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ASSET RETIREMENT OBLIGATION: (Details) (USD $)
 3 Months Ended 9 Months Ended 12 Months Ended 57 Months Ended
Sep. 30, 2014
Sep. 30, 2014
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Sep. 30, 2013
Dec. 31, 2012
Sep. 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 $ 27,800,000 $ 27,800,000      
Cumulative guarantee amount 14,200,000 14,200,000      
Number of units with future closure costs recognized as an asset retirement obligation   3      
Environmental costs          
Plant demolition and soil remediation budgeted cost         62,400,000
Environmental remediation 37,163,000 37,163,000     43,800,000
Additions       25,100,000  
Increase to net property due to closure of mine       20,300,000  
Asset retirement obligation activity          
Balance at the beginning of the period   124,800,000 122,300,000    
Changes in estimates   26,700,000      
Closure payments   (8,100,000) (2,300,000)    
Accretion expense   5,800,000 6,700,000    
Balance at the end of the period $ 149,200,000 $ 149,200,000 $ 126,700,000 $ 122,300,000