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Asset Retirement Obligations - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Sep. 30, 2017
Mar. 31, 2013
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Asset Retirement Obligations [Line Items]          
Increase (Decrease) in ARO Due to Early Retirement     $ 800 $ (410)  
Increase (Decrease) in ARO Due to Change in DOE Spent Fuel Acceptance Date     360    
Increase (Decrease) in ARO for Impacts of Revised Decommissioning Cost Estimates     (220)    
Asset Retirement Obligation, Revision of Estimate         $ 20
Increase (Decrease) in ARO Due to Change in Retirement Timing Probabilities       780  
Increase (Decrease) in ARO for Other Impacts       490  
Shortfall of decommissioning funds with recourse     $ 50    
Percent of additional decommissioning shortfall with recourse     5.00%    
Nuclear decommissioning trust funds     $ 14,464 13,190  
Estimated annual after tax return on nuclear decommissioning funds     2.00%    
Annual average accretion of the ARO     4.00%    
Historical five-year annual average after-tax return on NDT funds     9.00%    
Minimum [Member]          
Asset Retirement Obligations [Line Items]          
Estimated annual after tax return on nuclear decommissioning funds     5.60%    
Maximum [Member]          
Asset Retirement Obligations [Line Items]          
Estimated annual after tax return on nuclear decommissioning funds     6.10%    
Exelon Generation Co L L C [Member]          
Asset Retirement Obligations [Line Items]          
Increase (Decrease) in ARO Due to Early Retirement     $ 800 (410)  
Increase (Decrease) in ARO Due to Change in DOE Spent Fuel Acceptance Date     360    
Increase (Decrease) in ARO for Impacts of Revised Decommissioning Cost Estimates     (220)    
Asset Retirement Obligation, Revision of Estimate         0
Increase (Decrease) in ARO Due to Change in Retirement Timing Probabilities       780  
Increase (Decrease) in ARO for Other Impacts       490  
Shortfall of decommissioning funds with recourse     $ 50    
Percent of additional decommissioning shortfall with recourse     5.00%    
Nuclear decommissioning trust funds     $ 14,464 13,190  
Estimated annual after tax return on nuclear decommissioning funds     2.00%    
Number Of Years Used In Present Value Measurement     30 years    
Annual average accretion of the ARO     4.00%    
Historical five-year annual average after-tax return on NDT funds     9.00%    
Exelon Generation Co L L C [Member] | Minimum [Member]          
Asset Retirement Obligations [Line Items]          
Estimated annual after tax return on nuclear decommissioning funds     5.60%    
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     6.10%    
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
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     $ 1,022 864  
Asset Retirement Obligation     11,922 10,504 10,005
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     1,022 864  
Asset Retirement Obligation     $ 11,922 10,504 10,005
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%    
Zion Station [Member]          
Asset Retirement Obligations [Line Items]          
Asset Retirement Obligation     $ 175    
Zion Station [Member] | Exelon Generation Co L L C [Member]          
Asset Retirement Obligations [Line Items]          
Asset Retirement Obligation     175    
Nonnuclear Decommissioning Asset Retirement Obligation [Member]          
Asset Retirement Obligations [Line Items]          
Asset Retirement Obligation, Revision of Estimate     7 17  
Asset Retirement Obligation     461 460 471
Nonnuclear Decommissioning Asset Retirement Obligation [Member] | Exelon Generation Co L L C [Member]          
Asset Retirement Obligations [Line Items]          
Asset Retirement Obligation, Revision of Estimate     2 7  
Asset Retirement Obligation     212 216 238
Nonnuclear Decommissioning Asset Retirement Obligation [Member] | PECO Energy Co [Member]          
Asset Retirement Obligations [Line Items]          
Asset Retirement Obligation, Revision of Estimate     2 0  
Asset Retirement Obligation     29 28 $ 28
Nuclear Decommissioning Trust Fund Investments [Member]          
Asset Retirement Obligations [Line Items]          
Public Utilities, Approved Rate Increase (Decrease), Amount $ 4 $ 24      
Nuclear Decommissioning Trust Fund Investments [Member] | Exelon Generation Co L L C [Member]          
Asset Retirement Obligations [Line Items]          
Public Utilities, Approved Rate Increase (Decrease), Amount $ 4 $ 24      
Lessee, Operating Lease, Renewal Term 20 years        
Assets, Total [Member]          
Asset Retirement Obligations [Line Items]          
Nuclear decommissioning trust funds     14,599 13,353  
Assets, Total [Member] | Exelon Generation Co L L C [Member]          
Asset Retirement Obligations [Line Items]          
Nuclear decommissioning trust funds     14,599 13,353  
Assets, Total [Member] | Zion Station [Member]          
Asset Retirement Obligations [Line Items]          
Nuclear decommissioning trust funds     66    
Assets, Total [Member] | Zion Station [Member] | Exelon Generation Co L L C [Member]          
Asset Retirement Obligations [Line Items]          
Nuclear decommissioning trust funds     66    
Other Current Assets [Member]          
Asset Retirement Obligations [Line Items]          
Nuclear decommissioning trust funds     134 163  
Other Current Assets [Member] | Exelon Generation Co L L C [Member]          
Asset Retirement Obligations [Line Items]          
Nuclear decommissioning trust funds     134 163  
Operating Expense [Member]          
Asset Retirement Obligations [Line Items]          
Asset Retirement Obligation, Revision of Estimate     60 (150)  
Operating Expense [Member] | Exelon Generation Co L L C [Member]          
Asset Retirement Obligations [Line Items]          
Asset Retirement Obligation, Revision of Estimate     60 $ (150)  
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations [Member] | Nonnuclear Decommissioning Asset Retirement Obligation [Member]          
Asset Retirement Obligations [Line Items]          
Asset Retirement Obligation, Revision of Estimate     10    
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations [Member] | Nonnuclear Decommissioning Asset Retirement Obligation [Member] | Exelon Generation Co L L C [Member]          
Asset Retirement Obligations [Line Items]          
Asset Retirement Obligation, Revision of Estimate     $ 10