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Retirement Plans - Net Pension Income (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Pension Plans        
Defined Benefit Plan Disclosure        
Net pension expense (income) $ 78 $ (491) $ 157 $ (924)