Derivative Instruments and Hedging Activities (Notional Volumes) (Details) (Economic Hedges)	Jun. 30, 2012 GWh	Dec. 31, 2011 GWh
Electricity options, swaps and forwards
Derivative [Line Items]
Nonmonetary notional amount of derivative	26,597	30,881
Natural gas options, swaps and forwards
Derivative [Line Items]
Nonmonetary notional amount of derivative	213	300
Congestion revenue rights
Derivative [Line Items]
Nonmonetary notional amount of derivative	136,863	166,163
Tolling arrangements
Derivative [Line Items]
Nonmonetary notional amount of derivative	102,819	104,154