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Derivative Instruments (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
MWh
Dec. 31, 2014
MWh
Power [Member]    
Derivative [Line Items]    
Derivative, Nonmonetary Notional Amount, Energy Measure (79)us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= cpn_PowerMember		 (62)us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= cpn_PowerMember
Natural Gas [Member]    
Derivative [Line Items]    
Derivative, Nonmonetary Notional Amount, Energy Measure 693us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_NaturalGasReservesMember		 291us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_NaturalGasReservesMember
Environmental Credits [Member]    
Derivative [Line Items]    
Derivative, Nonmonetary Notional Amount, Mass 1us-gaap_DerivativeNonmonetaryNotionalAmountMass
/ us-gaap_DerivativeInstrumentRiskAxis
= cpn_EnvironmentalCreditsMember		 0us-gaap_DerivativeNonmonetaryNotionalAmountMass
/ us-gaap_DerivativeInstrumentRiskAxis
= cpn_EnvironmentalCreditsMember
Interest Rate Swap [Member]    
Derivative [Line Items]    
Derivative, Notional Amount 1,419invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember		 1,431invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember