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INVENTORIES: (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Metals at average cost:      
Finished goods $ 48.8 $ 130.5  
Work-in-process 308.0 231.6  
Ore stockpiles on leach pads 320.9 310.9  
Supplies at average cost 364.2 337.4  
Total current inventory 1,041.9 1,010.4  
Inventory, long-term:      
Ore stockpiles on leach pads 977.4 806.9  
Total leaching costs added as long-term inventory of ore stockpiles in leach pads 513.9 439.0 $ 506.9
Long-term leaching inventories recognized as cost of sales $ 333.5 $ 316.6 $ 274.1