Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Credit risk derivative liabilities, at fair value	$ 0	$ 0
Cash plus borrowing capacity to meet future liquidity needs	885	332
Delmarva Power & Light Co/De [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Credit risk derivative liabilities, at fair value	0	0
Cash plus borrowing capacity to meet future liquidity needs	$ 885	$ 332