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Employee Benefits - Components of Net Periodic Pension Cost and Other Changes in Plan Assets and Benefit (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Components of Net Periodic Pension Cost        
Interest cost on projected benefit obligation $ 967 $ 1,025 $ 1,934 $ 2,050
Expected return on plan assets (1,567) (1,582) (3,134) (3,164)
Amortization of prior service credit 545 475 1,089 949
Net Periodic Pension Expense $ (55) $ (82) $ (111) $ (165)