Derivative instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 330	$ 1,950	$ 6,179
Derivative liability fair value gross liability	12,186	7,483	12,445
Designated as hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments in hedges, assets, at fair value	0	0	1,623
Derivative instruments in hedges, liabilities, at fair value	0	0	10,986
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	0	1,623
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	5,994
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	534
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	0	4,458
Not designated as hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	330	1,950	4,556
Other derivatives not designated as hedging instruments liabilities at fair value	12,186	7,483	1,459
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	81	1,447	4,313
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	249	503	243
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	12,186	7,483	1,385
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	$ 0	$ 0	$ 74