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Exploration Expense - Summary of exploration expenses (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Disclosure of geographical areas [line items]      
Total exploration expense $ (60.6) $ (49.7) $ (84.4)
Australia      
Disclosure of geographical areas [line items]      
Total exploration expense (21.3) (16.9) (30.0)
Ghana      
Disclosure of geographical areas [line items]      
Total exploration expense (9.6) 0.0 0.0
Peru      
Disclosure of geographical areas [line items]      
Total exploration expense (1.6) (1.4) (4.4)
Chile      
Disclosure of geographical areas [line items]      
Total exploration expense (27.2) (30.1) (49.1)
Other      
Disclosure of geographical areas [line items]      
Total exploration expense $ (0.9) $ (1.3) $ (0.9)