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Finance Expense - Summary of Finance Expense (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Disclosure of transactions between related parties [line items]      
Interest expense – environmental rehabilitation $ (8.6) $ (10.7) $ (11.7)
Unwinding of discount rate on silicosis settlement costs (1.1) (1.5) (1.3)
Interest expense – lease liability (24.1) (22.4) (18.6)
Interest expense – borrowings (79.6) (105.3) (114.0)
Borrowing costs capitalised 12.5 13.2 43.4
Total finance expense (100.9) (126.7) (102.2)
Borrowing costs 12.5 13.2 43.4
Additions to property plant and equipment     612.5
General borrowings      
Disclosure of transactions between related parties [line items]      
Transaction costs 12.5 13.2 31.0
Specific borrowings      
Disclosure of transactions between related parties [line items]      
Transaction costs $ 0.0 $ 0.0 $ 12.4