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Derivatives (Narrative) (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2017
Oct. 01, 2016
Sep. 30, 2017
Oct. 01, 2016
Derivative [Line Items]        
Gain or (Loss) Recognized in Income on Derivatives (Ineffective Portion and Amount Excluded from Effectiveness Testing) $ 1,689,000 $ 323,000 $ 384,000 $ 537,000
Net income 8,684,000 $ 28,890,000 26,410,000 $ 65,544,000
Commodity Contract [Member]        
Derivative [Line Items]        
Forward purchase amount $ 58,600,000   58,600,000  
Cash Flow Hedging [Member]        
Derivative [Line Items]        
Cash flow hedge gain (loss) to be reclassified within 12 months     2,200,000  
Net income     $ 0