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Assets and Liabilities with Recurring Fair Value Measurements Quantitative Information about Level 3 Fair Value Measurements (Details) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Quantitative Information about Level 3 fair Value Measurements [Line Items]    
Derivative, Fair Value, Net $ 271,000,000us-gaap_DerivativeFairValueOfDerivativeNet $ (144,000,000)us-gaap_DerivativeFairValueOfDerivativeNet
Physical Power [Member]    
Quantitative Information about Level 3 fair Value Measurements [Line Items]    
Derivative, Fair Value, Net 74,000,000us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember		 7,000,000us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember
Natural Gas [Member]    
Quantitative Information about Level 3 fair Value Measurements [Line Items]    
Derivative, Fair Value, Net 5,000,000us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_NaturalGasReservesMember		 0us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_NaturalGasReservesMember
Power Congestion Products [Member]    
Quantitative Information about Level 3 fair Value Measurements [Line Items]    
Derivative, Fair Value, Net 9,000,000us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PowerCongestionProductsMember		 7,000,000us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PowerCongestionProductsMember
Minimum [Member] | Physical Power [Member]    
Quantitative Information about Level 3 fair Value Measurements [Line Items]    
Fair Value Inputs Quantitative Information 14.00cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 28.92cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Minimum [Member] | Natural Gas [Member]    
Quantitative Information about Level 3 fair Value Measurements [Line Items]    
Fair Value Inputs Quantitative Information 1.00cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_NaturalGasReservesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 0cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_NaturalGasReservesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Minimum [Member] | Power Congestion Products [Member]    
Quantitative Information about Level 3 fair Value Measurements [Line Items]    
Fair Value Inputs Quantitative Information (19.56)cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PowerCongestionProductsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 (8.79)cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PowerCongestionProductsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum [Member] | Physical Power [Member]    
Quantitative Information about Level 3 fair Value Measurements [Line Items]    
Fair Value Inputs Quantitative Information 122.79cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 53.15cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PhysicalPowerMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Maximum [Member] | Natural Gas [Member]    
Quantitative Information about Level 3 fair Value Measurements [Line Items]    
Fair Value Inputs Quantitative Information 10.86cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_NaturalGasReservesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 0cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_NaturalGasReservesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Maximum [Member] | Power Congestion Products [Member]    
Quantitative Information about Level 3 fair Value Measurements [Line Items]    
Fair Value Inputs Quantitative Information $ 19.56cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PowerCongestionProductsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 $ 11.53cpn_FairValueInputsQuantitativeInformation
/ us-gaap_FairValueByLiabilityClassAxis
= cpn_PowerCongestionProductsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember