Derivative Instruments and Hedging Activities - Schedule of Cash Collateral Offset Against Derivative Positions (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Cash collateral pledged to counterparities with the right reclaim	$ 4
Cash collateral received from counterparties with the obligation to return		(1)
Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Cash collateral pledged to counterparities with the right reclaim	4
Cash collateral received from counterparties with the obligation to return		$ (1)