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ASSET RETIREMENT OBLIGATION: (Details)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
item
Mar. 31, 2017
USD ($)
Dec. 31, 2017
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 $ 36.9    
Cumulative guarantee amount $ 32.3    
Number of units with future closure costs recognized as an asset retirement obligation | item 3    
Changes in estimates $ (5.2)   $ (11.6)
Asset retirement obligation activity      
Balance at the beginning of the year 222.5 $ 216.5 216.5
Changes in estimates (5.2)   (11.6)
Payments   (0.3)  
Accretion expense 3.2 6.1  
Balance at the end of the year 220.5 $ 222.3 $ 222.5
Mexico      
Environmental cost      
Plant demolition and soil remediation budgeted cost $ 66.2