074,000
Amortization of prior service cost		254,000	207,000	875,000		828,000
Amortization of net actuarial loss		6,387,000	$ 3,944,000	18,219,000		$ 16,529,000
Regulatory adjustment		(5,746,000)
Ending Balance AOCI March 31, 2018		$ (54,843,000)		$ (54,843,000)	$ (47,073,000)
Southwest Gas Corporation [Member] | Tax Cuts and Jobs Act [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Effective income tax rate	38.00%	24.00%
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		$ 895,000
Amounts reclassified into net income		635,000
Reclass of stranded deferred taxes due to TCJA (4)		(9,300,000)
Amounts reclassified to net income, net of tax (FSIRS)		635,000
Reclass of stranded deferred taxes due to TCJA		(9,300,000)
Southwest Gas Corporation [Member] | Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclass of stranded deferred taxes due to TCJA (4)		$ (9,300)