Revenue - Schedule of Gas Operating Revenue (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Other revenues	$ (11,454)	$ 1,194	$ (8,568)	$ 2,998
Tax Reform Savings Revenue Adjustments [Member] | Southwest Gas Holdings, Inc. [Member]
Disaggregation of Revenue [Line Items]
Other revenues	$ 14,000		$ 14,000