Property, Plant and Equipment - Capital Lease Assets Recorded within Property, Plant and Equipment (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Leases [Line Items]
Original Cost	$ 152	$ 155
Accumulated Amortization	92	86
Net Book Value	60	69
Potomac Electric Power Co [Member]
Leases [Line Items]
Original Cost	152	155
Accumulated Amortization	92	86
Net Book Value	60	69
Transmission [Member]
Leases [Line Items]
Original Cost	76	76
Accumulated Amortization	46	41
Net Book Value	30	35
Transmission [Member] | Potomac Electric Power Co [Member]
Leases [Line Items]
Original Cost	76	76
Accumulated Amortization	46	41
Net Book Value	30	35
Distribution [Member]
Leases [Line Items]
Original Cost	76	76
Accumulated Amortization	46	42
Net Book Value	30	34
Distribution [Member] | Potomac Electric Power Co [Member]
Leases [Line Items]
Original Cost	76	76
Accumulated Amortization	46	42
Net Book Value	30	34
General [Member]
Leases [Line Items]
Original Cost		3
Accumulated Amortization		3
General [Member] | Potomac Electric Power Co [Member]
Leases [Line Items]
Original Cost		3
Accumulated Amortization		$ 3