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INVENTORIES - Additional information (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of inventories    
Inventory write-down $ 14.1 $ 15.8
Operating expense 371.9 325.4
Stockpile Ore at Rainy River [Member]    
Disclosure of inventories    
Inventory write-down 19.8  
Operating expense 14.1  
Depreciation expense 5.7  
Inventories recognized [Member]    
Disclosure of inventories    
Operating expense $ 358.0 $ 311.6