Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Derivative Liability Fair Value Gross Liability	$ 18,863	$ 30,911
Derivative Asset, Fair Value, Gross Asset	30,455	19,227
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Instruments in Hedges, Liabilities, at Fair Value	18,426	21,552
Derivative Instruments in Hedges, Assets, at Fair Value	30,455	19,227
Designated as Hedging Instrument [Member] | Commodity derivative instruments - current assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Instruments in Hedges, Assets, at Fair Value	27,687	15,123
Designated as Hedging Instrument [Member] | Other assets noncurrent [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Instruments in Hedges, Assets, at Fair Value	2,768	4,104
Designated as Hedging Instrument [Member] | Commodity derivative instruments - current liabilities [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Instruments in Hedges, Liabilities, at Fair Value	12,727	15,069
Designated as Hedging Instrument [Member] | Other accrued liabilities [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative Instruments in Hedges, Liabilities, at Fair Value	827	0
Designated as Hedging Instrument [Member] | Other liabilities noncurrent [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative Instruments in Hedges, Liabilities, at Fair Value	3,935	0
Designated as Hedging Instrument [Member] | Other liabilities noncurrent [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Instruments in Hedges, Liabilities, at Fair Value	937	6,483
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Other Derivatives Not Designated as Hedging Instruments Liabilities at Fair Value	437	9,359
Other Derivatives Not Designated as Hedging Instruments Assets at Fair Value	0	0
Not Designated as Hedging Instrument [Member] | Commodity derivative instruments - current assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Other Derivatives Not Designated as Hedging Instruments Assets at Fair Value	0	0
Not Designated as Hedging Instrument [Member] | Other assets noncurrent [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Other Derivatives Not Designated as Hedging Instruments Assets at Fair Value	0	0
Not Designated as Hedging Instrument [Member] | Commodity derivative instruments - current liabilities [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Other Derivatives Not Designated as Hedging Instruments Liabilities at Fair Value	437	9,359
Not Designated as Hedging Instrument [Member] | Other liabilities noncurrent [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Other Derivatives Not Designated as Hedging Instruments Liabilities at Fair Value	$ 0	$ 0