Quarterly report pursuant to Section 13 or 15(d)

Employee Retirement Plans (Details)

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Employee Retirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Pension plan          
Components of net periodic cost:          
Service cost $ 102 $ 89 $ 204   $ 178
Interest cost 389 403 778   806
Expected return on plan assets (475) (448) (950)   (896)
Amortization of net (gain) loss 176 181 352   362
Net periodic pension cost 192 225 384   450
Employer contributions during the period     31   $ 655
Pension plan | Minimum          
Components of net periodic cost:          
Estimated total required contributions 72   72    
Other postretirement benefit cost          
Components of net periodic cost:          
Service cost 47 51 94 $ 102  
Interest cost 58 70 116 140  
Amortization of net (gain) loss (52) (27) (104) (54)  
Net periodic pension cost $ 53 $ 94 $ 106 $ 188