Variable Interest Entities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 18, 2011 MW
Delmarva Power & Light Co/De [Member]
Variable Interest Entity [Line Items]
Power of fuel cell facility	30	15		30
Delmarva Power & Light Co/De [Member] | Fuel Cell Facility [Member]
Variable Interest Entity [Line Items]
Amount billed to distribution customers	$ 36	$ 23	$ 4
Delmarva Power & Light Co/De [Member] | Fuel Cell Facility [Member] | October 18, 2011 [Member]
Variable Interest Entity [Line Items]
Term of agreement	Through 2033
Delmarva Power & Light Co/De [Member] | Land-Based Wind PPA [Member]
Variable Interest Entity [Line Items]
Megawatts received from power purchase agreements (PPAs)	128
Number of purchase power agreements	3
Delmarva Power & Light Co/De [Member] | Solar PPA [Member]
Variable Interest Entity [Line Items]
Megawatts received from power purchase agreements (PPAs)	10
Number of purchase power agreements	1
Energy purchase maximum to be purchased	19
Term of agreement, years	20 years
Term of agreement	Through 2030
Obligated purchase amount of energy produced at the facility	70.00%
Solar energy purchases	6	3	2
Delmarva Power & Light Co/De [Member] | Wind PPA [Member]
Variable Interest Entity [Line Items]
Purchased energy	31	30	27
Delmarva Power & Light Co/De [Member] | Wind PPA [Member] | Wind Facility One [Member]
Variable Interest Entity [Line Items]
Energy purchase maximum to be purchased	50
Delmarva Power & Light Co/De [Member] | Wind PPA [Member] | Wind Facility Two [Member]
Variable Interest Entity [Line Items]
Energy purchase maximum to be purchased	40
Delmarva Power & Light Co/De [Member] | Wind PPA [Member] | Wind Facility Three [Member]
Variable Interest Entity [Line Items]
Energy purchase maximum to be purchased	38
Atlantic City Electric Co [Member]
Variable Interest Entity [Line Items]
Net power purchases with non-utility generators	233	221	206
Equity ownership percentage	100.00%
Atlantic City Electric Co [Member] | Non-Utility Generators [Member]
Variable Interest Entity [Line Items]
Megawatts received from power purchase agreements (PPAs)	459
Number of purchase power agreements	3
Purchased energy	$ 208	$ 206	$ 201