Earnings Per Share and Equity - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Statement Of Equity Line Item [Line Items]
Treasury Stock, Shares	1,832,457		1,832,457
Treasury Stock, Value	$ 123		$ 123
Employee Stock Option [Member]
Statement Of Equity Line Item [Line Items]
Stock options not included in the calculation of diluted common shares outstanding	5,000,000	9,000,000
Pepco Holdings [Member]
Statement Of Equity Line Item [Line Items]
Stock options not included in the calculation of diluted common shares outstanding	0	0