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Environmental Trust Funds (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Schedule of Environmental Trust Funds [Line Items]      
Balance at beginning of the year $ 98.8 $ 88.1  
Contributions 10.7 11.1  
Interest earned 1.2 1.1 $ 0.8
Translation adjustment (1.1) (1.5)  
Balance at end of the year 109.6 98.8 $ 88.1
South Africa      
Schedule of Environmental Trust Funds [Line Items]      
Term deposit 20.7 20.0  
Ghana      
Schedule of Environmental Trust Funds [Line Items]      
Cash deposit $ 88.9 $ 78.8