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Share capital (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2021
USD ($)
shares
Dec. 31, 2020
USD ($)
shares
Dec. 31, 2020
$ / shares
Disclosure of classes of share capital [abstract]      
Beginning balance (in shares) | shares 680,250,000 675,957,000  
Common shares, beginning of period $ 3,154.0 $ 3,144.5  
Issuance of common shares under First Nations agreements (in shares) | shares 250,000 350,000  
Issuance of shares under First Nations agreements and land purchases $ 0.4 $ 0.7  
Issuance of flow through shares (in shares) | shares   3,824,000  
Issuance of flow through shares   $ 8.6  
Exercise of options and vested performance share units (in shares) | shares 646,000 119,000  
Exercise of options and vested performance share units $ 1.0 $ 0.2  
Ending balance (in shares) | shares 681,146,000 680,250,000  
Common shares, end of period $ 3,155.4 $ 3,154.0  
Proceeds received from issuance of shares $ 0.2 $ 10.2  
Shares issued in equity offering, price per share (in dollars per share) | $ / shares     $ 3.40