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INVENTORIES: (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Metals at average cost:      
Finished goods $ 82.1   $ 87.2
Work-in-process 263.8   259.7
Ore stockpiles on leach pads 385.1   358.4
Supplies at average cost 348.4   363.2
Total current inventory 1,079.4   1,068.5
Inventory, long-term:      
Ore stockpiles on leach pads 1,149.0   $ 1,230.3
Total leaching costs added as long-term inventory of ore stockpiles in leach pads 50.7 $ 122.3  
Long-term leaching inventories recognized as cost of sales $ 105.9 $ 94.1