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Employee Retirement Plans (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 151 $ 140
Interest cost 549 548
Expected return on plan assets (646) (542)
Net amortization and deferral 159 120
Net periodic pension cost $ 213 $ 266