Commitments and Contingencies - Tax Legislation, Guarantees, Indemnifications, and Performance Contracts - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	0 Months Ended
	Mar. 31, 2015	Apr. 23, 2015	Mar. 31, 2015
Commitment and Contingencies [Line Items]
Pro-rata dividend payable, description	The Board of Directors also declared a pro-rata dividend payable in lieu of the Second Quarter Dividend in the event the Merger is completed before the close of business on June 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 March 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		Apr. 23, 2015
Dividend declared, payment date		Jun. 30, 2015
Dividends payable, date of record		Jun. 10, 2015
Pro-rata dividend payable, period		20 days
Pro-rata dividend rate, per day		0.2967%
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		$ 15
Accrued liability on contracts	1		1
Pepco Energy Services [Member] | Completed Performance Contracts Associated With Savings Guarantees [Member]
Commitment and Contingencies [Line Items]
Value of guarantees completed projects	318		318
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	60		$ 60
Maximum term of project under construction			15 years