Derivative instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 6,179	$ 18,304	$ 27,280
Derivative liability fair value gross liability	12,445	6,255	29,651
Designated as hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, assets, at fair value	1,623	18,084	26,097
Derivative instruments in hedges, liabilities, at fair value	10,986	6,255	28,383
Not designated as hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	4,556	220	1,183
Other derivatives not designated as hedging instruments liabilities at fair value	1,459	0	1,268
Interest rate swap [Member] | Designated as hedging instrument [Member] | Other accrued liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, liabilities, at fair value	4,458	6,255	1,168
Interest rate swap [Member] | Designated as hedging instrument [Member] | Other liabilities noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, liabilities, at fair value	0	0	2,153
Commodity contract [Member] | Designated as hedging instrument [Member] | Commodity derivative instruments - current assets [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, assets, at fair value	1,623	18,084	25,560
Commodity contract [Member] | Designated as hedging instrument [Member] | Other assets noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, assets, at fair value	0	0	537
Commodity contract [Member] | Designated as hedging instrument [Member] | Commodity derivative instruments - current liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, liabilities, at fair value	5,994	0	18,964
Commodity contract [Member] | Designated as hedging instrument [Member] | Other liabilities noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, liabilities, at fair value	534	0	6,098
Commodity contract [Member] | Not designated as hedging instrument [Member] | Commodity derivative instruments - current assets [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	4,313	220	1,138
Commodity contract [Member] | Not designated as hedging instrument [Member] | Other assets noncurrent [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	243	0	45
Commodity contract [Member] | Not designated as hedging instrument [Member] | Commodity derivative instruments - current liabilities [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments liabilities at fair value	1,385	0	1,219
Commodity contract [Member] | Not designated as hedging instrument [Member] | Other liabilities noncurrent [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments liabilities at fair value	$ 74	$ 0	$ 49