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Investments - Summary of Investments (Parenthetical) (Detail) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2016
Disclosure of detailed information about investment property [line items]      
Redeemable preference shares 174,939,999 164,939,999  
Redeemable preference shares, par value $ 174,939,999 $ 164,939,999  
Fair value adjustments through other comprehensive income $ 47.4    
Marverix Metals Incorporated [Member]      
Disclosure of detailed information about investment property [line items]      
Common share purchase warrants 4,125,000 10,000,000 10,000,000
Asanko Gold [member]      
Disclosure of detailed information about investment property [line items]      
Investment Fairvalue $ 21.2    
Cardinal Resources Limited [member]      
Disclosure of detailed information about investment property [line items]      
Investment Fairvalue 17.1    
Magmatic Resources Limited [member]      
Disclosure of detailed information about investment property [line items]      
Investment Fairvalue 3.0    
Chakana Copper Corp [member]      
Disclosure of detailed information about investment property [line items]      
Investment Fairvalue $ 3.0