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Derivative Instruments Income Statement Location (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, gain (loss) on derivative, net $ (1,550) $ (2,122) $ 417
Interest Rate Contract | Interest expense | Derivatives not designated as hedging instrument      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, gain (loss) on derivative, net 0 0 (24)
OTC commodity swaps | Cost of goods sold | Derivatives not designated as hedging instrument      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, gain (loss) on derivative, net $ (1,550) $ (2,122) $ 441