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INCOME TAXES: (Details 2) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2012
Minimum
  
Income Taxes  
Increase in unrecognized tax benefit $ 30
Maximum
  
Income Taxes  
Increase in unrecognized tax benefit 50
Peru
  
Income Taxes  
Operating income margin (as a percent) 10.00%
Increase in special mining tax rate for each 5% increase up to 85% increase in operating income margin (as a percent) 0.40%
Provision for special mining tax $ 11.9
Peru | Minimum
  
Income Taxes  
Special mining tax (as a percent) 2.00%
Increment in operating income margin (as a percent) 5.00%
Peru | Maximum
  
Income Taxes  
Special mining tax (as a percent) 8.40%
Increment in operating income margin (as a percent) 85.00%