Regulatory Matters Regulatory Matters - Schedule of Regulatory Asset Impairment (Details) $ in Millions	Dec. 31, 2017 USD ($)
Amount of Impairment to Carrying Amount of Regulatory Assets	$ 35
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
Potomac Electric Power Company [Member]
Amount of Impairment to Carrying Amount of Regulatory Assets	14
Delmarva Power & 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