Derivative Instruments (Details) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accumulated Other Comprehensive Loss
Expected derivative payments during next twelve months		$ 5,300,000
Unrealized losses, before tax, on contracts previously designated as cash flow hedges	(70,700,000)	(8,900,000)	(70,700,000)
Unrealized losses, net of tax, on contracts previously designated as cash flow hedges	(43,806,000)	(5,517,000)	(43,806,000)
Reclassified unrealized losses, before tax from AOCL into earnings		(61,800,000)	(26,700,000)
Reclassified unrealized losses, after tax from AOCL into earnings		(38,289,000)	(16,566,000)
Termination of Interest Rate Derivatives
Cash paid to counterparties to unwind or terminate interest rate derivative contracts during the period	10,800,000
Fair value gain related to interest rate derivative contracts that were terminated during the period	8,900,000
Realized and unrealized (gain) loss on derivatives, net		13,908,000	(31,847,000)	(7,756,000)
2012 | Oil Bbl/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)		21,000
2012 | Natural Gas MMBtu/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)		15,000
2013 | Oil Bbl/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)		15,000
2013 | Natural Gas MMBtu/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)		0
2014 | Oil Bbl/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)		2,000
2014 | Natural Gas MMBtu/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)		0
Interest rate contracts
Termination of Interest Rate Derivatives
Realized and unrealized (gain) loss on derivatives, net	$ (1,900,000)