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Exploration Expense - Summary of exploration expenses (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of geographical areas [line items]      
Expense arising from exploration for and evaluation of mineral resources $ (49.7) $ (84.4) $ (104.2)
Australia      
Disclosure of geographical areas [line items]      
Expense arising from exploration for and evaluation of mineral resources (16.9) (30.0) (37.7)
Ghana      
Disclosure of geographical areas [line items]      
Expense arising from exploration for and evaluation of mineral resources 0.0 0.0 (0.4)
Peru      
Disclosure of geographical areas [line items]      
Expense arising from exploration for and evaluation of mineral resources (1.4) (4.4) (1.1)
Chile      
Disclosure of geographical areas [line items]      
Expense arising from exploration for and evaluation of mineral resources (30.1) (49.1) (61.5)
Other      
Disclosure of geographical areas [line items]      
Expense arising from exploration for and evaluation of mineral resources $ (1.3) $ (0.9) $ (3.5)