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Derivatives and Fair Value Measurements - Fair Values of Swaps in Condensed Consolidated Balance Sheets (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Derivatives, Fair Value [Line Items]    
Asset derivatives not designated as hedging instruments $ 0us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue $ 0us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue
Liability derivatives not designated as hedging instruments (3,569)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsLiabilitiesAtFairValue (5,425)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsLiabilitiesAtFairValue
Net total not designated as hedging instruments (3,569)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsAtFairValueNet (5,425)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsAtFairValueNet
Swaps [Member] | Other current liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Asset derivatives not designated as hedging instruments 0us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentLiabilitiesMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 0us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentLiabilitiesMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember
Liability derivatives not designated as hedging instruments (3,346)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsLiabilitiesAtFairValue
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentLiabilitiesMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 (5,062)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsLiabilitiesAtFairValue
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentLiabilitiesMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember
Net total not designated as hedging instruments (3,346)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsAtFairValueNet
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentLiabilitiesMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 (5,062)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsAtFairValueNet
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentLiabilitiesMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember
Swaps [Member] | Other deferred credits [Member]    
Derivatives, Fair Value [Line Items]    
Asset derivatives not designated as hedging instruments 0us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue
/ us-gaap_BalanceSheetLocationAxis
= swx_OtherDeferredCreditsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 0us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue
/ us-gaap_BalanceSheetLocationAxis
= swx_OtherDeferredCreditsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember
Liability derivatives not designated as hedging instruments (223)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsLiabilitiesAtFairValue
/ us-gaap_BalanceSheetLocationAxis
= swx_OtherDeferredCreditsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 (363)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsLiabilitiesAtFairValue
/ us-gaap_BalanceSheetLocationAxis
= swx_OtherDeferredCreditsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember
Net total not designated as hedging instruments $ (223)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsAtFairValueNet
/ us-gaap_BalanceSheetLocationAxis
= swx_OtherDeferredCreditsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 $ (363)us-gaap_OtherDerivativesNotDesignatedAsHedgingInstrumentsAtFairValueNet
/ us-gaap_BalanceSheetLocationAxis
= swx_OtherDeferredCreditsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember