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Employee benefit plans - Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Employee benefit plans      
Employer Service cost $ 1,263 $ 1,079  
Interest cost 84 71  
Expected return on plan assets (60) (50)  
Amortization of prior service cost 45 45  
Amortization of actuarial losses 66 66  
Net periodic benefit cost $ 1,398 $ 1,211  
Location of costs excluding the service component us-gaap:OtherNonoperatingIncomeExpense us-gaap:OtherNonoperatingIncomeExpense  
Expected contributions $ 3,500    
Accrued pension obligation $ 15,800   $ 15,700