Derivative Instruments and Hedging Activities - Fair Values of Derivative Instruments by Balance Sheet Location (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Gross Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)	$ 3	$ 3
Derivative liabilities (current liabilities)	(2)	(4)
Net Derivative (liability) asset	1	(1)
Net Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)	3	3
Net Derivative (liability) asset	3	3
Other Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)	3	3
Derivative liabilities (current liabilities)	(2)	(4)
Net Derivative (liability) asset	1	(1)
Delmarva Power & Light Co/De [Member] | Gross Derivative Instruments [Member]
Derivative [Line Items]
Derivative liabilities (current liabilities)	(2)	(4)
Delmarva Power & Light Co/De [Member] | Other Derivative Instruments [Member]
Derivative [Line Items]
Derivative liabilities (current liabilities)	(2)	(4)
Effects of Cash Collateral and Netting [Member]
Derivative [Line Items]
Derivative liabilities (current liabilities)	2	4
Net Derivative (liability) asset	2	4
Effects of Cash Collateral and Netting [Member] | Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Derivative liabilities (current liabilities)	$ 2	$ 4