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Debt and Credit Agreements - Narrative (Details)
$ in Millions
12 Months Ended
Dec. 31, 2019
USD ($)
MW
Dec. 31, 2018
USD ($)
Mar. 31, 2016
USD ($)
Debt Instrument [Line Items]      
Outstanding letters of credit $ 1,443    
Collateral amount of debt 2,800    
Long-term Line of Credit/Facility draws 0    
Line of Credit Facility, Maximum Borrowing Capacity 25    
Long-term debt $ 35,950    
Maximum [Member]      
Debt Instrument [Line Items]      
Basis points adders for prime-based borrowings 0.65%    
Basis points adders for LIBOR-based borrowings 1.65%    
Maximum [Member] | Transition Bonds [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 5.55%    
Maximum [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 7.60%    
Maximum [Member] | Junior Subordinated Debt [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 3.50%    
Minimum [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 2.45%    
Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity     $ 500
Exelon Corporate [Member]      
Debt Instrument [Line Items]      
Outstanding letters of credit $ 6 $ 9  
Long-term debt $ 7,039    
Exelon Corporate [Member] | Maximum [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 7.60%    
Exelon Corporate [Member] | Maximum [Member] | Junior Subordinated Debt [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 3.50%    
Exelon Corporate [Member] | Minimum [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 2.45%    
Exelon Corporate [Member] | Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity $ 600    
Exelon Generation Co L L C [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity 25    
Long-term debt 7,943    
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | DOE Project Financing, 2.82% January 5, 2037 [Member]      
Debt Instrument [Line Items]      
Outstanding letters of credit 38    
Non-Recourse Debt 485    
Long-term Line of Credit/Facility draws $ 646    
Debt, Weighted Average Interest Rate 2.82%    
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | SolGen [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount $ 150    
Interest rate on long-term debt 3.93%    
Non-Recourse Debt $ 131    
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | Renewable Power Generation [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount $ 150    
Interest rate on long-term debt 4.11%    
Non-Recourse Debt $ 106    
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | Continetal Wind [Member]      
Debt Instrument [Line Items]      
Aggregate Bank Commitments Under Unsecured Revolving Credit Facilities 10    
Outstanding letters of credit 115    
Debt Instrument, Face Amount $ 613    
Interest rate on long-term debt 6.00%    
Non-Recourse Debt $ 447    
Generation capacity of portfolio | MW 667    
Line of Credit Facility, Maximum Borrowing Capacity $ 131    
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | ExGenRenewablesIVNov2024 [Member]      
Debt Instrument [Line Items]      
Non-Recourse Debt 796    
Non-recourse debt, interest rate swap $ 636    
Non-recourse debt, hedge percentage 2.32%    
Long-term debt $ 850    
Proceeds from Issuance of Debt 785    
Proceeds from (Repayments of) Restricted Cash, Financing Activities $ 50    
Exelon Generation Co L L C [Member] | Maximum [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 7.60%    
Exelon Generation Co L L C [Member] | Minimum [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 2.95%    
Exelon Generation Co L L C [Member] | Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Outstanding letters of credit $ 1,400 545  
Commonwealth Edison Co [Member]      
Debt Instrument [Line Items]      
Outstanding letters of credit 2    
Long-term Line of Credit/Facility draws 0    
Long-term debt 8,791    
PECO Energy Co [Member]      
Debt Instrument [Line Items]      
Outstanding letters of credit 0    
Long-term Line of Credit/Facility draws 0    
Long-term debt 3,634    
Baltimore Gas and Electric Company [Member]      
Debt Instrument [Line Items]      
Outstanding letters of credit 0    
Long-term Line of Credit/Facility draws 0    
Long-term debt $ 3,300    
Baltimore Gas and Electric Company [Member] | Maximum [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 6.35%    
Baltimore Gas and Electric Company [Member] | Minimum [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 2.40%    
Pepco Holdings LLC [Member]      
Debt Instrument [Line Items]      
Outstanding letters of credit $ 0    
Long-term Line of Credit/Facility draws 0    
Long-term debt $ 5,995    
Pepco Holdings LLC [Member] | Maximum [Member] | Transition Bonds [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 5.55%    
Pepco Holdings LLC [Member] | Minimum [Member] | Transition Bonds [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt    
Pepco Holdings LLC [Member] | Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity $ 900 1,500  
Pepco Holdings LLC [Member] | Revolving Credit Facility [Member] | Maximum [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity   $ 900  
Potomac Electric Power Company [Member]      
Debt Instrument [Line Items]      
Outstanding letters of credit 0    
Long-term Line of Credit/Facility draws 0    
Long-term debt 2,897    
Delmarva Power & Light Company [Member]      
Debt Instrument [Line Items]      
Outstanding letters of credit 0    
Long-term Line of Credit/Facility draws 0    
Long-term debt 1,578    
Atlantic City Electric Company [Member]      
Debt Instrument [Line Items]      
Outstanding letters of credit 0    
Long-term Line of Credit/Facility draws 0    
Long-term debt $ 1,335    
Atlantic City Electric Company [Member] | Maximum [Member] | Transition Bonds [Member]      
Debt Instrument [Line Items]      
Interest rate on long-term debt 5.55%    
Corporate, Non-Segment [Member] | Exelon Corporate [Member]      
Debt Instrument [Line Items]      
Short-term Bank Loans and Notes Payable $ 500    
London Interbank Offered Rate (LIBOR) [Member] | ExGenRenewablesIVNov2024 [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Interest Rate During Period 3.00%    
Debt Instrument, Interest Rate Terms 1.00%    
London Interbank Offered Rate (LIBOR) [Member] | Corporate, Non-Segment [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Interest Rate During Period 0.95%