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Note 5 - Derivative Instruments (Details) - Derivative Instruments on the Consolidated Statements of Income (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Designated as cash flow or fair value hedges      
Income tax (expense)/benefit $ 2us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesTax $ 1us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesTax $ 3us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesTax
Total designated as cash flow or fair value hedges, net of tax 3us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax (3)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax (6)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax
Not designated as hedges (1)      
Total gains (losses) on derivative instruments, net of tax 91gas_TotalGainLossOnDerivativeInstruments (57)gas_TotalGainLossOnDerivativeInstruments 13gas_TotalGainLossOnDerivativeInstruments
Cost of Sales [Member] | Cash Flow Hedging [Member]      
Designated as cash flow or fair value hedges      
Natural gas contracts - gain reclassified from OCI 4us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember		 (1)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember		 (5)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
Cost of Sales [Member] | Fair Value Hedging [Member]      
Not designated as hedges (1)      
Natural gas contracts - net value adjustments (7)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember		 [1],[2] 2us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember		 [1],[2] (4)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember		 [1],[2]
Operating Expense [Member] | Cash Flow Hedging [Member]      
Designated as cash flow or fair value hedges      
Natural gas contracts - gain reclassified from OCI 1us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OperatingExpenseMember		    
Interest Expense [Member] | Cash Flow Hedging [Member]      
Designated as cash flow or fair value hedges      
Interest rate swaps – net loss reclassified from OCI into interest expense   (3)us-gaap_DerivativeInstrumentsLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_InterestExpenseMember		 (4)us-gaap_DerivativeInstrumentsLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_InterestExpenseMember
Sales [Member] | Fair Value Hedging [Member]      
Not designated as hedges (1)      
Natural gas contracts - net value adjustments 149us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember		 [1] (90)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember		 [1] 34us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember		 [1]
Cash Flow Hedging [Member]      
Designated as cash flow or fair value hedges      
Income tax (expense)/benefit (2)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 1us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 3us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
Total designated as cash flow or fair value hedges, net of tax 3us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 (3)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 (6)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
Fair Value Hedging [Member]      
Not designated as hedges (1)      
Income tax (expense)/benefit (54)gas_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNetTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [1] 34gas_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNetTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [1] (11)gas_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNetTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [1]
Total not designated as hedges, net of tax $ 88gas_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNetAfterTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [1] $ (54)gas_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNetAfterTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [1] $ 19gas_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNetAfterTax
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 [1]
[1] Associated with the fair value of derivative instruments held at December 31, 2014, 2013 and 2012.
[2] Excludes losses recorded in cost of goods sold associated with weather derivatives of $7 million, $5 million and $14 million for the years ended December 31, 2014, 2013 and 2012, respectively.