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PROPERTY, PLANT AND EQUIPMENT - Rollforward (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance $ 663,828  
Property write downs   $ 2,466
Ending balance 658,629 663,828
Gross carrying amount    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance 1,041,565 645,736
Acquisition of Seabee Gold Operation (note 3)   331,381
Additions 111,203 94,637
Disposals (14,555) (19,583)
Impairment reversal 24,357  
Property write downs (1,646)  
Change in estimate of close down and restoration provision (9,128) (42)
Transfers 0 (10,564)
Ending balance 1,151,796 1,041,565
Accumulated depreciation and amortisation    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance (377,737) (297,024)
Charge for the year (127,254) (84,879)
Disposals 11,824 4,166
Ending balance (493,167) (377,737)
Plant and equipment    
Disclosure of detailed information about property, plant and equipment [line items]    
Construction in progress 17,307 6,113
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance 232,838  
Ending balance 229,829 232,838
Plant and equipment | Gross carrying amount    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance 509,008 425,157
Acquisition of Seabee Gold Operation (note 3)   52,318
Additions 33,738 42,625
Disposals (13,555) (10,948)
Impairment reversal 24,357  
Property write downs 0  
Change in estimate of close down and restoration provision (8,458) 0
Transfers 0 (144)
Ending balance 545,090 509,008
Plant and equipment | Accumulated depreciation and amortisation    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance (276,170) (233,023)
Charge for the year (50,915) (47,313)
Disposals 11,824 4,166
Ending balance (315,261) (276,170)
Mineral properties subject to depreciation    
Disclosure of detailed information about property, plant and equipment [line items]    
Construction in progress 3,715 1,833
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance 204,710  
Ending balance 215,367 204,710
Mineral properties subject to depreciation | Gross carrying amount    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance 306,277 142,397
Acquisition of Seabee Gold Operation (note 3)   62,229
Additions 43,118 48,940
Disposals 0 0
Impairment reversal 0  
Property write downs (747)  
Change in estimate of close down and restoration provision (670) (42)
Transfers 45,295 52,753
Ending balance 393,273 306,277
Mineral properties subject to depreciation | Accumulated depreciation and amortisation    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance (101,567) (64,001)
Charge for the year (76,339) (37,566)
Disposals 0 0
Ending balance (177,906) (101,567)
Mineral properties not yet subject to depreciation    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance 89,288  
Ending balance 77,582 89,288
Mineral properties not yet subject to depreciation | Gross carrying amount    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance 89,288 0
Acquisition of Seabee Gold Operation (note 3)   128,100
Additions 33,589 946
Disposals 0 0
Impairment reversal 0  
Property write downs 0  
Change in estimate of close down and restoration provision 0 0
Transfers (45,295) (39,758)
Ending balance 77,582 89,288
Mineral properties not yet subject to depreciation | Accumulated depreciation and amortisation    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance 0 0
Charge for the year 0 0
Disposals 0 0
Ending balance 0 0
Exploration and evaluation assets    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance 136,992  
Ending balance 135,851 136,992
Exploration and evaluation assets | Gross carrying amount    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance 136,992 78,182
Acquisition of Seabee Gold Operation (note 3)   88,734
Additions 758 2,126
Disposals (1,000) (8,635)
Impairment reversal 0  
Property write downs (899)  
Change in estimate of close down and restoration provision 0 0
Transfers 0 (23,415)
Ending balance 135,851 136,992
Exploration and evaluation assets | Accumulated depreciation and amortisation    
Reconciliation of changes in property, plant and equipment [abstract]    
Beginning balance 0 0
Charge for the year 0 0
Disposals 0 0
Ending balance $ 0 $ 0