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Disclosure of leases and similar arrangements - Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of recognised finance lease as assets by lessee [line items]      
Lease expense $ 1,288 $ 1,409 $ 1,659
Average contract term (in years) 6 years    
Minimum operating lease payments recognised as expense $ 9,200 41,100 38,500
Minimum finance lease payments payable $ 522 595  
Lease term (in years) 10 years    
Lease Expense Capitalized      
Disclosure of recognised finance lease as assets by lessee [line items]      
Lease expense $ 0 $ 14,000 $ 6,800