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ASSET RETIREMENT OBLIGATION: (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
item
Sep. 30, 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 $ 7.0   $ 23.5
Asset retirement obligation activity      
Balance at the beginning of the year   $ 190.9 116.1
Payments   (1.7) (15.3)
Accretion expense   21.2 13.1
Balance at the end of the year $ 113.9 210.4 $ 113.9
Mexico      
Environmental cost      
Plant demolition and soil remediation budgeted cost   66.2  
Plant demolition and soil remediation spent   $ 65.7