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