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Long-term Incentive Plan - Summary of Long-term Incentive Plan (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of long-term incentive plan [abstract]      
Opening balance $ 11.5 $ 2.1  
Charge to income statement 51.3 9.1 $ 1.1
Salares Norte project costs capitalised 0.6    
Translation adjustment 3.8 0.3  
Balance at end of the year 67.2 11.5 $ 2.1
Current portion of long-term incentive plan (33.8) 0.0  
Non-current portion of long-term incentive plan $ 33.4 $ 11.5