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Borrowings - Schedule of Borrowings (Parenthetical) (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings $ 1,290,000,000    
Profit on buy back of notes 0 $ 17,700,000 $ 0
Carrying value of asset 4,892,900,000 4,524,600,000 [1] $ 4,295,600,000 [1]
Fleet assets and CIL plant [member] | Ghana [member]      
Disclosure of detailed information about borrowings [line items]      
Carrying value of asset 183,600,000 95,500,000  
US$70 million revolving senior secured credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 70,000,000    
Amount available under the facility 70,000,000    
US$100 million revolving senior secured credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 100,000,000    
Amount available under the facility 100,000,000    
US$1,510 Million Term Loan and Revolving Credit Facilities [Member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 1,510,000,000    
Borrowing financed, description     These facilities were cancelled and refinanced through the US$1,290 million term loan and revolving credit facilities on 6 June 2016, resulting in the total amount available to be US$nil at 31 December 2016.
Amount available under the facility   0  
US $1 Billion Notes [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 1,000,000,000    
Unamortized transaction costs 4,500,000 6,000,000  
Profit on buy back of notes 0 $ 17,700,000  
US$150 million revolving senior secured credit facility - old [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 150,000,000    
US$150 million revolving senior secured credit facility - new [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 150,000,000    
A$500 million syndicated revolving credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 500,000,000    
Facility A (US $75 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 75,000,000    
Facility A (US $45 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 45,000,000    
Facility B (US $720 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 720,000,000    
Facility C (US $670 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 670,000,000    
Facility A (US $380 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 380,000,000    
Facility B (US $360 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 360,000,000    
Facility C (US $550 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 550,000,000    
R 1,500 Million Nedbank Revolving Credit Facility [member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 1,500,000,000    
US$1,290 Million Term Loan and Revolving Credit Facilities [Member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 1,290,000,000    
US $147.6 Million Notes [Member]      
Disclosure of detailed information about borrowings [line items]      
Face amount of borrowings 1,510,000,000    
Purchase of notes, amount 147,600,000    
Purchase price per US$1,000 $ 880    
Borrowing financed, description The purchase of the notes amounting to US$147.6 million was financed by drawing down under the US$1,510 million term loan and revolving credit facilities. The Group recognised a profit of US$17.7 million on the buy back of the notes.    
[1] As Restated - Refer note 40 for further details.