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Asset Retirement Obligations - Unrealized Gain on NDT Funds (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Asset Retirement Obligations [Line Items]      
Net unrealized gains (losses) on decommissioning trust funds - Regulatory Agreement Units $ 180exc_UnrealizedGainLossInvestmentIncomeNuclearDecommissioningTrustFundsRegulatoryAccounting [1] $ 406exc_UnrealizedGainLossInvestmentIncomeNuclearDecommissioningTrustFundsRegulatoryAccounting [1] $ 386exc_UnrealizedGainLossInvestmentIncomeNuclearDecommissioningTrustFundsRegulatoryAccounting [1]
Net unrealized gains (losses) on decommissioning trust funds - Non-Regulatory Agreement Units 134exc_UnrealizedGainLossInvestmentIncomeNuclearDecommissioningTrustFundsNonregulatoryAccounting [2],[3] 146exc_UnrealizedGainLossInvestmentIncomeNuclearDecommissioningTrustFundsNonregulatoryAccounting [2],[3] 105exc_UnrealizedGainLossInvestmentIncomeNuclearDecommissioningTrustFundsNonregulatoryAccounting [2],[3]
Exelon Generation Co L L C [Member]      
Asset Retirement Obligations [Line Items]      
Unrealized Gain Loss Investment Income Pledged Assets $ 29exc_UnrealizedGainLossInvestmentIncomePledgedAssets
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember		 $ 7exc_UnrealizedGainLossInvestmentIncomePledgedAssets
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember		 $ 73exc_UnrealizedGainLossInvestmentIncomePledgedAssets
/ dei_LegalEntityAxis
= exc_ExelonGenerationCoLLCMember
[1] Net unrealized gains related to Generation’s NDT funds associated with Regulatory Agreement Units are included in Regulatory liabilities on Exelon’s Consolidated Balance Sheets and Noncurrent payables to affiliates on Generation’s Consolidated Balance Sheets.
[2] Excludes $29 million, $7 million and $73 million of net unrealized gains related to the Zion Station pledged assets in 2014, 2013 and 2012, respectively. Net unrealized gains related to Zion Station pledged assets are included in the Payable for Zion Station decommissioning on Exelon’s and Generation’s Consolidated Balance Sheets.
[3] Net unrealized gains related to Generation’s NDT funds with Non-Regulatory Agreement Units are included within Other, net in Exelon’s and Generation’s Consolidated Statements of Operations and Comprehensive Income.