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ASSET RETIREMENT OBLIGATION: (Details)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2020
USD ($)
item
Mar. 31, 2019
USD ($)
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   28.1  
Closure Payments (0.6) (0.2)  
Accretion expense 3.8 3.0  
Balance at the end of the year $ 265.5 $ 248.6 262.3
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