Commitments And Contingencies (PHI's Cross-Border Energy Lease Investments) (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	9 Months Ended	12 Months Ended
	Jan. 31, 2011	Sep. 30, 2012	Dec. 31, 2011
Commitments And Contingencies [Line Items]
Estimated current annual tax benefits		$ 43
Investment in finance leases held in trust		1,226	1,349
Federal and state income tax benefits		479
Tax payment made	74
Income tax penalties	1
Interest expense associated with the disallowed deductions	28
Additional federal and state taxes	760	622
Potential interest on potential tax liability related to disallowed tax benefits		138
Maximum tax penalty percentage			20.00%
Potential tax liability in the event of a total disallowance		760
Pepco Holdings Inc [Member]
Commitments And Contingencies [Line Items]
Investment in finance leases held in trust		$ 1,200