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Employee Retirement Plans (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 142 $ 444 $ 282 $ 819
Interest cost 555 610 1,103 1,225
Expected return on plan assets (609) (630) (1,151) (1,215)
Net amortization and deferral 143 476 263 838
Net periodic benefit cost $ 231 $ 900 $ 497 $ 1,667