Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 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	333	89	599	355
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	8,596	5,914	26,338	31,546
Regulatory adjustment, Defined Benefit Plans Before-Tax	(7,787)	(5,178)	137,699	(121,224)
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	1,142	825	(15,671)	11,022
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	1,142
Ending Balance AOCI March 31, 2015, Defined Benefit Plans Before-Tax	(56,069)		(56,069)
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)	(228)	(135)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	(3,266)	(2,247)	(10,008)	(11,987)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,959	1,968	(52,326)	46,065
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(434)	(313)	5,954	(4,188)
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	(434)
Ending Balance AOCI March 31, 2015 Defined Benefit Plans Tax (Expense) Benefit	21,306		21,306
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	206	55	371	220
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	5,330	3,667	16,330	19,559
Regulatory adjustment, Defined Benefit Plans After-Tax	(4,828)
Net defined benefit pension plans	708	512	(9,717)	6,834
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	708
Ending Balance AOCI March 31, 2015, Defined Benefit Plans After-Tax	(34,763)		(34,763)
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	3,345	3,345
Net current period other comprehensive income (loss), FSIRS - Before-Tax	836	836	3,345	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	836
Ending Balance AOCI March 31, 2015, FSIRS - Before-Tax	(21,852)		(21,852)
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)	(1,271)	(1,271)
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	(317)	(318)	(1,271)	(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	(317)
Ending Balance AOCI March 31, 2015, FSIRS - Tax (Expense) Benefit	8,304		8,304
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	2,074	2,074
Net forward-starting interest rate swaps	519	518	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	519
Ending Balance AOCI March 31, 2015, FSIRS - After-Tax	(13,548)		(13,548)
Beginning Balance AOCI December 31, 2014,Foreign Currency Items Before-Tax	(637)
Translation adjustments, Foreign Currency Items Before-Tax	(1,272)		(1,931)
Other comprehensive income before reclassifications, Foreign Currency Items Before-Tax	(1,272)		(1,931)
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	(1,272)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency Items Before-Tax	(43)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items Before-Tax	(1,229)
Ending Balance AOCI March 31, 2015, Foreign Currency Items Before-Tax	(1,866)		(1,866)
Beginning Balance AOCI December 31, 2013,Foreign Currency items - Tax (Expense) Benefit	0
Translation adjustments,Foreign Currency items - Tax (Expense) Benefit	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
Ending Balance AOCI December 31, 2014,Foreign Currency items - Tax (Expense) Benefit	0		0
Beginning Balance AOCI December 31, 2013, Foreign Currency items - After-Tax	(637)
Translation adjustments, Foreign Currency items - After-Tax	(1,272)		(1,931)
Other comprehensive income before reclassifications, Foreign Currency items - After-Tax	(1,272)		(1,931)
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	(1,272)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - After-Tax	(43)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items - After-Tax	(1,229)
Ending Balance AOCI December 31, 2014, Foreign Currency items - After-Tax	(1,866)		(1,866)
Beginning Balance AOCI December 31, 2014, AOCI	(50,175)
Translation adjustments, AOCI	(1,272)		(1,272)
Other comprehensive income before reclassifications, AOCI	(1,272)
Amounts reclassified into net income	519	518	2,074	2,074
Amortization of prior service cost, AOCI	206	55	371	220
Amortization of net actuarial loss	5,330	3,667	16,330	19,559
Regulatory adjustment, AOCI	(4,828)
Total other comprehensive income (loss), net of tax	(45)	1,030	(9,574)	8,908
Less: Translation adjustment attributable to redeemable noncontrolling interest, AOCI	(43)		(43)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, AOCI	(2)		(2)
Ending Balance AOCI March 31, 2015, AOCI	$ (50,177)		$ (50,177)