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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
segment
item
$ / lb
Dec. 31, 2017
$ / lb
Dec. 31, 2016
$ / lb
Jan. 01, 2019
USD ($)
Significant Accounting Policies        
Increase in total reserves at a property to qualify as a major expansion (as a percent) 10.00%      
Period considered for calculation of average metal price 3 years      
Current price of copper (in dollars per pound) | $ / lb 2.64 2.50 2.61  
U.S. statutory rate (as a percent) 21.00% 35.00% 35.00%  
Impairment in the carrying value of long-term assets $ 0.0      
Number of years of average prices for copper and molybdenum used for impairment analysis 3 years      
Number of reportable segments | segment 3      
Long-term contracts with promises to deliver mineral 1,724,840      
ASU 2016-02 | Restatement        
Significant Accounting Policies        
Right of use assets       $ 1,020.4
Lease obligations       $ 1,115.9
Minimum        
Financial information related to segments        
Estimated useful lives of buildings and equipment 5 years      
Significant Accounting Policies        
Percentage Of Metal Production Sold Under Annual Or Longer Term Contracts 80.00%      
Maximum        
Financial information related to segments        
Maximum period after shipping within which pricing is based by customer contracts in most cases 3 months      
Estimated useful lives of buildings and equipment 40 years      
Significant Accounting Policies        
Percentage Of Metal Production Sold Under Annual Or Longer Term Contracts 90.00%      
Peruvian Operations        
Financial information related to segments        
Number of open-pit copper mines | item 2      
Mexican IMMSA Unit        
Financial information related to segments        
Number of underground poly metal mines | item 5