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ASSET RETIREMENT OBLIGATION: (Details)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2021
USD ($)
item
Sep. 30, 2020
USD ($)
Dec. 31, 2020
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 $ 56.5      
Asset retirement obligation activity        
Balance at the beginning of the year 545.0 $ 262.3 $ 262.3  
Closure Payments (3.7) (1.0)    
Accretion expense 18.3 11.1    
Balance at the end of the year $ 559.6 $ 272.4 545.0 $ 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
Mexico        
Asset retirement obligation activity        
Changes in estimates     $ 269.3