Background, Organization, and Summary of Significant Accounting Policies- Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Other income (deductions)
Total other income (deductions)	$ 1,146	$ (2,706)	$ 7,985	$ (7,040)	$ (2,401)	$ (9,270)
Southwest Gas Corporation
Other income (deductions)
Total other income (deductions)	1,592	(2,094)	7,538	(6,697)	(3,005)	(9,036)
Operating Segments | Natural Gas Operations | Southwest Gas Corporation
Other income (deductions)
Change in COLI policies	3,400	2,000	11,000	1,300	6,500	6,900
Interest income	1,822	1,377	3,419	2,795	6,644	4,401
Equity AFUDC	1,007	357	1,967	586	5,008	1,773
Other components of net periodic benefit cost	(3,765)	(5,264)	(7,530)	(10,529)	(18,060)	(20,241)
Miscellaneous income and (expense)	(872)	(564)	(1,318)	(849)	(3,097)	(1,869)
Total other income (deductions)	1,592	(2,094)	7,538	(6,697)	(3,005)	(9,036)
Operating Segments | Utility Infrastructure Services | Centuri
Other income (deductions)
Interest income	0	1	0	2	86	4
Foreign transaction gain (loss)	21	202	552	349	(19)	(207)
Miscellaneous income and (expense)	(498)	(835)	(154)	(720)	462	(69)
Total other income (deductions)	(477)	(632)	398	(369)	529	(272)
Corporate and administrative
Other income (deductions)
Total other income (deductions)	$ 31	$ 20	$ 49	$ 26	$ 75	$ 38