Debt and Credit Agreements - Commercial Paper Borrowings (Details) - Commercial Paper [Member] - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Outstanding Commercial Paper	$ 629	$ 89
Average Interest Rate on Commercial Paper Borrowings	2.63%	2.15%
Exelon Generation Co L L C [Member]
Short-term Debt [Line Items]
Outstanding Commercial Paper	$ 0	$ 0
Commonwealth Edison Co [Member]
Short-term Debt [Line Items]
Outstanding Commercial Paper	$ 322	$ 0
Average Interest Rate on Commercial Paper Borrowings	2.64%	2.14%
PECO Energy Co [Member]
Short-term Debt [Line Items]
Outstanding Commercial Paper	$ 0	$ 0
Baltimore Gas and Electric Company [Member]
Short-term Debt [Line Items]
Outstanding Commercial Paper	$ 106	$ 35
Average Interest Rate on Commercial Paper Borrowings	2.59%	2.18%
Pepco Holdings LLC [Member]
Short-term Debt [Line Items]
Outstanding Commercial Paper	$ 201	$ 54
Average Interest Rate on Commercial Paper Borrowings	2.62%	2.15%
Potomac Electric Power Company [Member]
Short-term Debt [Line Items]
Outstanding Commercial Paper	$ 105	$ 40
Average Interest Rate on Commercial Paper Borrowings	2.62%	2.24%
Delmarva Power and Light Company [Member]
Short-term Debt [Line Items]
Outstanding Commercial Paper	$ 5	$ 0
Average Interest Rate on Commercial Paper Borrowings	2.61%	2.07%
Atlantic City Electric Company [Member]
Short-term Debt [Line Items]
Outstanding Commercial Paper	$ 91	$ 14
Average Interest Rate on Commercial Paper Borrowings	2.62%	2.21%