Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Mar. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 2,715	$ 4,514

The cash outflow for derivative instruments during the period, which are classified as operating activities, excluding those designated as hedging instruments.

No definition available.

Name:	swx_PaymentsForDerivativeInstrumentOperatingActivities
Namespace Prefix:	swx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration