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INVENTORIES: (Details) (USD $)
 3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Dec. 31, 2011
Metals at lower of average cost or market:      
Finished goods $ 76,500,000   $ 93,300,000
Work-in-process 258,500,000   296,100,000
Supplies at average cost 274,100,000   262,500,000
Total current inventory 609,079,000   651,896,000
Inventory, long-term      
Leachable material, net 153,990,000   122,985,000
Leaching costs capitalized as long-term inventory of leachable material 52,700,000 25,200,000  
Long-term leaching inventories recognized as cost of sales $ 18,400,000 $ 10,600,000