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ASSET RETIREMENT OBLIGATION: (Details) (USD $)
In Millions, unless otherwise specified
6 Months Ended 12 Months Ended 54 Months Ended
Jun. 30, 2014
item
Y
Jun. 30, 2013
Dec. 31, 2012
Jun. 30, 2014
Mexican operations
ASSET RETIREMENT OBLIGATION:        
Period after which successive reviews are required by the law (in years) 5      
Accepted value of the Lima office building $ 27.8      
Cumulative guarantee amount 14.2      
Number of units with future closure costs recognized as an asset retirement obligation 3      
Environmental costs        
Plant demolition and soil remediation budgeted cost       62.4
Plant demolition and soil cost recognized       39.3
Additions     25.1  
Increase to net property due to closure of mine     20.3  
Asset retirement obligation activity        
Balance at the beginning of the period 124.8 122.3    
Changes in estimates 26.7      
Closure payments (3.6) (0.9)    
Accretion expense 3.6 4.2    
Balance at the end of the period $ 151.5 $ 125.6 $ 122.3