Commitments and Contingencies - Tax Legislation, Guarantees, Indemnifications, and Performance Contracts - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended		1 Months Ended
	Oct. 24, 2013 Subsequent Event [Member]	Sep. 30, 2013 Energy Savings or Combined Heat and Power Performance [Member] Maximum [Member]	Sep. 30, 2013 Pepco Energy Services [Member] Completed Performance Contracts Associated With Savings Guarantees [Member]	Sep. 30, 2013 Pepco Energy Services [Member] Energy Savings or Combined Heat and Power Performance [Member]	Sep. 30, 2013 Pepco Energy Services [Member] Uncompleted Performance Contracts Associated With Savings Guarantees [Member]
Commitment and Contingencies [Line Items]
Contract life, maximum remaining term			14 years	20 years
Value of guarantees completed projects			$ 242
Value of guarantees on projects under construction					194
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	$ 0.27
Dividend declared, payable date	Dec. 31, 2013
Dividends payable, date of record	Dec. 10, 2013