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Derivative Instruments - Income Statement Location (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Commodity derivatives [Member] | Cost of goods sold | Derivatives not designated as hedging instrument [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Recognized gain (loss) $ 1,583 $ 510