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Deferred income tax - Composition of the provision for income taxes (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of Deferred income tax [Line Items]      
Current $ 11,911 $ 16,882 $ 23,713
Deferred (37,501) 9,997 (5,825)
Tax expense (income), continuing operations 25,590 (26,879) (17,888)
Income Tax Expense [Member]      
Disclosure of Deferred income tax [Line Items]      
Current (11,851) (12,433) (18,780)
Deferred 37,617 (10,029) 5,984
Tax expense (income), continuing operations 25,766 (22,462) (12,796)
Mining Royalties and Special Mining Tax [Member]      
Disclosure of Deferred income tax [Line Items]      
Current (60) (4,449) (4,933)
Deferred (116) 32 (159)
Tax expense (income), continuing operations $ (176) $ (4,417) $ (5,092)