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Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Non-capital losses carried forward | $ (477.0) | $ (291.0) |
Deferred income tax liability | 477.0 | 291.0 | $ 284.1
Accounting value of mineral property, plant and equipment in excess of tax value |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred tax assets | 409.0 | 270.8 |
Accounting value of inventories in excess of tax value |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred tax assets | 34.7 | 36.8 |
Other taxable temporary differences |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred tax assets | 58.6 | 18.1 |
Non-capital losses carried forward |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Non-capital losses carried forward | $ (25.3) | $ (34.7) |

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