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DERIVATIVE INSTRUMENTS (Details 3) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
(LOSS) Gain on swap contracts    
Realized $ 12.6 $ (18.8)
Unrealized 0.9 (6.1)
Total gain (loss) 13.5 (24.9)
Gold [Member]    
(LOSS) Gain on swap contracts    
Realized 1.3 (2.0)
Unrealized (0.8) (0.3)
Total gain (loss) 0.5 (2.3)
Copper [Member]    
(LOSS) Gain on swap contracts    
Realized 11.3 (16.8)
Unrealized 1.7 (5.8)
Total gain (loss) $ 13.0 $ (22.6)