Derivative instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 27,280	$ 30,455	$ 16,398
Derivative liability fair value gross liability	29,651	18,863	58,581
Designated as hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, assets, at fair value	26,097	30,455	16,398
Derivative instruments in hedges, liabilities, at fair value	28,383	18,426	54,072
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	25,560	27,687	13,250
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	537	2,768	3,148
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	18,964	12,727	35,990
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	1,168	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	6,098	937	18,082
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	2,153	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	1,183	0	0
Other derivatives not designated as hedging instruments liabilities at fair value	1,268	437	4,509
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	1,138	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	45	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	1,219	437	4,509
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	$ 49	$ 0	$ 0