Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) - Swaps [Member] - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Prepaids and other current assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 3,425	$ 5,062
Deferred charges and other assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 1,462	$ 363

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