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Acquisition of Richmond Mines Ltd. (Schedule of Asset Acquisition) (Details) - USD ($) $ in Millions | Nov. 23, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | --- | ---
Net assets acquired |  |  |  |
Cash and cash equivalents |  | $ 200.8 | $ 252.2 | $ 282.9
Equity securities |  | 35.8 | 14.1 |
Mineral property, plant and equipment |  | 2,753.4 | 1,918.2 |
Current liabilities |  | (106.7) | (99.6) |
Deferred tax liabilities |  | $ (477.0) | $ (291.0) |
Richmont Mines Ltd [Member] |  |  |  |
Consideration transferred |  |  |  |
Common shares issued | $ 615.3 |  |  |
Replacement of long-term incentive instruments | 12.1 |  |  |
Consideration transferred | 627.4 |  |  |
Net assets acquired |  |  |  |
Cash and cash equivalents | 46.2 |  |  |
Equity securities | 11.7 |  |  |
Current assets, excluding cash and cash equivalents and equity securities | 22.5 |  |  |
Mineral property, plant and equipment | 774.3 |  |  |
Current liabilities | (21.2) |  |  |
Long-term liabilities | (4.6) |  |  |
Deferred tax liabilities | (201.5) |  |  |
Net assets acquired | $ 627.4 |  |  |

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