Stock-Based Compensation Plans - Narrative (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Common Stock Narrative Information [Line Items]
Obligations of outstanding restricted stock not settled	$ 83		$ 108
Settlement of restricted stock	106		$ 88	$ 68
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 33
Weighted Average Period Non Vested Performance Share Awards Are Expected To Be Recognized Over	1 year 8 months
Performance Shares [Member]
Common Stock Narrative Information [Line Items]
Granted, Weighted Average Grant Date Fair Value (in usd per share)	$ 38.15	[1]	$ 35.00	$ 28.85
Percentage to be settled as common stock	50.00%
Percentage to be settled as cash	50.00%
Percentage to be settled as cash if ownership requirements are met	100.00%
Performance Shares [Member] | Pepco Holdings LLC [Member]
Common Stock Narrative Information [Line Items]
Granted, Weighted Average Grant Date Fair Value (in usd per share)				26.10
Employee Stock Option [Member]
Common Stock Narrative Information [Line Items]
Requisite service period	4 years
Restricted Stock [Member]
Common Stock Narrative Information [Line Items]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	1 year 8 months 12 days
Restricted Stock Units (RSUs) [Member]
Common Stock Narrative Information [Line Items]
Granted, Weighted Average Grant Date Fair Value (in usd per share)	$ 38.60		$ 34.98	$ 28.14
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 38
Minimum [Member] | Restricted Stock [Member]
Common Stock Narrative Information [Line Items]
Requisite service period	3 years
Maximum [Member] | Restricted Stock [Member]
Common Stock Narrative Information [Line Items]
Requisite service period	5 years
LTIP [Member]
Common Stock Narrative Information [Line Items]
Shares authorized	11

[1] (a)As of December 31, 2018, $33 million of total unrecognized compensation costs related to nonvested performance shares are expected to be recognized over the remaining weighted-average period of 1.7 years.