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Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) (Swaps [Member], USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Prepaids and other current assets [Member]
    
Derivatives, Fair Value [Line Items]    
Net Total $ 3,346swx_DerivativeFairValueOfRegulatoryAssetsLiabilitiesOffsetAmount
/ us-gaap_BalanceSheetLocationAxis
= swx_PrepaidAndOtherCurrentAssetsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 $ 5,062swx_DerivativeFairValueOfRegulatoryAssetsLiabilitiesOffsetAmount
/ us-gaap_BalanceSheetLocationAxis
= swx_PrepaidAndOtherCurrentAssetsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember
Deferred charges and other assets [Member]
    
Derivatives, Fair Value [Line Items]    
Net Total $ 223swx_DerivativeFairValueOfRegulatoryAssetsLiabilitiesOffsetAmount
/ us-gaap_BalanceSheetLocationAxis
= swx_DeferredChargesAndOtherAssetsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 $ 363swx_DerivativeFairValueOfRegulatoryAssetsLiabilitiesOffsetAmount
/ us-gaap_BalanceSheetLocationAxis
= swx_DeferredChargesAndOtherAssetsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember