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Mining and Income Taxation - Summary of Components of Mining and Income Tax (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2014
May 30, 2018
Disclosure of Components of Mining and Income Tax [Line Items]          
South African statutory mining tax rate 34.00% 34.00% 34.00%    
South African Revenue Service [Member]          
Disclosure of Components of Mining and Income Tax [Line Items]          
Gross recognised capital allowance disallowed       $ 182.2  
Legal settlement amount         $ 185.1
Capital allowance recognized       $ 925.5 $ 53.7
Deferred tax assets not recognized [member] | Cerro Cerona and Damang [member]          
Disclosure of Components of Mining and Income Tax [Line Items]          
Deferred Tax assets at Cerro Cerona and Damang $ 14.9 $ 12.9 $ 34.9    
Deferred tax assets not recognized [member] | Cerro Cerona [member]          
Disclosure of Components of Mining and Income Tax [Line Items]          
Deferred Tax assets at Cerro Cerona and Damang 14.9 12.9 33.5    
Deferred tax assets not recognized [member] | Damang [member]          
Disclosure of Components of Mining and Income Tax [Line Items]          
Deferred Tax assets at Cerro Cerona and Damang 0.0 0.0 $ 1.4    
Deferred tax assets recognized [member] | Cerro Cerona [member]          
Disclosure of Components of Mining and Income Tax [Line Items]          
Deferred Tax assets at Cerro Cerona and Damang 0.0 17.3      
Deferred tax assets recognized [member] | Damang [member]          
Disclosure of Components of Mining and Income Tax [Line Items]          
Deferred Tax assets at Cerro Cerona and Damang $ 6.5 $ 2.5