Derivative Instruments and Hedging Activities - Fair Values of Derivative Instruments by Balance Sheet Location (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Gross Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)	$ 3,000,000	$ 1,000,000
Derivative liabilities (current liabilities)	(4,000,000)
Net Derivative (liability) asset	(1,000,000)
Net Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)	3,000,000
Net Derivative (liability) asset	3,000,000
Other Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)	3,000,000	1,000,000
Derivative liabilities (current liabilities)	(4,000,000)
Net Derivative (liability) asset	(1,000,000)
Delmarva Power & Light Co/De [Member] | Gross Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)		1,000,000
Derivative liabilities (current liabilities)	(4,000,000)
Delmarva Power & Light Co/De [Member] | Other Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)		1,000,000
Derivative liabilities (current liabilities)	(4,000,000)
Effects of Cash Collateral and Netting [Member]
Derivative [Line Items]
Derivative assets (current assets)		(1,000,000)
Derivative liabilities (current liabilities)	4,000,000
Net Derivative (liability) asset	4,000,000
Effects of Cash Collateral and Netting [Member] | Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Derivative assets (current assets)		(1,000,000)
Derivative liabilities (current liabilities)	$ 4,000,000