Mergers, Acquisitions, and Dispositions Merger Transaction Table (Details) - USD ($) $ in Millions			9 Months Ended
		Mar. 23, 2016	Sep. 30, 2017	Sep. 30, 2016
Payments to Acquire Businesses, Gross			$ 208	$ 6,896
Pepco Holdings LLC [Member]
Payments to Acquire Businesses, Gross		$ 7,142
Pepco Holdings LLC [Member] | Common Stock [Member]
Payments to Acquire Businesses, Gross		6,933
Pepco Holdings LLC [Member] | Preferred Stock [Member]
Payments to Acquire Businesses, Gross		180
Pepco Holdings LLC [Member] | Deferred Compensation, Share-based Payments [Member]
Payments to Acquire Businesses, Gross	[1]	$ 29

[1] PHI’s unvested time-based restricted stock units and performance-based restricted stock units issued prior to April 29, 2014 were immediately vested and paid in cash upon the close of the merger. PHI’s remaining unvested time-based restricted stock units as of the close of the merger were cancelled. There were no remaining unvested performance-based restricted stock units as of the close of the merger.