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