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Nuclear Decommissioning - Rollforward of Nuclear Decommissioning ARO (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2017
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]          
Asset Retirement Obligation, Accretion Expense [1]     $ 350 $ 333  
Asset Retirement Obligation, Period Increase (Decrease)       0  
Exelon Generation Co L L C [Member]          
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]          
Asset Retirement Obligation, Accretion Expense [1]     350 332  
Asset Retirement Obligation, Period Increase (Decrease)     632    
Asset Retirement Obligation, Period Increase (Decrease)       $ 0  
Recovery Stranded Costs $ 4       $ 24
Current portion of ARO         $ 10
Exelon Generation Co L L C [Member] | Nuclear Decommissioning [Member]          
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]          
Asset Retirement Obligation, Accretion Expense     342    
Asset Retirement Obligation, Period Increase (Decrease)     (148)    
Asset Retirement Obligation, Liabilities Settled     (6)    
Asset Retirement Obligation, Revision of Estimate   $ 417 444    
Current portion of ARO     $ 12    
[1] Included in Operating and maintenance expense on the Registrants’ Consolidated Statements of Operations and Comprehensive Income