Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) - Swaps [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 5,864	$ 4,457
Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 2,397	$ 1,323

The net amount as of the balance sheet date of the fair value of derivative assets and (liabilities) that in accordance with regulatory treatment is recoverable in utility rates.

No definition available.

Name:	swx_DerivativeFairValueOfRegulatoryAssetsLiabilitiesOffsetAmount
Namespace Prefix:	swx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant