Mergers, Acquisitions and Dispositions Merger Transaction Table (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
	Mar. 23, 2016	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Payments to Acquire Businesses, Gross			$ 154	$ 208	$ 6,923
Pepco Holdings [Member]
Payments to Acquire Businesses, Gross	$ 7,142
Pepco Holdings LLC [Member] | Pepco Holdings LLC [Member] | Deferred Compensation, Share-based Payments [Member]
Stockholders' Equity, Other		$ 29