Derivative Instruments Location and fair value of derivative instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Derivative asset, fair value, gross asset	$ 18,304	$ 30,455
Derivative liability, fair value, gross liability	6,255	18,863
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative instruments in hedges, liabilities, at fair value	6,255	18,426
Derivative instruments in hedges, assets, at fair value	18,084	30,455
Not designated as hedging instruments
Derivative [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	220	0
Other derivatives not designated as hedging instruments liabilities at fair value	0	437
Commodity derivatives | Designated as Hedging Instrument [Member] | Commodity derivative instruments liabilities, current [Member]
Derivative [Line Items]
Derivative instruments in hedges, liabilities, at fair value	0	12,727
Commodity derivatives | Designated as Hedging Instrument [Member] | Commodity derivative instruments assets, current [Member]
Derivative [Line Items]
Derivative instruments in hedges, assets, at fair value	18,084	27,687
Commodity derivatives | Designated as Hedging Instrument [Member] | Other assets noncurrent [Member]
Derivative [Line Items]
Derivative instruments in hedges, assets, at fair value	0	2,768
Commodity derivatives | Designated as Hedging Instrument [Member] | Other liabilities noncurrent [Member]
Derivative [Line Items]
Derivative instruments in hedges, liabilities, at fair value	0	937
Commodity derivatives | Not designated as hedging instruments | Commodity derivative instruments liabilities, current [Member]
Derivative [Line Items]
Other derivatives not designated as hedging instruments liabilities at fair value	0	437
Commodity derivatives | Not designated as hedging instruments | Commodity derivative instruments assets, current [Member]
Derivative [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	220	0
Interest rate derivative instruments | Designated as Hedging Instrument [Member] | Other liabilities noncurrent [Member]
Derivative [Line Items]
Derivative instruments in hedges, liabilities, at fair value	0	3,935
Interest rate derivative instruments | Designated as Hedging Instrument [Member] | Other accrued liabilities [Member]
Derivative [Line Items]
Derivative instruments in hedges, liabilities, at fair value	$ 6,255	$ 827