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INVENTORIES: (Details) (USD $)
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2012
Metals at lower of average cost or market:      
Finished goods $ 93,500,000   $ 101,100,000
Work-in-process 303,600,000   297,400,000
Supplies at average cost 295,300,000   284,200,000
Total current inventory 692,369,000   682,749,000
Inventory, long-term      
Leach stockpiles 284,290,000   262,795,000
Leaching costs capitalized as long-term inventory of leachable material 68,200,000 52,700,000  
Leachable material inventories recognized as cost of sales $ 26,700,000 $ 18,400,000