Discontinued Operations - Net Outstanding Commodity Forward Contracts that Did Not Qualify for Hedge Accounting (Detail) (Discontinued Operations [Member])	12 Months Ended
	Dec. 31, 2012 MWh
Financial Transmission Rights (MWh) [Member]
Derivative [Line Items]
Quantity	181,008
Net Position	Long
Electricity (MWh) [Member]
Derivative [Line Items]
Quantity	261,240
Net Position	Long
Natural Gas (One Million British Thermal Units (MMBtu)) [Member]
Derivative [Line Items]
Quantity	2,867,500
Net Position	Long