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Employee Benefit Plans (Details 2) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Components of net pension cost      
Service cost - benefits earned during the year $ 1,343 $ 1,179 $ 1,325
Interest costs on projected benefit obligations 1,009 956 838
Expected return on plan assets (1,127) (1,057) (957)
Expected administrative expenses 88 70 40
Amortization of prior service cost 79 79 79
Amortization of unrecognized net loss 11 0 66
Net periodic pension expense $ 1,403 $ 1,227 $ 1,391