Stock-Based Compensation Plans - Summary of Nonvested Restricted Stock Unit Activity (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares		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]
Beginning Balance	[1],[2]	3,824,416
Granted		2,266,199
Vested		(1,736,965)
Forfeited		(92,938)
Undistributed vested awards	[3]	(871,209)
Ending Balance	[2]	3,389,503	3,824,416	[1]
Nonvested, Weighted Average Exercise Price (in usd per share)	[1],[2]	$ 30.49
Granted, Weighted Average Grant Date Fair Value (in usd per share)		34.98	$ 28.14		$ 36.55
Vested, Weighted Average Exercise Price (in usd per share)		30.98
Forfeited, Weighted Average Exercise Price (in usd per share)		33.12
Undistributed Vested Awards, Weighted Average Grant Date Fair Value (in usd per share)	[3]	34.09
Nonvested, Weighted Average Exercise Price (in usd per share)	[2]	$ 32.24	$ 30.49	[1]
Fully Vested Stock Based Compensation Issued To Retirement Eligible Employees		1,488,383	1,319,372
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)					$ 27.40

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

[2] Excludes 1,488,383 and 1,319,372 of restricted stock units issued to retirement-eligible employees as of December 31, 2017 and 2016, respectively, as they are fully vested.

[3] Represents restricted stock units that vested but were not distributed to retirement-eligible employees during 2017.