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Retirement plans - Defined benefit pension plans (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of net defined benefit liability (asset) [line items]    
Balance, beginning of year $ 22,170  
Current service cost 3,016 $ 2,639
Interest cost on accrued benefit obligations / (income) on assets 534 800
Balance, end of year 31,852 22,170
Unfunded status 31,852 22,170
Minimum funding requirement 0 0
Accrued benefit obligations    
Disclosure of net defined benefit liability (asset) [line items]    
Balance, beginning of year 66,061 60,618
Current service cost 3,016 2,639
Interest cost on accrued benefit obligations / (income) on assets 1,794 2,196
Benefit payments (2,227) (7,092)
Actuarial loss 7,120 8,041
Foreign exchange gain 3,046 (341)
Balance, end of year 78,810 66,061
Fair values of plan assets    
Disclosure of net defined benefit liability (asset) [line items]    
Balance, beginning of year 43,891 40,955
Interest cost on accrued benefit obligations / (income) on assets 1,260 1,396
Contributions 1,182 4,056
Benefit payments (2,227) (7,092)
Return on plan assets 1,940 2,500
Foreign exchange gain 912 2,076
Balance, end of year $ 46,958 $ 43,891