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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Other income (deductions)
Total other income (deductions)	$ (3,112)	$ 1,799	$ (3,975)	$ (11,046)	$ 282	$ (7,788)
Southwest Gas Corporation
Other income (deductions)
Total other income (deductions)	(4,287)	1,751	(4,902)	(10,947)	(545)	(7,615)
Revenues from external customers | Natural Gas Operations | Southwest Gas Corporation
Other income (deductions)
Change in COLI policies	0	4,500	5,800	1,000	14,000	7,200
Interest income	1,365	1,412	3,312	3,214	4,113	4,630
Equity AFUDC	0	1,232	0	3,413	1,311	4,395
Other components of net periodic benefit cost	(3,506)	(5,005)	(10,516)	(15,016)	(15,522)	(18,780)
Miscellaneous income and (expense)	(2,146)	(388)	(3,498)	(3,558)	(4,447)	(5,060)
Total other income (deductions)	(4,287)	1,751	(4,902)	(10,947)	(545)	(7,615)
Revenues from external customers | Utility Infrastructure Services Segment | Centuri
Other income (deductions)
Foreign transaction gain (loss)	(7)	0	(19)	(16)	(19)	(16)
Miscellaneous income and (expense)	1,182	48	946	(91)	846	(194)
Total other income (deductions)	1,175	48	927	(107)	827	(210)
Corporate and administrative
Other income (deductions)
Total other income (deductions)	$ 0	$ 0	$ 0	$ 8	$ 0	$ 37