Stock-Based Compensation Plans - Summary of Nonvested Performance Share Awards Activity (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options		$ 41
Weighted Average Period Non Vested Performance Share Awards Are Expected To Be Recognized Over		1 year 6 months
Fair Value Of Settled Performance Share Awards		$ 72	[1]	$ 45		$ 46
Performance Share Awards Obligations Settled In Cash		$ 56	[1]	$ 28		$ 29
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning Balance	[2],[3]	3,116,261
Granted		1,632,186
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Period Increase (Decrease)		545,793
Vested		(1,111,751)
Forfeited		(18,034)
Undistributed vested awards	[4]	(1,207,489)
Ending Balance	[3]	2,956,966		3,116,261	[2]
Nonvested, Weighted Average Exercise Price (in usd per share)	[3]	$ 32.65		$ 30.77	[2]
Granted, Weighted Average Grant Date Fair Value (in usd per share)		35.00	[1]	$ 28.85		$ 35.88
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Change in Performance, Weighted Average Grant Date Fair Value		30.97
Vested, Weighted Average Exercise Price (in usd per share)		29.11
Forfeited, Weighted Average Exercise Price (in usd per share)		33.74
Undistributed Vested Awards, Weighted Average Grant Date Fair Value (in usd per share)	[4]	$ 33.46
Fully Vested Stock Based Compensation Issued To Retirement Eligible Employees		2,723,440		2,443,409
Performance Shares [Member] | Pepco Holdings LLC [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted, Weighted Average Grant Date Fair Value (in usd per share)						$ 26.10

[1] (a)As of December 31, 2017, $41 million of total unrecognized compensation costs related to nonvested performance shares are expected to be recognized over the remaining weighted-average period of 1.5 years.

[2] (c)2016 amounts include activity related to stock-based compensation granted to eligible PHI employees since the merger date of March 23, 2016.

[3] Excludes 2,723,440 and 2,443,409 of performance share awards issued to retirement-eligible employees as of December 31, 2017 and 2016, respectively, as they are fully vested.

[4] Represents performance share awards that vested but were not distributed to retirement-eligible employees during 2017.