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Finance Expense - Summary of Finance Expense (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of finance expense [abstract]      
Interest expense - environmental rehabilitation $ (11.7) $ (12.1) $ (10.7)
Unwinding of discount on silicosis settlement costs (2.0) (0.9) 0.0
Interest expense - finance lease liability (0.2) 0.0 0.0
Interest expense - borrowings (91.6) (91.2) (82.5)
Borrowing costs capitalised 17.5 22.9 15.1
Total finance expense $ (88.0) $ (81.3) $ (78.1)