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Investments - Summary of Investments (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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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 |

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