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Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Non-capital losses carried forward $ (559.9) $ (513.7)  
Deferred income tax liability 559.9 513.7 $ 491.5
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 591.8 557.0  
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 8.8 7.9  
Other taxable temporary differences      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Non-capital losses carried forward (25.5) (19.8)  
Non-capital losses carried forward      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Non-capital losses carried forward $ (15.2) $ (31.4)