Property, Plant and Equipment - Capital Lease Assets Recorded within Property, Plant and Equipment (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Leases [Line Items]
Original Cost	$ 155	$ 155
Accumulated Amortization	86	77
Net Book Value	69	78
Transmission [Member]
Leases [Line Items]
Original Cost	76	76
Accumulated Amortization	41	37
Net Book Value	35	39
Distribution [Member]
Leases [Line Items]
Original Cost	76	76
Accumulated Amortization	42	37
Net Book Value	34	39
General [Member]
Leases [Line Items]
Original Cost	3	3
Accumulated Amortization	3	3
Net Book Value
Potomac Electric Power Co [Member]
Leases [Line Items]
Original Cost	155	155
Accumulated Amortization	86	77
Net Book Value	69	78
Potomac Electric Power Co [Member] | Transmission [Member]
Leases [Line Items]
Original Cost	76	76
Accumulated Amortization	41	37
Net Book Value	35	39
Potomac Electric Power Co [Member] | Distribution [Member]
Leases [Line Items]
Original Cost	76	76
Accumulated Amortization	42	37
Net Book Value	34	39
Potomac Electric Power Co [Member] | General [Member]
Leases [Line Items]
Original Cost	3	3
Accumulated Amortization	3	3
Net Book Value