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Stated Capital - Schedule of Stated Capital (Detail)
$ in Millions, R in Billions
 12 Months Ended
Feb. 12, 2020
USD ($)
shares
Feb. 12, 2020
ZAR (R)
shares
Dec. 31, 2020
USD ($)
shares
Dec. 31, 2019
USD ($)
shares
Dec. 31, 2018
USD ($)
shares
Disclosure of classes of share capital [line items]          
Beginning Balance | $     $ 3,622.5    
Share issue $ 249.0 R 3.7 249.0 $ 0.0 $ 0.0
Ending balance | $     3,871.5 3,622.5  
Placing of ordinary shares 41,431,635 41,431,635      
Ordinary Shares [member]          
Disclosure of classes of share capital [line items]          
Beginning Balance | $     3,622.5 3,622.5  
Share issue | $     249.0 0.0  
Ending balance | $     $ 3,871.5 $ 3,622.5 $ 3,622.5
In issue at 1 January     828,632,707 821,532,707  
Placing of ordinary shares     41,431,635 0  
Exercise of employee share options     13,269,176 7,100,000  
In issue at 31 December     883,333,518 828,632,707 821,532,707
Authorised     2,000,000,000 2,000,000,000