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Gains and Losses from Derivative Instruments (Details) (Commodity Contracts, USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Designated as Hedging Instruments | Fuel and purchased power |  |
Gains and losses from derivative instruments |  |
Loss Reclassified from Accumulated OCI into Income (Effective Portion Realized) | $ (2,343)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember | $ (4,439)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember
Not Designated as Hedging Instruments |  |
Gains and losses from derivative instruments |  |
Net Gain (Loss) Recognized in Income from Derivative Instruments | (44,851)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 18,015 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Not Designated as Hedging Instruments | Revenue |  |
Gains and losses from derivative instruments |  |
Net Gain (Loss) Recognized in Income from Derivative Instruments | (48)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesMember | (92)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesMember
Not Designated as Hedging Instruments | Fuel and purchased power |  |
Gains and losses from derivative instruments |  |
Net Gain (Loss) Recognized in Income from Derivative Instruments | (44,803)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember | 18,107 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember
Other comprehensive income | Designated as Hedging Instruments |  |
Gains and losses from derivative instruments |  |
Gain (loss) recognized in OCI on derivative instruments (effective portion) | $ (327)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember / us-gaap_OtherComprehensiveIncomeLocationAxis = us-gaap_OtherComprehensiveIncomeMember | $ 177 us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember / us-gaap_OtherComprehensiveIncomeLocationAxis = us-gaap_OtherComprehensiveIncomeMember

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- Definition The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624177-113959 + Details
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- Definition The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
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