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Net Periodic Pension Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jul. 01, 2017
Jun. 30, 2018
Jul. 01, 2017
Retirement Benefits [Abstract]        
Service cost $ 133 $ 183 $ 266 $ 366
Interest cost on projected benefit obligation 963 1,178 1,926 2,356
Expected return on plan assets (1,327) (1,584) (2,654) (3,168)
Amortization of unrecognized loss 271 268 542 536
Net periodic pension cost $ 40 $ 45 $ 80 $ 90