Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Parenthetical) (Detail)	9 Months Ended
Sep. 30, 2016
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%

Other comprehensive income loss effective income tax rate.

No definition available.

Name:	swx_OtherComprehensiveIncomeLossEffectiveIncomeTaxRate
Namespace Prefix:	swx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration