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Other (Loss) Gain (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Other Gain (Loss) [Abstract]    
Reduction of obligation to renounce flow-through exploration expenditures $ 1.0 $ 0.6
Unrealized gains on non-hedged derivatives 0.7 (0.6)
Fair value adjustment on prepayment option embedded derivative 3.5 2.0
Loss on disposal of assets (1.0) (0.7)
Reclamation, care and maintenance 0.6 8.0
Other (5.0) (2.2)
Other gains (losses) $ (3.7) $ 5.1