Regulatory Matters Schedule of Regulatory Asset Impairment (Details) $ in Millions	Dec. 31, 2017 USD ($)
Amount of Impairment to Carrying Amount of Regulatory Assets	$ 35	[1]
Commonwealth Edison Co [Member]
Amount of Impairment to Carrying Amount of Regulatory Assets	3
Baltimore Gas and Electric Company [Member]
Amount of Impairment to Carrying Amount of Regulatory Assets	5
Pepco Holdings LLC [Member]
Amount of Impairment to Carrying Amount of Regulatory Assets	27	[1]
Potomac Electric Power Company [Member]
Amount of Impairment to Carrying Amount of Regulatory Assets	14
Delmarva Power and Light Company [Member]
Amount of Impairment to Carrying Amount of Regulatory Assets	6
Atlantic City Electric Company [Member]
Amount of Impairment to Carrying Amount of Regulatory Assets	$ 7

[1] Exelon reflects the consolidated regulatory asset impairments of ComEd, BGE, Pepco, DPL and ACE, and PHI reflects the consolidated regulatory asset impairments of Pepco, DPL and ACE.