Mergers, Acquisitions, and Dispositions Merger Transaction Table (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016	Mar. 24, 2016
Payments to Acquire Businesses, Gross		$ 212	$ 6,645
Pepco Holdings LLC [Member]
Business Acquisition, Share Price				$ 27.25
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	$ 29