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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
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)