Derivative instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 6,144	$ 18,304	$ 22,719
Derivative liability fair value gross liability	9,889	6,255	9,934
Designated as hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, assets, at fair value	0	18,084	22,082
Derivative instruments in hedges, liabilities, at fair value	0	6,255	9,820
Designated as hedging instrument [Member] | Commodity contract [Member] | Commodity derivative instruments - current assets [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, assets, at fair value	0	18,084	18,619
Designated as hedging instrument [Member] | Commodity contract [Member] | Other assets noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, assets, at fair value	0	0	3,463
Designated as hedging instrument [Member] | Commodity contract [Member] | Commodity derivative instruments - current liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, liabilities, at fair value	0	0	1,958
Designated as hedging instrument [Member] | Commodity contract [Member] | Other liabilities noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, liabilities, at fair value	0	0	83
Designated as hedging instrument [Member] | Interest rate swap [Member] | Other accrued liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, liabilities, at fair value	0	6,255	7,779
Not designated as hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	6,144	220	637
Other derivatives not designated as hedging instruments liabilities at fair value	9,889	0	114
Not designated as hedging instrument [Member] | Commodity contract [Member] | Commodity derivative instruments - current assets [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	4,373	220	574
Not designated as hedging instrument [Member] | Commodity contract [Member] | Other assets noncurrent [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	1,771	0	63
Not designated as hedging instrument [Member] | Commodity contract [Member] | Commodity derivative instruments - current liabilities [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments liabilities at fair value	9,740	0	114
Not designated as hedging instrument [Member] | Commodity contract [Member] | Other liabilities noncurrent [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments liabilities at fair value	$ 149	$ 0	$ 0