Derivative instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 260	$ 1,950	$ 11,244
Derivative liability fair value gross liability	17,449	7,483	1,388
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	129	1,447	9,797
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	131	503	1,447
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	$ 17,449	$ 7,483	$ 1,388