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Deferred income tax (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) $ 13,551 $ 28,912  
Deferred tax expense (income) (5,825) 14,060 $ 541
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss (7,963) 1,301 (3,372)
Deferred tax liability (asset) 27,339 13,551 28,912
Deferred asset [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 179,884 169,171  
Deferred tax expense (income) 8,972 12,014  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 7,963 (1,301)  
Deferred tax liability (asset) 196,819 179,884 169,171
Deferred asset [Member] | Less - Allowance for deferred asset [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (37,012) (18,166)  
Deferred tax expense (income) (1,898) (18,846)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) (38,910) (37,012) (18,166)
Deferred asset [Member] | Deferred asset for income tax before allowance [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 215,713 185,037  
Deferred tax expense (income) 10,790 30,676  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) 226,503 215,713 185,037
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) 93,050 78,409  
Deferred tax expense (income) 1,889 14,641  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) 94,939 93,050 78,409
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) 60,883 52,377  
Deferred tax expense (income) 734 8,506  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) 61,617 60,883 52,377
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) 39,538 32,644  
Deferred tax expense (income) 5,030 6,894  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) 44,568 39,538 32,644
Deferred asset [Member] | Deferred asset for income tax before allowance [Member] | Environmental liability for Santa Barbara mine [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 1,569 1,556  
Deferred tax expense (income) (273) 13  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) 1,296 1,569 1,556
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) 13,081 14,866  
Deferred tax expense (income) 1,082 (1,785)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) 14,163 13,081 14,866
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) 7,592 5,185  
Deferred tax expense (income) 2,328 2,407  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) 9,920 7,592 5,185
Deferred asset [Member] | Deferred asset included in retained earnings [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 179,841 169,312  
Deferred tax expense (income) 8,892 11,830  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 7,963 (1,301)  
Deferred tax liability (asset) 196,696 179,841 169,312
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) 1,140 2,441  
Deferred tax expense (income) 0 0  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 7,963 (1,301)  
Deferred tax liability (asset) 9,103 1,140 2,441
Deferred asset [Member] | Deferred assets for mining royalties [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 43 (141)  
Deferred tax expense (income) 80 184  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) 123 43 (141)
Deferred asset [Member] | Deferred assets for mining royalties [Member] | Other temporary differences [member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 5 185  
Deferred tax expense (income) 118 (180)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) 123 5 185
Deferred asset [Member] | Deferred assets for mining royalties [Member] | Exploration expenses [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 38 (326)  
Deferred tax expense (income) (38) 364  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) 0 38 (326)
Deferred asset [Member] | Deferred asset for income tax after allowance [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) 178,701 166,871  
Deferred tax expense (income) 8,892 11,830  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) 187,593 178,701 166,871
Deferred liability [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (166,333) (140,259)  
Deferred tax expense (income) (3,147) (26,074)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) (169,480) (166,333) (140,259)
Deferred liability [Member] | Deferred liability for income tax [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (166,298) (140,423)  
Deferred tax expense (income) (3,021) (25,875)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) (169,319) (166,298) (140,423)
Deferred liability [Member] | Deferred liability for income tax [Member] | Other temporary differences [member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (43,985) (34,582)  
Deferred tax expense (income) (33,618) (9,403)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) (77,603) (43,985) (34,582)
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) (70,525) (73,537)  
Deferred tax expense (income) 24,502 3,012  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) (46,023) (70,525) (73,537)
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) (51,788) (32,304)  
Deferred tax expense (income) 6,095 (19,484)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) (45,693) (51,788) (32,304)
Deferred liability [Member] | Deferred liability for mining royalties [Member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (35) 164  
Deferred tax expense (income) (126) (199)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) (161) (35) 164
Deferred liability [Member] | Deferred liability for mining royalties [Member] | Other temporary differences [member]      
Disclosure of Deferred income tax [Line Items]      
Deferred tax liability (asset) (35) 164  
Deferred tax expense (income) (126) (199)  
Income tax relating to components of other comprehensive income that will be reclassified to profit or loss 0 0  
Deferred tax liability (asset) $ (161) $ (35) $ 164