Commitments And Contingencies (Tax Legislation, Guarantees, Indemnifications, And Performance Contracts) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Commitments And Contingencies [Line Items]
Value of guarantees on both completed projects and projects under construction	$ 449
Value of performance contracts issued	15
Dividend declared on common stock, per share	$ 27
Dividend, declared date	Oct. 25, 2012
Dividend, payable date	Dec. 31, 2012
Dividend, record date	Dec. 10, 2012
Performance Contracts Associated With Savings Guarantees [Member] | Pepco Energy Services [Member]
Commitments And Contingencies [Line Items]
Contract life, maximum remaining term	13 years
Accrued liability	$ 1
Performance Guarantee Contracts Associated With Production At Heat And Power Facilities [Member] | Pepco Energy Services [Member]
Commitments And Contingencies [Line Items]
Contract life, maximum remaining term	20 years