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Assets and Liabilities with Recurring Fair Value Measurements Quantitative Info on Level 3 (Details) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Fair Value, Net $ 354,000,000us-gaap_DerivativeFairValueOfDerivativeNet $ 271,000,000us-gaap_DerivativeFairValueOfDerivativeNet
Power Contracts [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Fair Value, Net 201,000,000us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember
74,000,000us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember
Power Contracts [Member] | Minimum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs Quantitative Information 13.43cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
14.00cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Power Contracts [Member] | Maximum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs Quantitative Information 123.25cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
122.79cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Natural Gas [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Fair Value, Net 0us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_NaturalGasReservesMember
5,000,000us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_NaturalGasReservesMember
Natural Gas [Member] | Minimum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs Quantitative Information   1.00cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_NaturalGasReservesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Natural Gas [Member] | Maximum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs Quantitative Information   10.86cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_NaturalGasReservesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Power Congestion Products [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Fair Value, Net 0us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PowerCongestionProductsMember
9,000,000us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PowerCongestionProductsMember
Power Congestion Products [Member] | Minimum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs Quantitative Information   (19.56)cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PowerCongestionProductsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Power Congestion Products [Member] | Maximum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Inputs Quantitative Information   $ 19.56cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PowerCongestionProductsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember