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Derivative instruments (Details 3) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Sep. 30, 2013
Derivatives, Fair Value [Line Items]      
Derivative asset, fair value, gross asset $ 11,581 $ 1,950 $ 6,144
Derivative liability fair value gross liability 44 7,483 9,889
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 11,322 1,447 4,373
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 259 503 1,771
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 44 7,483 9,740
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 $ 0 $ 0 $ 149