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Discontinued Operation - Additional Information (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Oct. 02, 2017
USD ($)
Oct. 02, 2017
AUD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2019
AUD ($)
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2018
AUD ($)
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2017
AUD ($)
shares
Oct. 31, 2018
AUD ($)
Disclosure Of Noncurrent Assets Held For Sale And Discontinued Operations [line items]                  
Profit on the sale assets     $ 1.2   $ (51.6)   $ 4.0    
Percentage of shares held     19.90% 19.90%          
Percentage of share holdings     19.90% 19.90%          
Discontinued operations     $ 174.7   (344.8)   $ (7.7)    
Mining Contractor Higher Market Value Percentage     25.00% 25.00%          
Percentage of Premiums Payout Value     25.00% 25.00%          
Red 5 Limited [Member]                  
Disclosure Of Noncurrent Assets Held For Sale And Discontinued Operations [line items]                  
Sale of investments     $ 20.9 $ 29.6          
Discontinued operations     12.4 17.2          
Darlot [Member]                  
Disclosure Of Noncurrent Assets Held For Sale And Discontinued Operations [line items]                  
Consideration from disposal               $ 18.5  
Cash consideration from disposal               $ 12.0  
Number of shares received from disposal | shares             130,000,000 130,000,000  
Upfront cash consideration received     $ 0.0 $ 7.0 $ 0.0 [1]   $ 5.4 [1]    
Cash consideration receivable               $ 5.0  
Cash consideration received                 $ 5.0
Profit on the sale assets $ 23.5 $ 30.8              
Percentage of shares held 19.90% 19.90%              
Percentage of share holdings 19.90% 19.90%              
Darlot [Member] | Red 5 Limited [Member]                  
Disclosure Of Noncurrent Assets Held For Sale And Discontinued Operations [line items]                  
Shares received under underwriting agreement related to disposal of discontinued operation | shares         116,875,821 116,875,821      
Consideration received under underwriting agreement related to disposal of discontinued operation           $ 5.8      
[1] Refer note 42.