Fair value derivative instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivatives, fair value [Line Items]
Derivative asset, fair value, gross asset	$ 1,950	$ 18,304
Derivative liability, fair value, gross liability	7,483	6,255
Designated as hedging instrument [Member]
Derivatives, fair value [Line Items]
Derivative instruments in hedges, assets, at fair value	0	18,084
Derivative instruments in hedges, liabilities, at fair value	0	6,255
Designated as hedging instrument [Member] | Commodity derivatives | Commodity derivative instruments assets, current [Member]
Derivatives, fair value [Line Items]
Derivative instruments in hedges, assets, at fair value	0	18,084
Designated as hedging instrument [Member] | Interest rate derivative instruments | Other accrued liabilities [Member]
Derivatives, fair value [Line Items]
Derivative instruments in hedges, liabilities, at fair value	0	6,255
Not designated as hedging instruments [Member]
Derivatives, fair value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	1,950	220
Other derivatives not designated as hedging instruments liabilities at fair value	7,483	0
Not designated as hedging instruments [Member] | Commodity derivatives | Commodity derivative instruments assets, current [Member]
Derivatives, fair value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	1,447	220
Not designated as hedging instruments [Member] | Commodity derivatives | Other assets noncurrent [Member]
Derivatives, fair value [Line Items]
Other derivatives not designated as hedging instruments assets at fair value	503	0
Not designated as hedging instruments [Member] | Commodity derivatives | Commodity derivative instruments liabilities [Member]
Derivatives, fair value [Line Items]
Other derivatives not designated as hedging instruments liabilities at fair value	$ 7,483	$ 0