Background, Organization, and Summary of Significant Accounting Policies - Other Income (Deductions) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Other income (deductions)
Total other income (deductions)	$ 7,925	$ 1,146	$ (12,845)	$ 7,985	$ (10,745)	$ (2,401)
Southwest Gas Corporation
Other income (deductions)
Total other income (deductions)	7,838	1,592	(12,698)	7,538	(10,719)	(3,005)
Operating Segments | Natural Gas Operations | Southwest Gas Corporation
Other income (deductions)
Change in COLI policies	12,000	3,400	(3,500)	11,000	2,900	6,500
Interest income	414	1,822	1,802	3,419	4,739	6,644
Equity AFUDC	1,120	1,007	2,181	1,967	4,375	5,008
Other components of net periodic benefit cost	(5,006)	(3,765)	(10,011)	(7,530)	(17,540)	(18,060)
Miscellaneous income and (expense)	(690)	(872)	(3,170)	(1,318)	(5,193)	(3,097)
Total other income (deductions)	7,838	1,592	(12,698)	7,538	(10,719)	(3,005)
Operating Segments | Utility Infrastructure Services | Centuri
Other income (deductions)
Interest income	0	0	0	0	0	86
Foreign transaction gain (loss)	(6)	21	(16)	552	(22)	(19)
Miscellaneous income and (expense)	93	(498)	(139)	(154)	(65)	462
Total other income (deductions)	87	(477)	(155)	398	(87)	529
Corporate and administrative
Other income (deductions)
Total other income (deductions)	$ 0	$ 31	$ 8	$ 49	$ 61	$ 75