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Investments - Summary of Investments (Parenthetical) (Detail) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2016
Disclosure of detailed information about investment property [line items]      
Redeemable preference shares 137,439,999 174,939,999  
Redeemable preference shares, par value $ 137,439,999 $ 174,939,999  
Fair value adjustments through other comprehensive income $ 34.6 $ 47.4  
Discount rates - real 8.40% 8.50%  
Marverix Metals Incorporated [member]      
Disclosure of detailed information about investment property [line items]      
Common share purchase warrants 4,125,000 4,125,000 10,000,000
Asanko Gold [member]      
Disclosure of detailed information about investment property [line items]      
Investment Fairvalue $ 24.7 $ 21.2  
Cardinal Resources Limited [member]      
Disclosure of detailed information about investment property [line items]      
Investment Fairvalue 0.0 17.1  
Magmatic Resources Limited [member]      
Disclosure of detailed information about investment property [line items]      
Investment Fairvalue 2.5 3.0  
Chakana Copper Corp [member]      
Disclosure of detailed information about investment property [line items]      
Investment Fairvalue 7.4 3.0  
Lefroy Exploration Limited [member]      
Disclosure of detailed information about investment property [line items]      
Investment Fairvalue $ 3.8 $ 2.4