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Pension Plan (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Retirement Benefits [Abstract]        
Service cost $ 169 $ 151 $ 506 $ 452
Interest cost on projected benefit obligation 701 693 2,104 2,080
Expected return on plan assets (1,354) (1,325) (4,063) (3,975)
Net amortization and deferral 672 794 2,017 2,381
Net periodic pension cost $ 188 $ 313 $ 564 $ 938