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DERIVATIVE INSTRUMENTS - Swap contracts (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
(LOSS) GAIN ON SWAP CONTRACTS    
Realized $ (3.0) $ 12.6
Unrealized (1.3) 0.9
Total (loss) gain (4.3) 13.5
Gold    
(LOSS) GAIN ON SWAP CONTRACTS    
Realized (3.2) 1.3
Unrealized (0.4) (0.8)
Total (loss) gain (3.6) 0.5
Copper    
(LOSS) GAIN ON SWAP CONTRACTS    
Realized 0.2 11.3
Unrealized (0.9) 1.7
Total (loss) gain $ (0.7) $ 13.0