Debt and Credit Agreements - Commercial Paper Borrowings (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Commercial paper borrowings		$ 1,154	$ 427
Exelon Generation Co L L C [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		165	0
Commonwealth Edison Co [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		317	0
PECO Energy Co [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		220	0
Baltimore Gas and Electric Company [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		45	77
Pepco Holdings LLC [Member]
Short-term Debt [Line Items]
Commercial paper borrowings	[1]	407	350
Potomac Electric Power Company [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		60	26
Delmarva Power and Light Company [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		211	216
Atlantic City Electric Company [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		$ 136	$ 108

[1] PHI reflects the commercial paper borrowings outstanding of Pepco, DPL and ACE.