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PROPERTY, PLANT AND MINE DEVELOPMENT (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Property, plant and mine development    
Property, plant and equipment at beginning of period $ 5,106,036 $ 5,088,967
Additions 1,044,540 576,209
Gain on impairment reversal   120,161
Disposals (16,104) (19,548)
Amortization (507,920) (659,753)
Property, plant and equipment at end of period 5,626,552 5,106,036
Mining Properties    
Property, plant and mine development    
Property, plant and equipment at beginning of period 1,605,536 1,665,610
Additions 174,374 53,072
Gain on impairment reversal   83,992
Disposals (6,750) (1,890)
Amortization (127,579) (207,383)
Transfers between categories 19,946 12,135
Property, plant and equipment at end of period 1,665,527 1,605,536
Plant and Equipment    
Property, plant and mine development    
Property, plant and equipment at beginning of period 2,024,283 2,064,406
Additions 221,924 244,018
Gain on impairment reversal   36,169
Disposals (9,354) (17,658)
Amortization (276,493) (342,208)
Transfers between categories 30,761 39,556
Property, plant and equipment at end of period 1,991,121 2,024,283
Mine Development Costs    
Property, plant and mine development    
Property, plant and equipment at beginning of period 1,476,217 1,358,951
Additions 648,242 279,119
Amortization (103,848) (110,162)
Transfers between categories (50,707) (51,691)
Property, plant and equipment at end of period 1,969,904 1,476,217
Assets under construction    
Property, plant and mine development    
Property, plant and equipment at beginning of period 532,300  
Property, plant and equipment at end of period 910,600 532,300
Gross carrying amount    
Property, plant and mine development    
Property, plant and equipment at beginning of period 8,878,584  
Property, plant and equipment at end of period 10,033,352 8,878,584
Gross carrying amount | Mining Properties    
Property, plant and mine development    
Property, plant and equipment at beginning of period 2,593,659  
Property, plant and equipment at end of period 2,782,732 2,593,659
Gross carrying amount | Plant and Equipment    
Property, plant and mine development    
Property, plant and equipment at beginning of period 4,233,945  
Property, plant and equipment at end of period 4,602,106 4,233,945
Gross carrying amount | Mine Development Costs    
Property, plant and mine development    
Property, plant and equipment at beginning of period 2,050,980  
Property, plant and equipment at end of period 2,648,514 2,050,980
Accumulated depreciation, amortization and impairment    
Property, plant and mine development    
Property, plant and equipment at beginning of period (3,772,548)  
Property, plant and equipment at end of period (4,406,800) (3,772,548)
Accumulated depreciation, amortization and impairment | Mining Properties    
Property, plant and mine development    
Property, plant and equipment at beginning of period (988,123)  
Property, plant and equipment at end of period (1,117,205) (988,123)
Accumulated depreciation, amortization and impairment | Plant and Equipment    
Property, plant and mine development    
Property, plant and equipment at beginning of period (2,209,662)  
Property, plant and equipment at end of period (2,610,985) (2,209,662)
Accumulated depreciation, amortization and impairment | Mine Development Costs    
Property, plant and mine development    
Property, plant and equipment at beginning of period (574,763)  
Property, plant and equipment at end of period $ (678,610) $ (574,763)