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Interest in Joint Operation - Summary of Share of Joint Operation and Includes Inter-company Transactions and Balances (Parenthetical) (Detail)
$ in Millions, $ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2017
AUD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2016
AUD ($)
Disclosure of joint operations [abstract]        
Assets capitalised   $ 372.4 $ 268.6 $ 372.4
Additions to property, plant and equipment $ 275.9 372.4    
Cash additions to property, plant and equipment 197.1 266.0    
Non-cash additions to property, plant and equipment $ 78.8 $ 106.4