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Employee benefit plans - Summary of Amounts Recognised in the Balance Sheet for All Pension Plans (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Disclosure of defined benefit plans [line items]    
Present value of obligations $ (38,548) $ (53,314)
Fair value of plan assets 26,568 48,059
Asset (liability) on statement of financial position (11,980) (5,255)
Employee benefit plans    
Disclosure of defined benefit plans [line items]    
Present value of obligations (20,182) (16,239)
Fair value of plan assets 1,958 1,864
Asset (liability) on statement of financial position (18,224) (14,375)
Supplemental Pension Plan    
Disclosure of defined benefit plans [line items]    
Present value of obligations (18,366) (37,075)
Fair value of plan assets 24,610 46,195
Asset (liability) on statement of financial position $ 6,244 $ 9,120