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ASSET RETIREMENT OBLIGATION: (Details)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2020
USD ($)
item
Jun. 30, 2019
USD ($)
Dec. 31, 2019
USD ($)
ASSET RETIREMENT OBLIGATION:      
Maximum annual guarantees secured 50.00%    
Percentage of guarantee for Lima complex 50.00%    
Percentage of security for LOC 50.00%    
Total guarantees $ 47.1    
Asset retirement obligation activity      
Balance at the beginning of the year 262.3 $ 217.7 $ 217.7
Changes in estimates   25.2  
Closure Payments (0.8) (0.4)  
Accretion expense 7.4 6.2  
Balance at the end of the year $ 268.9 $ 248.7 262.3
Peru      
ASSET RETIREMENT OBLIGATION:      
Period after which successive reviews are required by the law (in years) 5 years    
Number of units with future closure costs recognized as an asset retirement obligation | item 3    
Asset retirement obligation activity      
Changes in estimates     $ 28.1