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Borrowings - Schedule of Borrowings (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
May 27, 2019
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 1,526.9 $ 1,845.8  
Current borrowings (83.5) (684.9)  
Non-current borrowings 1,443.4 1,160.9  
US $1 Billion Notes [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 601.4 $ 250.0
Name of borrower Orogen    
Nominal interest rate 4.875    
Maturity date 7 October 2020    
US$500 million 5-year notes issue (the 5-year notes) [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 497.0 496.3  
Name of borrower Orogen    
Nominal interest rate 5.125    
Maturity date 15 May 2024    
US$500 million 10-year notes issue (the 10-year notes) [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 496.4 496.1  
Name of borrower Orogen    
Nominal interest rate 6.125    
Maturity date 15 May 2029    
US$150 million revolving senior secured credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 83.5 83.5  
Name of borrower La Cima    
Nominal interest rate LIBOR plus 2.80    
Commitment fee 0.50%    
Maturity date 19 September 2021    
US$100 million revolving credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0  
Name of borrower Ghana    
Nominal interest rate LIBOR plus 3.50    
Commitment fee 1.40%    
Maturity date 30 November 2021    
Facility A (US $380 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0  
Name of borrower Orogen    
Nominal interest rate LIBOR plus 2.25    
Maturity date 6 June 2020    
Facility B (US $360 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0  
Name of borrower Orogen    
Nominal interest rate LIBOR plus 1.95    
Commitment fee 0.68%    
Maturity date 6 June 2021    
Facility C (US $550 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0  
Name of borrower Orogen    
Nominal interest rate LIBOR plus 2.20    
Commitment fee 0.77%    
Maturity date 6 June 2021    
US$1,200 million revolving credit facilities [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 250.0 0.0  
Facility A (US$600 million 3-year revolving credit facility) [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 250.0 0.0  
Name of borrower Orogen/Ghana    
Nominal interest rate LIBOR plus 1.45    
Commitment fee 0.51%    
Maturity date 25 July 2022    
Facility B (US$600 million 5-year revolving credit facility) [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0  
Name of borrower Orogen/Ghana    
Nominal interest rate LIBOR plus 1.70    
Commitment fee 0.60%    
Maturity date 25 July 2024    
R 1,500 Million Nedbank Revolving Credit Facility [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0  
Name of borrower GFIJVH/GFO    
Nominal interest rate JIBAR plus 2.80    
Commitment fee 0.90%    
Maturity date 8 May 2023    
R 500 million Absa Bank revolving credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0  
Name of borrower GFIJVH/GFO    
Nominal interest rate JIBAR plus 2.20    
Commitment fee 0.77%    
Maturity date 15 April 2023    
AU $ 500 Million Syndicated Revolving Credit Facility Old [Member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 168.5  
Name of borrower Gruyere    
Nominal interest rate BBSY plus 2.175    
Commitment fee 0.87%    
Maturity date 24 May 2021    
AU $ 500 Million Syndicated Revolving Credit Facility New [Member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 200.0 0.0  
Name of borrower Gruyere    
Nominal interest rate BBSY plus 2.200    
Commitment fee 0.88%    
Maturity date 19 November 2023    
R 500 Million Rand Merchant Bank Revolving Credit Facility [Member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0  
Name of borrower GFIJVH/GFO    
Nominal interest rate JIBAR plus 2.15    
Commitment fee 0.71%    
Maturity date 15 April 2023    
Short-term Rand Uncommitted Credit Facilities [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 $ 0.0  
Commitment fee 0.00%