Commitments and Contingencies - Tax Legislation, Guarantees, Indemnifications, and Performance Contracts - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		9 Months Ended
	Oct. 22, 2015	Sep. 30, 2015
Commitment and Contingencies [Line Items]
Pro-rata dividend payable, description		On October 22, 2015, Pepco Holdings' Board of Directors also declared a pro-rata dividend in the event the Merger is completed before the close of business on December 10, 2015. The pro-rata dividend is payable 20 days after the Merger is completed to holders of common stock of record as of the day immediately prior to the day the Merger is completed, at a rate of $0.002967 per share per day beginning September 11, 2015, and ending the day before the Merger is completed.
Subsequent Event [Member]
Commitment and Contingencies [Line Items]
Dividend declared on common stock, per share	$ 0.27
Dividend declared date	Oct. 22, 2015
Dividend declared, payment date	Dec. 31, 2015
Dividends payable, date of record	Dec. 10, 2015
Pro-rata dividend payable, period	20 days
Pro-rata dividend rate, per share per day	$ 0.002967
Pepco Energy Services [Member] | Energy Savings or Combined Heat and Power Performance [Member]
Commitment and Contingencies [Line Items]
Contract life, maximum remaining term		20 years
Value of guarantees under construction projects		$ 15
Accrued liability on contracts		1
Pepco Energy Services [Member] | Completed Performance Contracts Associated With Savings Guarantees [Member]
Commitment and Contingencies [Line Items]
Value of guarantees completed projects		$ 230
Contract life, maximum remaining term		23 years
Pepco Energy Services [Member] | Uncompleted Performance Contracts Associated With Savings Guarantees [Member]
Commitment and Contingencies [Line Items]
Value of guarantees on projects under construction		$ 57
Maximum term of project under construction		15 years