Debt and Credit Agreements - Commercial Paper Borrowings (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2016
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 25
Average Interest Rate on Commercial Paper Borrowings	2.25%	2.15%
Letters of Credit Outstanding, Amount	$ 1,443
Credit facility agreements with minority and community banks	142	$ 135
Line of Credit Facility, Remaining Borrowing Capacity	9,116
Commercial Paper [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	9,000	9,000
Commercial Paper	$ 870	$ 89
Average Interest Rate on Commercial Paper Borrowings	2.25%	2.15%
Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity			$ 500
Exelon Corporate [Member]
Short-term Debt [Line Items]
Letters of Credit Outstanding, Amount	$ 6	$ 9
Exelon Corporate [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	600
Exelon Corporate [Member] | Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	600
Exelon Generation Co L L C [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 25
Average Interest Rate on Commercial Paper Borrowings	1.84%	1.96%
Credit facility agreements with minority and community banks	$ 44	$ 49
Exelon Generation Co L L C [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	5,300	5,300
Commercial Paper	$ 320	$ 0
Average Interest Rate on Commercial Paper Borrowings	1.84%	1.96%
Exelon Generation Co L L C [Member] | Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Letters of Credit Outstanding, Amount	$ 1,400	$ 545
Exelon Generation Co L L C [Member] | Secured Debt [Member]
Short-term Debt [Line Items]
Letters of Credit Outstanding, Amount	159
Exelon Generation Co L L C [Member] | Letter of Credit [Member]
Short-term Debt [Line Items]
Letters of Credit Outstanding, Amount	$ 5
Commonwealth Edison Co [Member]
Short-term Debt [Line Items]
Average Interest Rate on Commercial Paper Borrowings	2.38%	2.14%
Letters of Credit Outstanding, Amount	$ 2
Credit facility agreements with minority and community banks	33	$ 33
Line of Credit Facility, Remaining Borrowing Capacity	998
Commonwealth Edison Co [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	1,000	1,000
Commercial Paper	$ 130	$ 0
Average Interest Rate on Commercial Paper Borrowings	2.38%	2.14%
Commonwealth Edison Co [Member] | Letter of Credit [Member]
Short-term Debt [Line Items]
Letters of Credit Outstanding, Amount	$ 5
PECO Energy Co [Member]
Short-term Debt [Line Items]
Average Interest Rate on Commercial Paper Borrowings	2.39%	2.24%
Letters of Credit Outstanding, Amount	$ 0
Credit facility agreements with minority and community banks	33	$ 34
Line of Credit Facility, Remaining Borrowing Capacity	600
PECO Energy Co [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	600	600
Commercial Paper	$ 0	$ 0
Average Interest Rate on Commercial Paper Borrowings	2.39%	2.24%
Baltimore Gas and Electric Company [Member]
Short-term Debt [Line Items]
Average Interest Rate on Commercial Paper Borrowings	2.46%	2.18%
Letters of Credit Outstanding, Amount	$ 0
Credit facility agreements with minority and community banks	8	$ 5
Line of Credit Facility, Remaining Borrowing Capacity	600
Baltimore Gas and Electric Company [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	600	600
Commercial Paper	$ 76	$ 35
Average Interest Rate on Commercial Paper Borrowings	2.46%	2.18%
Baltimore Gas and Electric Company [Member] | Letter of Credit [Member]
Short-term Debt [Line Items]
Letters of Credit Outstanding, Amount	$ 2
Pepco Holdings LLC [Member]
Short-term Debt [Line Items]
Letters of Credit Outstanding, Amount	0
Line of Credit Facility, Remaining Borrowing Capacity	900
Pepco Holdings LLC [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	900	$ 900
Commercial Paper	208	54
Pepco Holdings LLC [Member] | Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 900	$ 1,500
Exelon Corporate [Member]
Short-term Debt [Line Items]
Average Interest Rate on Commercial Paper Borrowings	1.92%	1.93%
Exelon Corporate [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Commercial Paper	$ 136
Potomac Electric Power Company [Member]
Short-term Debt [Line Items]
Average Interest Rate on Commercial Paper Borrowings	2.56%	2.24%
Letters of Credit Outstanding, Amount	$ 0
Credit facility agreements with minority and community banks	8	$ 5
Line of Credit Facility, Remaining Borrowing Capacity	300
Potomac Electric Power Company [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	300	300
Commercial Paper	$ 82	$ 40
Average Interest Rate on Commercial Paper Borrowings	2.56%	2.24%
Line of Credit Facility, Remaining Borrowing Capacity	$ 500
Delmarva Power & Light Company [Member]
Short-term Debt [Line Items]
Average Interest Rate on Commercial Paper Borrowings	2.02%	2.07%
Letters of Credit Outstanding, Amount	$ 0
Credit facility agreements with minority and community banks	8	$ 5
Line of Credit Facility, Remaining Borrowing Capacity	300
Delmarva Power & Light Company [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	300	300
Commercial Paper	$ 56	$ 0
Average Interest Rate on Commercial Paper Borrowings	2.02%	2.07%
Line of Credit Facility, Remaining Borrowing Capacity	$ 500
Atlantic City Electric Company [Member]
Short-term Debt [Line Items]
Average Interest Rate on Commercial Paper Borrowings	2.43%	2.21%
Letters of Credit Outstanding, Amount	$ 0
Credit facility agreements with minority and community banks	8	$ 5
Line of Credit Facility, Remaining Borrowing Capacity	300
Atlantic City Electric Company [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	300	300
Commercial Paper	$ 70	$ 14
Average Interest Rate on Commercial Paper Borrowings	2.43%	2.21%
Line of Credit Facility, Remaining Borrowing Capacity	$ 350
Maximum [Member] | Pepco Holdings LLC [Member] | Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity		$ 900