Mergers, Acquisitions and Dispositions Merger Transaction Table (Details) - USD ($) $ / shares in Units, $ in Millions		9 Months Ended	12 Months Ended
	Mar. 23, 2016	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition, Share Price	$ 27.25
Payments to Acquire Businesses, Gross			$ 208	$ 6,934	$ 40
Preferred Stock [Member]
Payments to Acquire Businesses, Gross			$ 180
Pepco Holdings [Member]
Payments to Acquire Businesses, Gross	$ 7,142
Pepco Holdings LLC [Member] | Preferred Stock [Member]
Payments to Acquire Businesses, Gross	180
Pepco Holdings LLC [Member] | Common Stock [Member]
Payments to Acquire Businesses, Gross	$ 6,933
Pepco Holdings LLC [Member] | Pepco Holdings LLC [Member] | Deferred Compensation, Share-based Payments [Member]
Stockholders' Equity, Other		$ 29