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Income and other taxes - Unused tax losses (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Tax effect $ (7,013) $ (6,945)
Unused tax losses    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount 1,424,583  
Tax effect 327,654  
Expire | Unused tax losses    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount 1,052,399  
Tax effect 242,052  
Losses generated in 2014 (expires 2034) | Unused tax losses    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount 33,252  
Tax effect 7,648  
Losses generated in 2015 (expires 2035) | Unused tax losses    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount 351,625  
Tax effect 80,874  
Losses generated in 2016 (expires 2036) | Unused tax losses    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount 432,581  
Tax effect 99,494  
Losses generated in 2017 (expires 2037) | Unused tax losses    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount 234,941  
Tax effect 54,036  
Losses generated in 2019 | Unused tax losses    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount 232,163  
Tax effect 53,397  
Losses generated in 2020 | Unused tax losses    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount 140,021  
Tax effect $ 32,205