Derivative Instruments and Hedging Activities (Outstanding Energy Commodity Contracts Employed as Cash Flow Hedges) (Detail) (Pepco Energy Services [Member], Electricity (Megawatt Hours (MWh)) [Member])	Sep. 30, 2012 MWh	Dec. 31, 2011 MWh
Forecasted Purchases Hedges [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount		614,560
Forecasted Sales Hedges [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount		614,560