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Investments - Summary of Investments - Asanko preference shares (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Disclosure of detailed information about financial instruments [line items]      
Fair value at beginning of the year $ 92.6 $ 95.5  
Redemption of preference shares (5.0) (37.5)  
Net change in fair value (recognised in OCI) 6.9 34.6  
Fair value at end of the year $ 94.5 $ 92.6  
Nominal discount rate 9.00% 8.40%  
Maverix Metals Incorporated ("Maverix")      
Disclosure of detailed information about financial instruments [line items]      
Common share purchase warrants 0 4,125,000 4,125,000