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CURRENT AND DEFERRED INCOME TAX - Deferred Income Tax Assets and Liabilities (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Net balance at beginning of year $ (114,576) $ (115,791)
Recognized in statement of income (78) (27)
Recognized in OCI 14,717 1,242
AMT refund (449)  
Net, ending balance (100,386) (114,576)
Deferred tax assets (7,523) 0
Deferred tax liabilities (107,909) (114,576)
Marketable securities    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Net balance at beginning of year (13,205) (16,949)
Recognized in statement of income 376 0
Recognized in OCI 13,867 3,744
AMT refund 0  
Net, ending balance 1,038 (13,205)
Inventory    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Net balance at beginning of year (7,244) (1,984)
Recognized in statement of income 3,733 (5,260)
Recognized in OCI 0 0
AMT refund 0  
Net, ending balance (3,511) (7,244)
Property, plant and equipment    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Net balance at beginning of year (81,294) (96,614)
Recognized in statement of income (10,452) 15,320
Recognized in OCI 0 0
AMT refund 0  
Net, ending balance (91,746) (81,294)
Close down and restoration provision    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Net balance at beginning of year 1,787 3,917
Recognized in statement of income 1,567 (2,130)
Recognized in OCI 0 0
AMT refund 0  
Net, ending balance 3,354 1,787
Convertible notes    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Net balance at beginning of year (4,056) (5,497)
Recognized in statement of income 1,820 1,441
Recognized in OCI 0 0
AMT refund 0  
Net, ending balance (2,236) (4,056)
Carry forward tax loss and tax credits    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Net balance at beginning of year 21,830 20,682
Recognized in statement of income (6,469) 3,404
Recognized in OCI 0 (2,256)
AMT refund 0  
Net, ending balance 15,361 21,830
Mining and foreign withholding tax    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Net balance at beginning of year (39,227) (37,151)
Recognized in statement of income (1,916) (2,076)
Recognized in OCI 0 0
AMT refund 0  
Net, ending balance (41,143) (39,227)
Executive compensation plans    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Net balance at beginning of year 2,065 2,582
Recognized in statement of income 1,918 (517)
Recognized in OCI 0 0
AMT refund 0  
Net, ending balance 3,983 2,065
Deductibility of other taxes    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Net balance at beginning of year 9,021 9,459
Recognized in statement of income 424 (438)
Recognized in OCI 0 0
AMT refund 0  
Net, ending balance 9,445 9,021
Other    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Net balance at beginning of year (4,253) 5,764
Recognized in statement of income 8,921 (9,771)
Recognized in OCI 850 (246)
AMT refund (449)  
Net, ending balance 5,069 (4,253)
Before Offset Amount    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets (39,321) (38,614)
Deferred tax liabilities (139,707) (153,190)
Before Offset Amount | Marketable securities    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets (1,038) 0
Deferred tax liabilities 0 (13,205)
Before Offset Amount | Inventory    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets 0 0
Deferred tax liabilities (3,511) (7,244)
Before Offset Amount | Property, plant and equipment    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets (940) (1,016)
Deferred tax liabilities (92,686) (82,310)
Before Offset Amount | Close down and restoration provision    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets (3,354) (1,787)
Deferred tax liabilities 0 0
Before Offset Amount | Convertible notes    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets 0
Deferred tax liabilities (2,236) (4,056)
Before Offset Amount | Carry forward tax loss and tax credits    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets (15,361) (21,830)
Deferred tax liabilities 0 0
Before Offset Amount | Mining and foreign withholding tax    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets 0 0
Deferred tax liabilities (41,143) (39,227)
Before Offset Amount | Executive compensation plans    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets (3,983) (2,065)
Deferred tax liabilities 0 0
Before Offset Amount | Deductibility of other taxes    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets (9,445) (9,021)
Deferred tax liabilities 0 0
Before Offset Amount | Other    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets (5,200) (2,895)
Deferred tax liabilities (131) (7,148)
Offset Amount    
Reconciliation of changes in deferred tax liability (asset) [abstract]    
Deferred tax assets (31,798) (38,614)
Deferred tax liabilities $ (31,798) $ (38,614)