Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other income (deductions)
Total other income (deductions)	$ (2,706)	$ (2,810)	$ (7,040)	$ (3,800)	$ (9,270)	$ (8,401)
Southwest Gas Corporation [Member]
Other income (deductions)
Change in COLI policies	2,000	1,900	1,300	4,700	6,900	9,000
Interest income	1,377	614	2,795	1,178	4,401	2,269
Equity AFUDC	357	633	586	1,109	1,773	2,116
Other components of net periodic benefit cost	(5,264)	(4,857)	(10,529)	(9,712)	(20,241)	(19,591)
Miscellaneous income and (expense)	(564)	(1,095)	(849)	(1,324)	(1,869)	(3,637)
Total other income (deductions)	(2,094)	(2,805)	(6,697)	(4,049)	(9,036)	(9,843)
Centuri Construction Group Inc [Member]
Other income (deductions)
Interest income	1	1	2	1	4	1
Foreign transaction gain (loss)	202	(197)	349	(198)	(207)	(201)
Miscellaneous income and (expense)	(835)	190	(720)	445	(69)	1,641
Total other income (deductions)	(632)	(6)	(369)	248	(272)	1,441
Corporate and administrative	$ 20	$ 1	$ 26	$ 1	$ 38	$ 1