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Employee Retirement Plans (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 126 $ 141 $ 427 $ 423
Interest cost 554 552 1,652 1,655
Expected return on plan assets (741) (576) (2,033) (1,727)
Net amortization and deferral 157 132 476 395
Net periodic pension cost $ 96 $ 249 $ 522 $ 746