Background, Organization, and Summary of Significant Accounting Policies - Other Income (Deductions) (Details) - 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
Other income (deductions)
Total other income (deductions)	$ 1,799	$ (1,158)	$ (11,046)	$ 6,827	$ (7,788)	$ (4,448)
Southwest Gas Corporation
Other income (deductions)
Total other income (deductions)	1,751	(1,353)	(10,947)	6,185	(7,615)	(5,194)
Operating Segments | Natural Gas Operations | Southwest Gas Corporation
Other income (deductions)
Change in COLI policies	4,500	200	1,000	11,200	7,200	2,000
Interest income	1,412	1,521	3,214	4,940	4,630	6,659
Equity AFUDC	1,232	1,212	3,413	3,179	4,395	5,772
Other components of net periodic benefit cost	(5,005)	(3,765)	(15,016)	(11,295)	(18,780)	(16,560)
Miscellaneous income and (expense)	(388)	(521)	(3,558)	(1,839)	(5,060)	(3,065)
Total other income (deductions)	1,751	(1,353)	(10,947)	6,185	(7,615)	(5,194)
Operating Segments | Utility Infrastructure Services | Centuri
Other income (deductions)
Interest income	0	0	0	0	0	82
Foreign transaction gain (loss)	0	(6)	(16)	546	(16)	66
Miscellaneous income and (expense)	48	177	(91)	23	(194)	514
Total other income (deductions)	48	171	(107)	569	(210)	662
Corporate and administrative
Other income (deductions)
Total other income (deductions)	$ 0	$ 24	$ 8	$ 73	$ 37	$ 84