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Retirement Plans - Additional Information (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Contributions to defined benefit plan $ 160    
Pension plan      
Defined Benefit Plan Disclosure [Line Items]      
Service period 5 years    
Assumptions used in determining periodic pension cost:      
Discount rate 3.70% 3.90%  
Expected long-term rate of return on plan assets 7.50% 7.50%  
Pension plan | Equities      
Current investment policy targets:      
Current investment mix 65.00%   57.00%
Pension plan | Treasury/Corporate Notes      
Current investment policy targets:      
Current investment mix 34.00%   37.00%
Pension plan | Money market funds      
Current investment policy targets:      
Current investment mix 1.00%   6.00%