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Supplemental Cash Flow Information (Other Non-cash Items) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of Cash Flow Statement [Abstract]    
Unrealized gains on non-hedged derivatives $ 0.3 $ (1.6)
(Gain) loss on prepayment option fair value adjustment 0.0 (9.5)
Reduction of obligation to renounce flow-through exploration expenditures (2.3) (1.3)
Loss on disposal of assets 1.9 4.6
Net realizable value adjustments (5.9) 0.0
Gain on termination of retained interest royalty 1.3 0.0
Other non-cash items 0.8 (2.3)
Other adjustments for non-cash items $ (3.9) $ (10.1)