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Employee benefit plans - Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Employee benefit plans          
Employer Service cost $ 1,263 $ 1,041 $ 3,813 $ 3,152  
Interest cost 84 69 253 208  
Expected return on plan assets (60) (48) (180) (145)  
Amortization of prior service cost 54 45 161 135  
Amortization of actuarial losses 74 66 186 198  
Net periodic benefit cost $ 1,415 $ 1,173 $ 4,233 $ 3,548  
Location of costs excluding the service component us-gaap:OtherNonoperatingIncomeExpense us-gaap:OtherNonoperatingIncomeExpense us-gaap:OtherNonoperatingIncomeExpense us-gaap:OtherNonoperatingIncomeExpense  
Expected contributions $ 3,500   $ 3,500    
Accrued pension obligation $ 16,600   $ 16,600   $ 15,700