CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net income	$ 12,331	$ 10,420	$ 112,176	$ 97,546	$ 208,572	$ 163,240
Defined benefit pension plans:
Net actuarial gain (loss)	0	0	0	0	(32,701)	(14,118)
Amortization of prior service cost	254	207	762	621	969	828
Amortization of net actuarial loss	6,387	3,944	19,161	11,832	23,105	16,027
Regulatory adjustment	(5,746)	(3,555)	(17,236)	(10,667)	6,021	(2,741)
Net defined benefit pension plans	895	596	2,687	1,786	(2,606)	(4)
Forward-starting interest rate swaps:
Amounts reclassified into net income	636	518	1,907	1,554	2,426	2,073
Net forward-starting interest rate swaps	636	518	1,907	1,554	2,426	2,073
Foreign currency translation adjustments	599	1,012	(1,002)	1,861	(1,092)	1,408
Total other comprehensive income (loss), net of tax	2,130	2,126	3,592	5,201	(1,272)	3,477
Comprehensive income	14,461	12,546	115,768	102,747	207,300	166,717
Comprehensive income (loss) attributable to noncontrolling interests	0	198	(797)	181	(866)	679
Comprehensive income (loss)	14,461	12,348	116,565	102,566	208,166	166,038
Southwest Gas Corporation
Net income			79,301	82,436	153,683	148,130
Net income (loss) from continuing operations	(13,670)	(4,024)	79,301	82,436	153,683	134,323
Defined benefit pension plans:
Net actuarial gain (loss)	0	0	0	0	(32,701)	(14,118)
Amortization of prior service cost	254	207	762	621	969	828
Amortization of net actuarial loss	6,387	3,944	19,161	11,832	23,105	16,027
Regulatory adjustment	(5,746)	(3,555)	(17,236)	(10,667)	6,021	(2,741)
Net defined benefit pension plans	895	596	2,687	1,786	(2,606)	(4)
Forward-starting interest rate swaps:
Amounts reclassified into net income	636	518	1,907	1,554	2,426	2,073
Net forward-starting interest rate swaps	636	518	1,907	1,554	2,426	2,073
Total other comprehensive income (loss), net of tax	1,531	1,114	4,594	3,340	(180)	2,069
Comprehensive income	(12,139)	(2,910)	83,895	85,776	153,503	136,392
Discontinued operations-construction services, net income	0	0	0	0	0	13,293
Discontinued operations-construction services, foreign currency translation adjustments	0	0	0	0	0	(453)
Discontinued operations-construction services, comprehensive income	0	0	0	0	0	12,840
Discontinued operations-construction services, comprehensive income (loss) attributable to noncontrolling interests	0	0	0	0	0	(16)
Comprehensive income attributable to discontinued operations - construction services	0	0	0	0	0	12,856
Comprehensive income (loss)	$ (12,139)	$ (2,910)	$ 83,895	$ 85,776	$ 153,503	$ 149,248