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LONG-TERM DEBT AND CREDIT FACILITIES - Revolving credit and term loan facilities (Details) - USD ($)
$ in Millions
 1 Months Ended 12 Months Ended
Sep. 18, 2020
Jul. 24, 2020
Mar. 20, 2020
Dec. 16, 2019
Jul. 25, 2019
Feb. 29, 2020
Dec. 31, 2020
Dec. 31, 2019
Long-term debt                
Repayment of debt             $ 850.0 $ 200.0
Proceeds from drawdown of debt             950.0 300.0
Letters of credit and surety bonds             1,923.9 1,837.4
Tasiat Mauritamie Ltd. S.A                
Long-term debt                
Maximum borrowing capacity       $ 300.0        
Proceeds from drawdown of debt       200.0        
Remaining borrowing capacity       $ 100.0        
Amount held in separate bank account for repayments             $ 25.0  
Interest rate (as a percent)             4.38%  
Standby fee (as percent of unused availablity)             1.60%  
Revolving Credit Facility                
Long-term debt                
Facility utilized             $ 7.5 $ 119.1
Maximum borrowing capacity         $ 1,500.0   $ 1,500.0  
Repayment of debt $ 500.0 $ 250.0       $ 100.0    
Proceeds from drawdown of debt     $ 750.0          
Borrowings, maturity         August 10, 2024      
Interest rate (as a percent)             1.625%  
Standby fee (as percent of unused availablity)             0.325%  
Letter of credit | Minimum                
Long-term debt                
Interest rate (as a percent)             1.0833%  
Letter of credit | Maximum                
Long-term debt                
Interest rate (as a percent)             1.625%