CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net income	$ 21,063	$ 6,525	$ 133,949	$ 124,741	$ 225,855	$ 194,217
Defined benefit pension plans:
Net actuarial loss	0	0	0	0	(54,026)	(15,524)
Amortization of prior service cost	220	241	659	724	901	977
Amortization of net actuarial loss	7,187	4,442	21,563	13,325	26,004	19,713
Prior service cost	0	0	0	0	(1,426)	0
Regulatory adjustment	(6,380)	(4,065)	(19,140)	(12,193)	21,130	(1,214)
Net defined benefit pension plans	1,027	618	3,082	1,856	(7,417)	3,952
Forward-starting interest rate swaps (“FSIRS”):
Amounts reclassified into net income	783	635	2,054	1,906	2,689	2,540
Net forward-starting interest rate swaps	783	635	2,054	1,906	2,689	2,540
Foreign currency translation adjustments	1,024	(447)	(1,187)	1,131	(280)	(877)
Total other comprehensive income (loss), net of tax	2,834	806	3,949	4,893	(5,008)	5,615
Comprehensive income (loss)	23,897	7,331	137,898	129,634	220,847	199,832
Comprehensive income attributable to noncontrolling interest	2,790	1,172	5,169	2,523	5,357	2,695
Comprehensive income attributable to Southwest Gas Holdings, Inc.	21,107	6,159	132,729	127,111	215,490	197,137
Southwest Gas Corporation
Net income	(15,973)	(20,012)	79,568	86,746	155,993	146,287
Defined benefit pension plans:
Net actuarial loss	0	0	0	0	(54,026)	(15,524)
Amortization of prior service cost	220	241	659	724	901	977
Amortization of net actuarial loss	7,187	4,442	21,563	13,325	26,004	19,713
Prior service cost	0	0	0	0	(1,426)	0
Regulatory adjustment	(6,380)	(4,065)	(19,140)	(12,193)	21,130	(1,214)
Net defined benefit pension plans	1,027	618	3,082	1,856	(7,417)	3,952
Forward-starting interest rate swaps (“FSIRS”):
Amounts reclassified into net income	783	635	2,054	1,906	2,689	2,540
Net forward-starting interest rate swaps	783	635	2,054	1,906	2,689	2,540
Total other comprehensive income (loss), net of tax	1,810	1,253	5,136	3,762	(4,728)	6,492
Comprehensive income (loss)	$ (14,163)	$ (18,759)	$ 84,704	$ 90,508	$ 151,265	$ 152,779