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Employee Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Net periodic benefit cost      
Service cost $ 4,450 $ 2,836 $ 1,225
Interest cost 278 190 76
Expected return on plan assets (195) (138) (59)
Amortization of prior service cost 179 154  
Amortization of actuarial losses 265 141  
Net periodic benefit cost $ 4,977 $ 3,183 $ 1,242