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PROVISIONS (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Provisions reconciliation    
Provisions at beginning of period $ 896.5  
Additions 53.6  
Reductions (15.2)  
Reclamation spending (39.6)  
Accretion 23.0  
Reclamation expense 6.6  
Provisions at end of period 924.9 $ 896.5
Current portion 63.8  
Non-current portion 861.1  
Other operating expense 186.5 108.5
Reclamation and remediation obligations    
Provisions reconciliation    
Provisions at beginning of period 866.1  
Additions 45.0  
Reductions (5.0)  
Reclamation spending (39.6)  
Accretion 23.0  
Reclamation expense 6.6  
Provisions at end of period 896.1 866.1
Current portion 58.7  
Non-current portion 837.4  
Other operating expense 6.6 11.9
Letter of credit issued to various agencies 379.9 391.9
Surety bonds $ 289.3 $ 275.7
Reclamation and remediation obligations | Minimum    
Provisions reconciliation    
Discount rate used in estimating site restoration cost obligation (in percent) 0.40% 1.70%
Inflation rate used in estimating the site restoration cost obligation (in percent) 2.10% 2.20%
Reclamation and remediation obligations | Maximum    
Provisions reconciliation    
Discount rate used in estimating site restoration cost obligation (in percent) 13.30% 14.70%
Inflation rate used in estimating the site restoration cost obligation (in percent) 4.00% 4.00%
Other    
Provisions reconciliation    
Provisions at beginning of period $ 30.4  
Additions 8.6  
Reductions (10.2)  
Provisions at end of period 28.8 $ 30.4
Current portion 5.1  
Non-current portion $ 23.7