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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, 2020
Dec. 31, 2019
Disclosure of defined benefit plans [line items]    
Present value of obligations $ (24,695) $ (38,548)
Fair value of plan assets 8,470 26,568
Asset (liability) on statement of financial position (16,225) (11,980)
Employee benefit plans    
Disclosure of defined benefit plans [line items]    
Present value of obligations (21,974) (20,182)
Fair value of plan assets 0 1,958
Asset (liability) on statement of financial position (21,974) (18,224)
SERP    
Disclosure of defined benefit plans [line items]    
Present value of obligations (2,721) (18,366)
Fair value of plan assets 8,470 24,610
Asset (liability) on statement of financial position $ 5,749 $ 6,244