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Supplemental Cash Flow Information (Other Non-cash Items) (Details) - USD ($) $ in Millions | 12 Months Ended
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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)

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- Definition Noncash Gain on Termination of Retained Interest Royalty + References No definition available. + Details
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