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ASSET RETIREMENT OBLIGATION: (Details)
$ in Millions
12 Months Ended
Dec. 31, 2016
USD ($)
item
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Period after which successive reviews are required by the law (in years) 5 years    
Accepted value of the Lima office building $ 30.8    
Cumulative guarantee amount $ 26.9    
Number of units with future closure costs recognized as an asset retirement obligation | item 3    
Change in asset retirement obligation     $ 36.3
Environmental remediation   $ 45.0 91.4
Asset retirement obligation activity      
Balance at the beginning of the year $ 190.9 116.1  
Changes in estimates   3.8  
Additions 10.5 72.2  
Closure payments (2.0) (14.6)  
Accretion expense 17.1 13.4  
Balance at the end of the year 216.5 190.9 $ 116.1
Mexico      
Plant demolition and soil remediation budgeted cost 66.2    
Environmental remediation 66.0    
Toquepala Concentrator Expansion      
Asset retirement obligation activity      
Additions   $ 12.5  
Buenavista Project      
Asset retirement obligation activity      
Additions $ 9.5