Variable Interest Entities - Additional Information (Detail) $ in Millions		3 Months Ended		9 Months Ended
	Oct. 18, 2011 MW	Sep. 30, 2015 USD ($) MW	Sep. 30, 2014 USD ($) MW	Sep. 30, 2015 USD ($) Agreement MW	Sep. 30, 2014 USD ($) MW
Delmarva Power & Light Co/De [Member]
Variable Interest Entity [Line Items]
Power produced by fuel cell facility	30	56,388	56,335	170,028	165,878
Delmarva Power & Light Co/De [Member] | Fuel Cell Facility [Member]
Variable Interest Entity [Line Items]
Term of agreement				Through 2033
Amount billed to distribution customers, respect to energy produced by these facilities | $		$ 9	$ 8	$ 28	$ 26
Delmarva Power & Light Co/De [Member] | Wind PPA [Member]
Variable Interest Entity [Line Items]
Purchased energy | $		4	5	$ 20	$ 21
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
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 | Agreement				1
Energy purchase maximum to be purchased				28	19
Term of agreement				Through 2030
Term of agreement, years				20 years
Obligated purchase amount of energy produced at the facility				70.00%
Solar energy purchases | $		2	1	$ 4	$ 3
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 | Agreement				3
Delmarva Power & Light Co/De [Member] | 2015 PPA [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Expected time frame for facilities to be operational				1 year
Atlantic City Electric Co [Member]
Variable Interest Entity [Line Items]
Purchased energy | $		52	56	$ 160	182
Purchase activities with non utility generators per power purchase agreements | $		$ 50	$ 52	$ 151	$ 159
Equity ownership percentage		100.00%		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 | Agreement				3