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Impairment charge (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Jun. 30, 2021
Dec. 31, 2021
Dec. 31, 2020
Disclosure of impairment loss and reversal of impairment loss [line items]      
Pre-tax impairment charge   $ 224.3 $ 0.0
Deferred tax liability   (10.5)  
After-tax impairment charge $ 213.8 213.8  
Current assets      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Pre-tax impairment charge   5.5  
Plant and equipment      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Pre-tax impairment charge   0.3  
Mineral property - non-depletable      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Pre-tax impairment charge   142.4  
Exploration and evaluation      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Pre-tax impairment charge   $ 76.1