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Income and other taxes - Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Aug. 08, 2018
Dec. 31, 2018
Dec. 31, 2017
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unrecognized deferred tax assets   $ 172,954 $ 135,905
Income taxes paid   50,000 12,000
Canada | Capital Losses      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unrecognized deferred tax assets   423,000  
Fekola SA | Non-participating Share Capital      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Income tax relating to priority dividend   $ 18,000 $ 2,000
Priority dividend rate to non-controlling interest 10.00% 10.00%