Commitments and Contingencies - Tax Legislation, Guarantees, Indemnifications, and Performance Contracts - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	0 Months Ended	9 Months Ended
	Oct. 23, 2014 Subsequent Event [Member]	Sep. 30, 2014 Pepco Energy Services [Member] Energy Savings or Combined Heat and Power Performance [Member]	Sep. 30, 2014 Pepco Energy Services [Member] Completed Performance Contracts Associated With Savings Guarantees [Member]	Sep. 30, 2014 Pepco Energy Services [Member] Uncompleted Performance Contracts Associated With Savings Guarantees [Member]
Commitment and Contingencies [Line Items]
Value of guarantees completed projects			$ 323
Contract life, maximum remaining term		20 years	15 years
Value of guarantees on projects under construction				94
Maximum term of project under construction				23 years
Value of guarantees under construction projects		15
Accrued liability on contracts		$ 1
Dividend declared on common stock, per share	$ 27
Dividend declared, payment date	Dec. 31, 2014
Dividends payable, date of record	Dec. 10, 2014