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LONG-TERM DEBT AND CREDIT FACILITIES - Corporate revolving credit and term loan facilities (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 16, 2019
Mar. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Long-term debt        
Proceeds from drawdown of debt     $ 300.0 $ 80.0
Repayment of debt     200.0 80.0
Letters of credit and surety bonds     1,837.4 1,735.0
Tasiast Mauritanie Ltd. S.A.        
Long-term debt        
Maximum borrowing capacity $ 300.0      
Term of loan 8 years      
Interest rate (as a percent) 4.38%      
Standby fee (as percent of unused availablity) 1.60%      
Revolving Credit Facility        
Long-term debt        
Facility utilized     119.1 $ 19.7
Maximum borrowing capacity     1,500.0  
Proceeds from drawdown of debt     $ 200.0  
Borrowings, maturity     August 10, 2024  
Repayment of debt   $ 100.0 $ 300.0  
Standby fee (as percent of unused availablity)     0.325%  
Revolving Credit Facility | LIBOR        
Long-term debt        
Interest rate (as a percent)     1.625%  
Letter of credit        
Long-term debt        
Repayment of debt     $ 19.1  
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%