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Employee benefit plans - Summary of Amounts Recognised in the Income Statement (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [line items]    
Current service cost $ 2,353 $ 3,204
Interest cost 2,116 2,005
Past service cost 97 146
Loss on settlement 32 0
Expected return on plan assets (1,881) (1,800)
Employee benefit plans expense (recovery) 2,717 3,555
Employee Benefit Plan    
Disclosure of net defined benefit liability (asset) [line items]    
Current service cost 2,181 2,935
Interest cost 669 601
Past service cost 0 0
Loss on settlement 0 0
Expected return on plan assets (72) (73)
Employee benefit plans expense (recovery) 2,778 3,463
SERP    
Disclosure of net defined benefit liability (asset) [line items]    
Current service cost 172 269
Interest cost 1,447 1,404
Past service cost 97 146
Loss on settlement 32 0
Expected return on plan assets (1,809) (1,727)
Employee benefit plans expense (recovery) $ (61) $ 92