Derivative Instruments and Hedging Activities (Schedule of Cash Collateral Offset Against Derivative Positions) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Cash collateral pledged to counterparties with the right to reclaim	$ 27	[1]	$ 73	[1]
Cash collateral received from counterparties with the obligation to return	(1)
Delmarva Power & Light Co/De [Member]
Cash collateral pledged to counterparties with the right to reclaim			2
Cash collateral received from counterparties with the obligation to return	$ (1)

[1] Includes cash deposits on commodity brokerage accounts