Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Other income (deductions)
Total other income (deductions)	$ (4,334)	$ (990)	$ (9,374)	$ (8,062)
Southwest Gas Corporation [Member]
Other income (deductions)
Change in COLI policies	(700)	2,800	6,800	9,300
Interest income	1,418	564	3,638	2,045
Equity AFUDC	229	476	2,049	2,233
Other components of net periodic benefit cost	(5,265)	(4,855)	(19,834)	(19,675)
Miscellaneous income and (expense)	(285)	(229)	(2,400)	(3,446)
Total other income (deductions)	(4,603)	(1,244)	(9,747)	(9,543)
Centuri Construction Group Inc [Member]
Other income (deductions)
Interest income	1		4	1
Foreign transaction gain (loss)	147	(1)	(606)	(13)
Miscellaneous income and (expense)	115	255	956	1,493
Total other income (deductions)	263	$ 254	354	$ 1,481
Corporate and administrative	$ 6		$ 19