Debt and Credit Agreements - Commercial Paper Borrowings (Details) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Commercial paper borrowings		$ 628	$ 427
Commonwealth Edison Co [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		320	0
PECO Energy Co [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		50	0
Baltimore Gas and Electric Company [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		136	77
Pepco Holdings LLC [Member]
Short-term Debt [Line Items]
Commercial paper borrowings	[1]	122	350
Potomac Electric Power Company [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		0	26
Delmarva Power and Light Company [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		0	216
Atlantic City Electric Company [Member]
Short-term Debt [Line Items]
Commercial paper borrowings		$ 122	$ 108

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