Earnings Per Share Earnings Per Share - Narrative (Details) - shares shares in Millions		12 Months Ended
	Jun. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity units issued [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Stock Issued During Period, Shares, Treasury Stock Reissued	33
Pepco Holdings [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Stock options not included in the calculation of diluted common shares outstanding (less than 1 million related to PHI merger)		0	0	3
Employee Stock Option [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Stock options not included in the calculation of diluted common shares outstanding (less than 1 million related to PHI merger)		8	12	16