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COMMITTMENTS AND CONTINGENCIES: (Details 4) (USD $)
 3 Months Ended 12 Months Ended 13 Months Ended 1 Months Ended 12 Months Ended 12 Months Ended 2 Months Ended 1 Months Ended 3 Months Ended
Dec. 31, 1997
Y
Dec. 31, 2011
Peru
Dec. 31, 2011
Peru
Dec. 31, 2011
Peru
Maximum
Sep. 30, 2011
Peru
Maximum
Sep. 30, 2011
Peru
Minimum
Dec. 31, 2011
Peru
Minimum
Dec. 31, 2011
Toquepala and Cuajone mines
Peru
Dec. 31, 2010
Toquepala and Cuajone mines
Peru
Dec. 31, 2009
Toquepala and Cuajone mines
Peru
Dec. 31, 2011
Toquepala and Cuajone mines
Peru
Maximum
Dec. 31, 2011
Tia Maria
t
Dec. 31, 2011
Tia Maria
Peru
Dec. 31, 2011
Asarco LLC
Toquepala and Cuajone mines
Peru
Minimum
Jan. 31, 2005
Minera Mexico, S.A. de C.V. (the "Transaction")
lawsuit
Oct. 31, 2011
Americas Mining Corporation ("AMC") merger transaction
Oct. 31, 2010
Americas Mining Corporation ("AMC") merger transaction
lawsuit
Oct. 31, 2010
Americas Mining Corporation ("AMC") merger transaction
Asarco LLC
lawsuit
Litigation matter                                    
Number of purported class action derivative lawsuits filed                             3   4 2
Number of purported class action derivative lawsuits which are consolidated                             3      
Number of consolidated purported class action derivative lawsuits                             1      
Damages awarded by court                               $ 1,347,000,000    
Pre-adjustment interest                               684,600,000    
Attorney's fees and expenses awarded, option 1                               304,700,000    
Attorney's fees and expenses awarded, option 2 (as a percent)                               15.00%    
Other commitments:                                    
Estimated annual production of copper cathodes (in tons)                       120,000            
Percentage of royalty expense                     3.00%     1.00%        
Mining Royalty charge               52,500,000 65,500,000 43,700,000                
Mining royalty tax (as a percent)         12.00% 1.00%                        
Operating income margin (as a percent)     10.00%                              
Increment in operating income margin (as a percent)       85.00%     5.00%                      
Increase in royalty tax rate for each 5% increase in operating income margin (as a percent)     0.75%                              
Provision for mining royalty tax     19,300,000                              
Term of power purchase agreement related to sale of power plant (in years) 20                                  
Total budget for the project                         100,000,000          
Amount invested in mine and leach equipment and required studies                         434,100,000          
Provision for special mining tax   $ 16,400,000