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Mining and Income Taxation - Summary of Estimated Available for Set-off Against Future Income Pre Tax (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2014
May 30, 2018
South African Revenue Service [Member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Gross recognised capital allowance disallowed     $ 182.2  
Legal settlement amount       $ 185.1
Capital allowance recognized     $ 925.5 $ 53.7
Abosso Goldfields Limited [member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Tax losses carry forward period 5 years 5 years    
Exploration Entities [member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Total tax losses $ 337.7 $ 430.0    
Tax Losses Expire in 2 Years [Member] | Abosso Goldfields Limited [member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Tax losses 84.5 19.0    
Tax Losses Expire in 3 Years [Member] | Abosso Goldfields Limited [member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Tax losses expiration value 46.2 2.9    
Tax Losses Expire in 4 Years [Member] | Abosso Goldfields Limited [member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Tax losses expiration value 46.0 31.5    
Tax Losses Expire in 5 Years [Member] | Abosso Goldfields Limited [member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Tax losses expiration value 0.0 27.5    
Tax Losses Expiring Between 1 and 2 Years [Member] | Exploration Entities [member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Total tax losses 8.8 18.6    
Tax Losses Expiring Between 2 and 5 Years [Member] | Exploration Entities [member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Total tax losses 15.2 27.6    
Tax Losses Expiring Between 5 and 10 Years [Member] | Exploration Entities [member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Total tax losses 16.5 20.3    
Tax Losses Expiring After 10 Years [Member] | Exploration Entities [member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Total tax losses 33.0 42.3    
No Expiry Date [member] | Exploration Entities [member]        
Disclosure of Estimated Amount Available for Set Off Against Future Income Pretax [Line Items]        
Total tax losses $ 264.2 $ 320.9