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Discontinued Operation - Summary of Assets and Liabilities of Discontinued Operation (Detail) - Darlot [Member]
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2017
AUD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2017
AUD ($)
Disclosure Of Noncurrent Assets Held For Sale And Discontinued Operations [line items]        
Property, plant and equipment $ 3.3     $ 4.3
Inventories 7.2     9.4
Trade and other receivables 0.1     0.1
Trade and other payables (8.7)     (11.3)
Environmental rehabilitation costs provision (12.9)     (16.9)
Net liabilities (11.0)     $ (14.4)
Total consideration received less costs to sell 12.5 $ 16.4    
Gain on sale of discontinued operations $ 23.5 $ 30.8 $ 0.0