Variable Interest Entities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Oct. 18, 2011 MW
Delmarva Power & Light Co/De [Member]
Variable Interest Entity [Line Items]
Power producted by fuel cell facility	56,318	53,609	30
Delmarva Power & Light Co/De [Member] | Fuel Cell Facility [Member]
Variable Interest Entity [Line Items]
Amount billed to distribution customers, respect to energy produced by these facilities	$ 10	$ 9
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	1	1
Delmarva Power & Light Co/De [Member] | Wind PPA [Member]
Variable Interest Entity [Line Items]
Purchased energy	10	10
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]
Purchased energy	62	72
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
Purchase activities with non utility generators per power purchase agreements	$ 56	$ 59