Derivative Instruments and Hedging Activities - Net Outstanding Commodity Forward Contracts That Did Not Qualify For Hedge Accounting (Detail) - Natural Gas [Member] - Long [Member] - MMBTU	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Quantity	3,760,000	3,892,500
Delmarva Power & Light Co/De [Member]
Derivative [Line Items]
Quantity	3,760,000	3,892,500

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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