Related Party Transactions - Schedule of Related Party Transactions Included in Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Delmarva Power & Light Co/De [Member]
Related Party Transaction [Line Items]
Intercompany lease transactions	$ 1	$ 1	$ 2	$ 2
Atlantic City Electric Co [Member] | Millennium Account Services LLC (ACE Affiliate) [Member]
Related Party Transaction [Line Items]
Meter reading services provided by Millennium Account Services LLC (an ACE affiliate)	$ (1)	$ (1)	$ (2)	$ (2)