Derivative Instruments and Hedging Activities - Fair Values of Derivative Instruments by Balance Sheet Location (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)
Total Derivative asset
Derivative liabilities (current liabilities)
Total Derivative liability
Net Derivative asset
Gross Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)	3,000,000	1,000,000
Total Derivative asset	3,000,000	1,000,000
Derivative liabilities (current liabilities)	(1,000,000)
Total Derivative liability	(1,000,000)
Net Derivative asset	2,000,000
Net Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)	3,000,000
Total Derivative asset	3,000,000
Derivative liabilities (current liabilities)
Total Derivative liability
Net Derivative asset	3,000,000
Other Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)	3,000,000	1,000,000
Total Derivative asset	3,000,000	1,000,000
Derivative liabilities (current liabilities)	(1,000,000)
Total Derivative liability	(1,000,000)
Net Derivative asset	2,000,000
Delmarva Power & Light Co/De [Member] | Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Derivative liabilities (current liabilities)
Total Derivative liability
Delmarva Power & Light Co/De [Member] | Gross Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)		1,000,000
Total Derivative asset		1,000,000
Derivative liabilities (current liabilities)	(1,000,000)
Total Derivative liability	(1,000,000)
Delmarva Power & Light Co/De [Member] | Net Derivative Instruments [Member]
Derivative [Line Items]
Derivative liabilities (current liabilities)
Total Derivative liability
Delmarva Power & Light Co/De [Member] | Other Derivative Instruments [Member]
Derivative [Line Items]
Derivative assets (current assets)		1,000,000
Total Derivative asset		1,000,000
Derivative liabilities (current liabilities)	(1,000,000)
Total Derivative liability	(1,000,000)
Effects of Cash Collateral and Netting [Member]
Derivative [Line Items]
Derivative assets (current assets)		(1,000,000)
Total Derivative asset		(1,000,000)
Derivative liabilities (current liabilities)	1,000,000
Total Derivative liability	1,000,000
Net Derivative asset	1,000,000
Effects of Cash Collateral and Netting [Member] | Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Derivative assets (current assets)		(1,000,000)
Total Derivative asset		(1,000,000)
Derivative liabilities (current liabilities)	1,000,000
Total Derivative liability	$ 1,000,000