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Deferred income tax - Differences between the accounting and tax basis (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) $ 6,883 $ 27,339  
Deferred tax expense (income) (37,501) 9,997 $ (5,825)
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss (813) 9,916 (7,963)
Deferred income tan other income 400 (543)  
Deferred tax liability (asset) 45,597 6,883 27,339
Total deferred tax asset 290,600 250,349  
Unrecognized deferred tax asset (50,214) (40,948)  
Net deferred tax assets 240,386 209,401  
Unused tax losses [member]      
Disclosure of Deferred income tax [Line Items]      
Total deferred tax asset 139,614 106,223  
Unrecognized deferred tax asset (8,152) (761)  
Net deferred tax assets 131,462 105,462  
Difference in depreciation and amortization rates [Member]      
Disclosure of Deferred income tax [Line Items]      
Total deferred tax asset 68,151 68,114  
Unrecognized deferred tax asset (19,544) (19,544)  
Net deferred tax assets 48,607 48,570  
Provision for closure of mining units, net [Member]      
Disclosure of Deferred income tax [Line Items]      
Total deferred tax asset 53,418 47,859  
Unrecognized deferred tax asset (22,518) (20,643)  
Net deferred tax assets 30,900 27,216  
Other temporary differences [member]      
Disclosure of Deferred income tax [Line Items]      
Total deferred tax asset 21,369 20,681  
Net deferred tax assets 21,369 20,681  
Impairment loss of long-lived assets [Member]      
Disclosure of Deferred income tax [Line Items]      
Total deferred tax asset 8,048 7,472  
Net deferred tax assets 8,048 7,472  
Deferred assets for mining royalties [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 36 123  
Deferred tax expense (income) (6) (87)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 0    
Deferred tax liability (asset) 30 36 123
Deferred liability for mining royalties [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (2) (161)  
Deferred tax expense (income) (166) 159  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 0    
Deferred tax liability (asset) (168) (2) (161)
Deferred Liability For Derivative Financial Instruments [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (813) 0  
Deferred tax expense (income) 0 0  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 813 (813)  
Deferred income tan other income 0    
Deferred tax liability (asset) 0 (813) 0
Deferred asset [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 209,437 196,819  
Deferred tax expense (income) 30,579 22,264  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 (9,103)  
Deferred income tan other income 400 (543)  
Deferred tax liability (asset) 240,416 209,437 196,819
Deferred asset [Member] | Deferred asset for income tax before allowance [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 209,401 187,593  
Deferred tax expense (income) 30,585 22,351  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 400 (543)  
Deferred tax liability (asset) 240,386 209,401 187,593
Deferred asset [Member] | Deferred asset for income tax before allowance [Member] | Unused tax losses [member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 105,462 94,939  
Deferred tax expense (income) 25,866 10,919  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 0 (396)  
Deferred tax liability (asset) 131,328 105,462 94,939
Deferred asset [Member] | Deferred asset for income tax before allowance [Member] | Difference in depreciation and amortization rates [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 48,570 47,482  
Deferred tax expense (income) 37 1,088  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 0    
Deferred tax liability (asset) 48,607 48,570 47,482
Deferred asset [Member] | Deferred asset for income tax before allowance [Member] | Provision for closure of mining units, net [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 27,216 19,793  
Deferred tax expense (income) 3,684 7,423  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 0    
Deferred tax liability (asset) 30,900 27,216 19,793
Deferred asset [Member] | Deferred asset for income tax before allowance [Member] | Other temporary differences [member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 20,681 15,459  
Deferred tax expense (income) 422 5,369  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 400 (147)  
Deferred tax liability (asset) 21,503 20,681 15,459
Deferred asset [Member] | Deferred asset for income tax before allowance [Member] | Impairment loss of long-lived assets [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 7,472 9,920  
Deferred tax expense (income) 576 (2,448)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 0    
Deferred tax liability (asset) 8,048 7,472 9,920
Deferred asset [Member] | Deferred asset included in retained earnings [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 209,401 196,696  
Deferred tax expense (income) 30,585 22,351  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 (9,103)  
Deferred income tan other income 400 (543)  
Deferred tax liability (asset) 240,386 209,401 196,696
Deferred asset [Member] | Deferred asset included in retained earnings [Member] | Derivative financial instruments [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 0 9,103  
Deferred tax expense (income) 0 0  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 (9,103)  
Deferred income tan other income 0    
Deferred tax liability (asset) 0 0 9,103
Deferred asset [Member] | Deferred assets for mining royalties [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 0 0  
Deferred tax expense (income) 0 0  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 0    
Deferred tax liability (asset) 0 0 0
Deferred liability [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (202,554) (169,480)  
Deferred tax expense (income) 6,922 (32,261)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 813 (813)  
Deferred income tan other income 0    
Deferred tax liability (asset) (194,819) (202,554) (169,480)
Deferred liability [Member] | Deferred liability for income tax [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (201,739) (169,319)  
Deferred tax expense (income) 7,088 (32,420)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 0    
Deferred tax liability (asset) (194,651) (201,739) (169,319)
Deferred liability [Member] | Deferred liability for income tax [Member] | Other temporary differences [member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (74,480) (77,603)  
Deferred tax expense (income) (4,550) 3,123  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 0    
Deferred tax liability (asset) (79,030) (74,480) (77,603)
Deferred liability [Member] | Deferred liability for income tax [Member] | Effect of translation into U.S. dollars [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (61,271) (46,023)  
Deferred tax expense (income) 14,995 (15,248)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 0    
Deferred tax liability (asset) (46,276) (61,271) (46,023)
Deferred liability [Member] | Deferred liability for income tax [Member] | Differences in amortization rates for development costs [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (65,988) (45,693)  
Deferred tax expense (income) (3,357) (20,295)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred income tan other income 0    
Deferred tax liability (asset) $ (69,345) $ (65,988) $ (45,693)