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Nuclear Decommissioning - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Mar. 31, 2019
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Asset Retirement Obligation, Period Increase (Decrease) $ 280   $ (235)    
Asset Retirement Obligation, Revision of Estimate (300) $ 330   $ 20  
Decommissioning Fund Investments 12,706   12,706   $ 11,661
Assets held for sale 18   18   904
Exelon Generation Co L L C [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Asset Retirement Obligation, Period Increase (Decrease) 280   (235)    
Asset Retirement Obligation, Revision of Estimate (300) 330   0  
Decommissioning Fund Investments 12,706   12,706   11,661
Assets held for sale 18   18   904
Potomac Electric Power Company [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Asset Retirement Obligation, Revision of Estimate       $ 22  
Nuclear Decommissioning [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Asset Retirement Obligation, Revision of Estimate     211    
Nuclear Decommissioning [Member] | Exelon Generation Co L L C [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Asset Retirement Obligation, Revision of Estimate     211    
Three Mile Island [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Asset Retirement Obligation, Period Increase (Decrease)   (110)      
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)      
Assets, Total [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Decommissioning Fund Investments 12,862   12,862   12,695
Assets, Total [Member] | Exelon Generation Co L L C [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Decommissioning Fund Investments 12,862   12,862   12,695
Nuclear Decommissioning Trust Fund Investments [Member] | Oyster Creek [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Assets held for sale         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         890
Other Current Assets [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Decommissioning Fund Investments 156   156   144
Other Current Assets [Member] | Exelon Generation Co L L C [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Decommissioning Fund Investments 156   $ 156   $ 144
Operating Expense [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Asset Retirement Obligation, Revision of Estimate (65)        
Operating Expense [Member] | Exelon Generation Co L L C [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Asset Retirement Obligation, Revision of Estimate $ (65)        
Operating Expense [Member] | Three Mile Island [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Asset Retirement Obligation, Revision of Estimate   (85)      
Operating Expense [Member] | Three Mile Island [Member] | Exelon Generation Co L L C [Member]          
Nuclear Decommissioning Additional Narrative Information [Line Items]          
Asset Retirement Obligation, Revision of Estimate   $ (85)