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Regulatory Matters - Annual Electric Transmission Filings (Details)
$ in Millions
 12 Months Ended
May 15, 2020
USD ($)
Dec. 31, 2020
USD ($)
Commonwealth Edison Co [Member]    
Public Utilities, General Disclosures [Line Items]    
Additional basis points for RTO membership incentive   50
PECO Energy Co [Member]    
Public Utilities, General Disclosures [Line Items]    
Additional basis points for RTO membership incentive   50
Baltimore Gas and Electric Company [Member]    
Public Utilities, General Disclosures [Line Items]    
Additional basis points for RTO membership incentive 50  
Potomac Electric Power Company [Member]    
Public Utilities, General Disclosures [Line Items]    
Additional basis points for RTO membership incentive   50
Delmarva Power & Light Company [Member]    
Public Utilities, General Disclosures [Line Items]    
Additional basis points for RTO membership incentive   50
Atlantic City Electric Company [Member]    
Public Utilities, General Disclosures [Line Items]    
Additional basis points for RTO membership incentive   50
2020 Transmission Formula Rate Annual Update [Domain] | Commonwealth Edison Co [Member]    
Public Utilities, General Disclosures [Line Items]    
Public Utilities, Requested Rate Increase (Decrease), Amount   $ 18
Annual Reconciliation Increase (Decrease)   (4)
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount   $ 14
Public Utilities, Approved Equity Capital Structure, Percentage   8.17%
Public Utilities, Approved Return on Equity, Percentage   11.50%
Cap on Equity Component of Rate of Return on Common Equity in Federal Energy Regulatory Committee Complaint   55.00%
2020 Transmission Formula Rate Annual Update [Domain] | PECO Energy Co [Member]    
Public Utilities, General Disclosures [Line Items]    
Public Utilities, Requested Rate Increase (Decrease), Amount   $ 5
Annual Reconciliation Increase (Decrease)   (28)
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount   $ (23)
Public Utilities, Approved Equity Capital Structure, Percentage   7.47%
Public Utilities, Approved Return on Equity, Percentage   10.35%
Cap on Equity Component of Rate of Return on Common Equity in Federal Energy Regulatory Committee Complaint   55.75%
2020 Transmission Formula Rate Annual Update [Domain] | Baltimore Gas and Electric Company [Member]    
Public Utilities, General Disclosures [Line Items]    
Public Utilities, Requested Rate Increase (Decrease), Amount   $ 16
Annual Reconciliation Increase (Decrease)   (3)
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount   $ 4
Public Utilities, Approved Equity Capital Structure, Percentage   7.26%
Public Utilities, Approved Return on Equity, Percentage   10.50%
2020 Transmission Formula Rate Annual Update [Domain] | Potomac Electric Power Company [Member]    
Public Utilities, General Disclosures [Line Items]    
Public Utilities, Requested Rate Increase (Decrease), Amount   $ 2
Annual Reconciliation Increase (Decrease)   (46)
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount   $ (44)
Public Utilities, Approved Equity Capital Structure, Percentage   7.81%
Public Utilities, Approved Return on Equity, Percentage   10.50%
2020 Transmission Formula Rate Annual Update [Domain] | Delmarva Power & Light Company [Member]    
Public Utilities, General Disclosures [Line Items]    
Public Utilities, Requested Rate Increase (Decrease), Amount   $ (4)
Annual Reconciliation Increase (Decrease)   (40)
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount   $ (44)
Public Utilities, Approved Equity Capital Structure, Percentage   7.20%
Public Utilities, Approved Return on Equity, Percentage   10.50%
2020 Transmission Formula Rate Annual Update [Domain] | Atlantic City Electric Company [Member]    
Public Utilities, General Disclosures [Line Items]    
Public Utilities, Requested Rate Increase (Decrease), Amount   $ 5
Annual Reconciliation Increase (Decrease)   (25)
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount   $ (20)
Public Utilities, Approved Equity Capital Structure, Percentage   7.40%
Public Utilities, Approved Return on Equity, Percentage   10.50%
2020 Transmission Formula Rate Revenue Reduction [Member] | Baltimore Gas and Electric Company [Member]    
Public Utilities, General Disclosures [Line Items]    
Public Utilities, Approved Rate Increase (Decrease), Amount $ 9