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ASSET RETIREMENT OBLIGATION: (Details)
$ in Millions
3 Months Ended
Mar. 31, 2016
USD ($)
item
Mar. 31, 2015
USD ($)
ASSET RETIREMENT OBLIGATION:    
Period after which successive reviews are required by the law (in years) 5 years  
Accepted value of the Lima office building $ 27.8  
Cumulative guarantee amount $ 21.7  
Number of units with future closure costs recognized as an asset retirement obligation | item 3  
Environmental cost    
Environmental remediation   $ 5.9
Asset retirement obligation activity    
Balance at the beginning of the year $ 190.9 116.1
Payments (0.7) (4.2)
Accretion expense 6.8 2.9
Balance at the end of the year 197.0 $ 114.8
Mexico    
Environmental cost    
Plant demolition and soil remediation budgeted cost 66.2  
Environmental remediation $ 64.7