Fair Value Measurements (Details 3) (USD $) In Thousands, unless otherwise specified					3 Months Ended					3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Electricity forward contracts	Mar. 31, 2012 Electricity forward contracts Option Model	Mar. 31, 2012 Electricity forward contracts Maximum Discounted cash flows perMWh	Mar. 31, 2012 Electricity forward contracts Minimum Discounted cash flows perMWh	Mar. 31, 2012 Option Contracts Maximum Option Model perMMBTU perMWh	Mar. 31, 2012 Option Contracts Minimum Option Model perMMBTU perMWh	Mar. 31, 2012 Natural gas forward contracts	Mar. 31, 2012 Natural gas forward contracts Maximum Discounted cash flows perMMBTU	Mar. 31, 2012 Natural gas forward contracts Minimum Discounted cash flows perMMBTU
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	$ 142,000	$ 125,000	$ 106,000	$ 35,000					$ 1,000
Electricity prices (per MWh)					67.50	7.13	94.23	21.15
Natural gas prices (per mmbtu)							4.31	1.90		4.20	1.85
Electricity volatilities (as a percent)							83.00%	14.00%
Natural gas volatilities (as a percent)							50.00%	17.00%