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Nuclear Decommissioning - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Mar. 31, 2019
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2018
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Asset Retirement Obligation, Period Increase (Decrease) $ 324      
Asset Retirement Obligation, Revision of Estimate 330   $ 84  
Decommissioning Fund Investments 12,302     $ 11,661
Assets held for sale 890     904
Exelon Generation Co L L C [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Asset Retirement Obligation, Period Increase (Decrease) 324      
Asset Retirement Obligation, Revision of Estimate 330      
Decommissioning Fund Investments 12,302     11,661
Assets held for sale 890     904
Nuclear Decommissioning [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Asset Retirement Obligation, Revision of Estimate 223      
Nuclear Decommissioning [Member] | Exelon Generation Co L L C [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Asset Retirement Obligation, Revision of Estimate 223      
Oyster Creek [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Assets held for sale 888     897
Oyster Creek [Member] | Exelon Generation Co L L C [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Asset Retirement Obligation Revision Of Estimate In Operating And Maintenance Expense   $ (84)    
Assets held for sale 888     897
Three Mile Island [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Asset Retirement Obligation, Period Increase (Decrease) (110)      
Asset Retirement Obligation Revision Of Estimate In Operating And Maintenance Expense (85)      
Three Mile Island [Member] | Exelon Generation Co L L C [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Asset Retirement Obligation, Period Increase (Decrease) (110)      
Asset Retirement Obligation Revision Of Estimate In Operating And Maintenance Expense (85)      
Assets, Total [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Decommissioning Fund Investments 13,345     12,695
Assets, Total [Member] | Exelon Generation Co L L C [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Decommissioning Fund Investments 13,345     12,695
Nuclear Decommissioning Trust Fund Investments [Member] | Oyster Creek [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Assets held for sale 881     890
Nuclear Decommissioning Trust Fund Investments [Member] | Oyster Creek [Member] | Exelon Generation Co L L C [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Assets held for sale 881     890
Other Current Assets [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Decommissioning Fund Investments 163     144
Other Current Assets [Member] | Exelon Generation Co L L C [Member]        
Nuclear Decommissioning Additional Narrative Information [Line Items]        
Decommissioning Fund Investments $ 163     $ 144