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Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Non-capital losses carried forward $ (491.5) $ (477.0)  
Deferred income tax liability 491.5 477.0 $ 291.0
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 446.8 409.0  
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 13.8 34.7  
Other taxable temporary differences      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred tax assets 64.3 58.6  
Non-capital losses carried forward      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Non-capital losses carried forward $ (33.4) $ (25.3)