Nature of Operations and Basis of Presentation - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Other property	$ 13,218	$ 13,242
Total	442,771	428,180
Southwest Gas Corporation [Member]
Investment [Line Items]
Net cash surrender value of COLI policies	116,690	117,341
Other property	1,759	1,773
Total	118,449	119,114
Centuri Construction Group Inc [Member]
Investment [Line Items]
Centuri property, equipment, and intangibles	579,110	554,730
Centuri accumulated provision for depreciation and amortization	$ (268,006)	$ (258,906)