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Retirement Plans (Details) - Defined Benefit Plan [Member] - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 368 $ 461
Interest cost 289 355
Expected return on plan assets (245) (255)
Amortization of actuarial gain (27) (21)
Total defined benefit expense $ 385 $ 540