Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Equity [Abstract]
Beginning Balance AOCI December 31, 2014, Defined Benefit Plans Before-Tax			$ (57,211)
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	$ 89	1,001	$ 266	$ 1,090	$ 355
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	8,595	5,914	25,786	17,742	31,700	26,285
Regulatory adjustment, Defined Benefit Plans Before-Tax	(7,787)	(5,178)	(23,361)	(15,533)	132,480	(116,412)
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	1,142	825	3,426	2,475	(15,037)	10,573
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans Before-Tax			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans Before-Tax			3,426
Ending Balance AOCI September 30, 2015, Defined Benefit Plans Before-Tax	(53,785)		(53,785)		(53,785)
Beginning Balance AOCI December 31, 2014, Defined Benefit Plans Tax (Expense) Benefit			21,740
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	(127)	(34)	(380)	(101)	(414)	(134)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	(3,266)	(2,247)	(9,799)	(6,742)	(12,046)	(9,988)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,959	1,968	8,877	5,903	(50,343)	44,236
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(434)	(313)	(1,302)	(940)	5,713	(4,017)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans Tax (Expense) Benefit			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans Tax (Expense) Benefit			(1,302)
Ending Balance AOCI September 30, 2015, Defined Benefit Plans Tax (Expense) Benefit	20,438		20,438		20,438
Beginning Balance AOCI December 31, 2014, Defined Benefit Plans After-Tax			(35,471)
Translation adjustments, Defined Benefit Plans After-Tax			0
Other comprehensive income before reclassifications, Defined Benefit Plans After-Tax			0
Amortization of prior service cost, Defined Benefit Plans After-Tax	207	55	621	165	676	221
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	5,329	3,667	15,987	11,000	19,654	16,297
Regulatory adjustment, Defined Benefit Plans After-Tax			(14,484)
Net defined benefit pension plans	708	512	2,124	1,535	(9,324)	6,556
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans After-Tax			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans After-Tax			2,124
Ending Balance AOCI September 30, 2015, Defined Benefit Plans After-Tax	(33,347)		(33,347)		(33,347)
Beginning Balance AOCI December 31, 2014, FSIRS - Before-Tax			(22,688)
Translation adjustments, FSIRS - Before-Tax			0
Other comprehensive income before reclassifications, FSIRS - Before-Tax			0
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	836	836	2,508	2,509	3,344	3,345
Net current period other comprehensive income (loss), FSIRS - Before-Tax	836	836	2,508	2,509	3,344	3,345
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - Before-Tax			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Before-Tax			2,508
Ending Balance AOCI September 30, 2015, FSIRS - Before-Tax	(20,180)		(20,180)		(20,180)
Beginning Balance AOCI December 31, 2014, FSIRS - Tax (Expense) Benefit			8,621
Translation adjustments, FSIRS - Tax (Expense) Benefit			0
Other comprehensive income before reclassifications, FSIRS - Tax (Expense) Benefit			0
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	(317)	(318)	(952)	(954)	(1,270)	(1,271)
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	(317)	(318)	(952)	(954)	(1,270)	(1,271)
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - Tax (Expense) Benefit			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Tax (Expense) Benefit			(952)
Ending Balance AOCI September 30, 2015, FSIRS - Tax (Expense) Benefit	7,669		7,669		7,669
Beginning Balance AOCI December 31, 2014, FSIRS - After-Tax			(14,067)
Translation adjustments, FSIRS - After-Tax			0
Other comprehensive income before reclassifications, FSIRS - After-Tax			0
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	519	518	1,556	1,555	2,074	2,074
Net forward-starting interest rate swaps	519	518	1,556	1,555	2,074	2,074
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - After-Tax			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - After-Tax			1,556
Ending Balance AOCI September 30, 2015, FSIRS - After-Tax	(12,511)		(12,511)		(12,511)
Beginning Balance AOCI December 31, 2014, Foreign Currency Items Before-Tax			(637)
Translation adjustments, Foreign Currency Items Before-Tax	(510)	0	(1,573)	0	(2,232)	0
Other comprehensive income before reclassifications, Foreign Currency Items Before-Tax	(510)	0	(1,573)	0	(2,232)	0
FSIRS amounts reclassified from AOCI, Foreign Currency Items Before-Tax			0
Amortization of prior service cost, Foreign Currency Items Before-Tax			0
Amortization of net actuarial loss, Foreign Currency Items Before-Tax			0
Regulatory adjustment, Foreign Currency Items Before-Tax			0
Net current period other comprehensive income (loss), Foreign Currency Items Before-Tax			(1,573)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency Items Before-Tax			(55)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items Before-Tax			(1,518)
Ending Balance AOCI September 30, 2015, Foreign Currency Items Before-Tax	(2,155)		(2,155)		(2,155)
Beginning Balance AOCI December 31, 2014, Foreign Currency items - Tax (Expense) Benefit			0
Translation adjustments, Foreign Currency items - Tax (Expense) Benefit	0	0	0	0	0	0
Other comprehensive income before reclassifications, Foreign Currency items - Tax (Expense) Benefit			0
FSIRS amounts reclassified from AOCI, Foreign Currency items - Tax (Expense) Benefit			0
Amortization of prior service cost ,Foreign Currency items - Tax (Expense) Benefit			0
Amortization of net actuarial loss, Foreign Currency items - Tax (Expense) Benefit			0
Regulatory adjustment, Foreign Currency items - Tax (Expense) Benefit			0
Net current period other comprehensive income (loss), Foreign Currency items - Tax (Expense) Benefit			0
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - Tax (Expense) Benefit			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency items - Tax (Expense) Benefit	0	0	0	0	0	0
Ending Balance AOCI September 30, 2015, Foreign Currency items - Tax (Expense) Benefit	0		0		0
Beginning Balance AOCI December 31, 2014, Foreign Currency items - After-Tax			(637)
Translation adjustments, Foreign Currency items - After-Tax	(510)	0	(1,573)	0	(2,232)	0
Other comprehensive income before reclassifications, Foreign Currency items - After-Tax	(510)	0	(1,573)	0	(2,232)	0
FSIRS amounts reclassified from AOCI, Foreign Currency items - After-Tax			0
Amortization of prior service cost, Foreign Currency items - After-Tax			0
Amortization of net actuarial loss, Foreign Currency items - After-Tax			0
Regulatory adjustment, Foreign Currency items - After-Tax			0
Net current period other comprehensive income (loss), Foreign Currency items - After-Tax			(1,573)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - After-Tax			(55)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items - After-Tax			(1,518)
Ending Balance AOCI September 30, 2015, Foreign Currency items - After-Tax	(2,155)		(2,155)		(2,155)
Beginning Balance AOCI December 31, 2014, AOCI			(50,175)
Translation adjustments, AOCI	(1,573)		(1,573)		(1,573)
Other comprehensive income before reclassifications, AOCI			(1,573)
Amounts reclassified into net income	519	518	1,556	1,555	2,074	2,074
Amortization of prior service cost, AOCI	207	55	621	165	676	221
Amortization of net actuarial loss	5,329	3,667	15,987	11,000	19,654	16,297
Regulatory adjustment, AOCI			(14,484)
Total other comprehensive income (loss), net of tax	717	$ 1,030	2,107	$ 3,090	(9,482)	$ 8,630
Less: Translation adjustment attributable to redeemable noncontrolling interest, AOCI	(55)		(55)		(55)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, AOCI	2,162		2,162		2,162
Ending Balance AOCI September 30, 2015, AOCI	$ (48,013)		$ (48,013)		$ (48,013)