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Finance Expense - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Statement [LineItems]      
Borrowing costs [1] $ 43.4 $ 17.5 $ 22.9
Additions to property plant and equipment 612.5    
General Borrowings [Member]      
Statement [LineItems]      
Borrowing costs incurred 31.0    
Specific Borrowings [Member]      
Statement [LineItems]      
Borrowing costs incurred 12.4    
Gruyere [member]      
Statement [LineItems]      
Specific borrowings relates to related parties $ 12.4    
[1] Borrowing costs capitalised of US$43.4 million comprise borrowing costs relating to general borrowings of US$31.0 million and specific borrowings of US$12.4 million. The specific borrowings of US$12.4 million relate to the Gruyere project and are included in additions to property, plant and equipment of US$612.5 million (refer note 13).