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Asset Retirement Obligations - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 05, 2019
Jun. 27, 2017
Sep. 30, 2017
Mar. 31, 2013
Dec. 31, 2019
Dec. 31, 2018
Asset Retirement Obligations [Line Items]            
Increase (Decrease) in ARO Due to Change in Retirement Timing Probabilities         $ 780  
Increase (Decrease) in ARO for Other Impacts         490 $ 120
Increase (Decrease) in ARO for Impacts of Revised Decommissioning Cost Estimates           (205)
Asset Retirement Obligation, Revision of Estimate           20
Public Utilities, Approved Rate Increase (Decrease), Amount     $ 4 $ 24    
Shortfall of decommissioning funds with recourse         $ 50  
Percent of additional decommissioning shortfall with recourse         5.00%  
Nuclear decommissioning trust funds         $ 13,190 11,661
Disposal Group, Including Discontinued Operation, Assets, Current         0 904
Line of credit facility, maximum borrowing capacity         $ 25  
Estimated annual after tax return on nuclear decommissioning funds         2.00%  
Historical five-year annual average after-tax return on NDT funds         6.70%  
Annual average accretion of the ARO         5.00%  
Minimum [Member]            
Asset Retirement Obligations [Line Items]            
Estimated annual after tax return on nuclear decommissioning funds         5.40%  
Maximum [Member]            
Asset Retirement Obligations [Line Items]            
Estimated annual after tax return on nuclear decommissioning funds         6.50%  
Exelon Generation Co L L C [Member]            
Asset Retirement Obligations [Line Items]            
Increase (Decrease) in ARO Due to Change in Retirement Timing Probabilities         $ 780  
Increase (Decrease) in ARO for Other Impacts         490 120
Increase (Decrease) in ARO for Impacts of Revised Decommissioning Cost Estimates           (205)
Asset Retirement Obligation, Revision of Estimate           0
Public Utilities, Approved Rate Increase (Decrease), Amount     $ 4 $ 24    
Shortfall of decommissioning funds with recourse         $ 50  
Percent of additional decommissioning shortfall with recourse         5.00%  
Nuclear decommissioning trust funds         $ 13,190 11,661
Disposal Group, Including Discontinued Operation, Assets, Current         0 904
Line of credit facility, maximum borrowing capacity         $ 25  
Estimated annual after tax return on nuclear decommissioning funds         2.00%  
Historical five-year annual average after-tax return on NDT funds         6.70%  
Annual average accretion of the ARO         5.00%  
Number Of Years Used In Present Value Measurement         30 years  
Exelon Generation Co L L C [Member] | Minimum [Member]            
Asset Retirement Obligations [Line Items]            
Estimated annual after tax return on nuclear decommissioning funds         5.40%  
Number Of Years Used In Present Value Measurement         10 years  
Exelon Generation Co L L C [Member] | Maximum [Member]            
Asset Retirement Obligations [Line Items]            
Estimated annual after tax return on nuclear decommissioning funds         6.50%  
Number Of Years Used In Present Value Measurement         70 years  
PECO Energy Co [Member]            
Asset Retirement Obligations [Line Items]            
Asset Retirement Obligation, Revision of Estimate           0
Public Utilities, Approved Rate Increase (Decrease), Amount $ 14 $ 22        
Estimated annual after tax return on nuclear decommissioning funds         3.00%  
Nuclear decommissioning            
Asset Retirement Obligations [Line Items]            
Net increase/decrease due to changes in, and timing of, estimated future cash flows         $ 864 (77)
Increase (Decrease) in ARO for Impacts of Revised Decommissioning Cost Estimates         (410)  
Nuclear decommissioning | Exelon Generation Co L L C [Member]            
Asset Retirement Obligations [Line Items]            
Net increase/decrease due to changes in, and timing of, estimated future cash flows         864 (77)
Increase (Decrease) in ARO for Impacts of Revised Decommissioning Cost Estimates         $ (410)  
Oyster Creek [Member]            
Asset Retirement Obligations [Line Items]            
Increase (Decrease) in ARO Due to Early Retirement           115
Oyster Creek [Member] | Exelon Generation Co L L C [Member]            
Asset Retirement Obligations [Line Items]            
Increase (Decrease) in ARO Due to Early Retirement           115
Nine Mile Point [Member]            
Asset Retirement Obligations [Line Items]            
Percent of additional decommissioning shortfall with recourse         50.00%  
Nine Mile Point [Member] | Exelon Generation Co L L C [Member]            
Asset Retirement Obligations [Line Items]            
Percent of additional decommissioning shortfall with recourse         50.00%  
Assets, Total [Member]            
Asset Retirement Obligations [Line Items]            
Nuclear decommissioning trust funds         $ 13,353 12,695
Assets, Total [Member] | Exelon Generation Co L L C [Member]            
Asset Retirement Obligations [Line Items]            
Nuclear decommissioning trust funds         13,353 12,695
Nuclear Decommissioning Trust Fund Investments [Member] | Exelon Generation Co L L C [Member]            
Asset Retirement Obligations [Line Items]            
Lessee, Operating Lease, Renewal Term     20 years      
Nuclear Decommissioning Trust Fund Investments [Member] | Oyster Creek [Member]            
Asset Retirement Obligations [Line Items]            
Disposal Group, Including Discontinued Operation, Assets, Current           890
Nuclear Decommissioning Trust Fund Investments [Member] | Oyster Creek [Member] | Exelon Generation Co L L C [Member]            
Asset Retirement Obligations [Line Items]            
Disposal Group, Including Discontinued Operation, Assets, Current           890
Other Current Assets [Member]            
Asset Retirement Obligations [Line Items]            
Nuclear decommissioning trust funds         163 144
Other Current Assets [Member] | Exelon Generation Co L L C [Member]            
Asset Retirement Obligations [Line Items]            
Nuclear decommissioning trust funds         163 $ 144
Operating Expense [Member]            
Asset Retirement Obligations [Line Items]            
Asset Retirement Obligation, Revision of Estimate         (150)  
Operating Expense [Member] | Exelon Generation Co L L C [Member]            
Asset Retirement Obligations [Line Items]            
Asset Retirement Obligation, Revision of Estimate         $ (150)