v2.4.1.9
Assets and Liabilities with Recurring Fair Value Measurements (Textuals) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance, beginning of period $ 85us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue $ 14us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue    
Included in net income:        
Included in operating revenues 131cpn_FairValueAssetsMeasuredWithUnobservableInputsOnRecurringBasisGainLossIncludedInOperatingRevenue [1] (10)cpn_FairValueAssetsMeasuredWithUnobservableInputsOnRecurringBasisGainLossIncludedInOperatingRevenue [1]    
Fair Value, Assets Measured with Unobservable Inputs on Recurring Basis, Gain (Loss) Included In Fuel And Purchased Energy Expense 3cpn_FairValueAssetsMeasuredwithUnobservableInputsonRecurringBasisGainLossIncludedInFuelAndPurchasedEnergyExpense [2] 5cpn_FairValueAssetsMeasuredwithUnobservableInputsonRecurringBasisGainLossIncludedInFuelAndPurchasedEnergyExpense [2]    
Purchases, issuances and settlements:        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements (10)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements (3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements    
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount 0us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount 0us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount    
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount 0us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount 0us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount    
Transfers into level 3 (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 [3],[4] 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 [3],[4]    
Transfers out of Level 3 (7)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 [3],[5] 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 [3],[5]    
Balance, end of period 203us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue 8us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue    
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 134us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss (5)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss    
Assets and Liabilities with Recurring Fair Value Measurements (Textuals) [Abstract]        
Cash and Cash Equivalents, at Carrying Value 796us-gaap_CashAndCashEquivalentsAtCarryingValue 515us-gaap_CashAndCashEquivalentsAtCarryingValue 717us-gaap_CashAndCashEquivalentsAtCarryingValue 941us-gaap_CashAndCashEquivalentsAtCarryingValue
Restricted Cash and Cash Equivalents 209us-gaap_RestrictedCashAndCashEquivalents   244us-gaap_RestrictedCashAndCashEquivalents  
Fair Value Measurement [Domain]        
Assets and Liabilities with Recurring Fair Value Measurements (Textuals) [Abstract]        
Cash and Cash Equivalents, at Carrying Value 757us-gaap_CashAndCashEquivalentsAtCarryingValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_FairValueDisclosureItemAmountsDomain		   679us-gaap_CashAndCashEquivalentsAtCarryingValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_FairValueDisclosureItemAmountsDomain		  
Restricted Cash and Cash Equivalents $ 208us-gaap_RestrictedCashAndCashEquivalents
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_FairValueDisclosureItemAmountsDomain		   $ 217us-gaap_RestrictedCashAndCashEquivalents
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_FairValueDisclosureItemAmountsDomain		  
[1] For power contracts and other power-related products, included on our Consolidated Condensed Statements of Operations.
[2] For natural gas contracts, swaps and options, included on our Consolidated Condensed Statements of Operations.
[3] We transfer amounts among levels of the fair value hierarchy as of the end of each period. There were no transfers into or out of level 1 for each of the three months ended March 31, 2015 and 2014.
[4] We had $(1) million in losses transferred out of level 2 into level 3 for the three months ended March 31, 2015. There were no transfers out of level 2 into level 3 for the three months ended March 31, 2014.
[5] We had $(7) million in gains and $1 million in losses transferred out of level 3 into level 2 for the three months ended March 31, 2015 and 2014, respectively, due to changes in market liquidity in various power markets.