Derivative instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 48,657	$ 30,455	$ 20,499
Derivative liability fair value gross liability	8,000	18,863	25,421
Designated as hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, assets, at fair value	47,805	30,455	20,305
Derivative instruments in hedges, liabilities, at fair value	7,752	18,426	24,515
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	36,360	27,687	14,040
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	11,445	2,768	6,265
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	789	12,727	17,780
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	6,963	827	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	0	937	6,735
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	0	3,935	0
Not designated as hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	852	0	194
Other derivatives not designated as hedging instruments liabilities at fair value	248	437	906
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	640	0	194
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	212	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	248	437	906
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	$ 0