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ASSET RETIREMENT OBLIGATION: (Details)
$ in Thousands
3 Months Ended 6 Months Ended 66 Months Ended
Jun. 30, 2015
USD ($)
Jun. 30, 2015
USD ($)
item
Jun. 30, 2014
USD ($)
Jun. 30, 2015
USD ($)
ASSET RETIREMENT OBLIGATION:        
Period after which successive reviews are required by the law (in years)   5 years    
Accepted value of the Lima office building $ 27,800 $ 27,800   $ 27,800
Cumulative guarantee amount 17,900 $ 17,900   17,900
Number of units with future closure costs recognized as an asset retirement obligation | item   3    
Environmental costs        
Environmental remediation 10,532 $ 16,460    
Asset retirement obligation activity        
Balance at the beginning of the period   116,100 $ 124,800  
Changes in estimates     26,700  
Payments   (11,200) (3,600)  
Accretion expense   8,200 3,600  
Balance at the end of the period $ 113,100 $ 113,100 $ 151,500 113,100
Mexico        
Environmental costs        
Plant demolition and soil remediation budgeted cost       62,400
Environmental remediation       $ 58,100