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Finance Expense - Summary of Finance Expense (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
 [1]
Dec. 31, 2015
 [1]
Disclosure of finance expense [abstract]      
Interest expense - environmental rehabilitation $ (12.1) $ (10.7) $ (11.7)
Unwinding of discount on silicosis settlement costs (0.9) 0.0 0.0
Interest expense - borrowings (91.2) (82.5) (87.8)
Borrowing costs capitalised 22.9 15.1 16.6
Total finance expense $ (81.3) $ (78.1) $ (82.9)
[1] As Restated - Refer note 40 for further details.