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Borrowings - Schedule of Borrowings (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 2,011.6 $ 1,781.5  
Current borrowings (86.3) (193.6)  
Non-current borrowings 1,925.3 1,587.9  
US $1 Billion Notes [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 849.4 847.9 $ 846.4
Name of borrower Orogen    
Commitment fee 0.00%    
Maturity date 7 October 2020    
US$150 million revolving senior secured credit facility - old [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0  
Name of borrower La Cima    
Commitment fee 0.65%    
Maturity date 19 December 2017    
US$150 million revolving senior secured credit facility - old [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate LIBOR plus 1.63    
US$150 million revolving senior secured credit facility - new [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 83.5 83.5 0.0
Name of borrower La Cima    
Commitment fee 0.50%    
Maturity date 19 September 2020    
US$150 million revolving senior secured credit facility - new [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate LIBOR plus 1.20    
US$70 million revolving senior secured credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0 45.0
Name of borrower Ghana    
Commitment fee 1.00%    
Maturity date 6 May 2017    
US$70 million revolving senior secured credit facility [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate LIBOR plus 2.40    
US$100 million revolving senior secured credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 45.0 45.0 0.0
Name of borrower Ghana    
Commitment fee 1.40%    
Maturity date 30 November 2021    
US$100 million revolving senior secured credit facility [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate LIBOR plus 3.50    
A$500 million syndicated revolving credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 316.5 231.5  
Name of borrower Gruyere    
Commitment fee 0.94%    
Maturity date 24 May 2021    
A$500 million syndicated revolving credit facility [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate BBSY plus 2.35    
Facility A (US $380 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 380.0 380.0  
Name of borrower Orogen    
Commitment fee 0.00%    
Maturity date 6 June 2020    
Facility A (US $380 Million) [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate LIBOR plus 2.25    
Facility B (US $360 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 203.0 0.0  
Name of borrower Orogen    
Commitment fee 0.77%    
Maturity date 6 June 2021    
Facility B (US $360 Million) [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate LIBOR plus 1.95    
Facility C (US $550 Million) [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0  
Name of borrower Orogen    
Commitment fee 0.86%    
Maturity date 6 June 2021    
Facility C (US $550 Million) [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate LIBOR plus 2.20    
R 1,500 Million Nedbank Revolving Credit Facility [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 79.5 0.0
Name of borrower GFIJVH/GFO    
Commitment fee 0.85%    
Maturity date 7 March 2018    
R 1,500 Million Nedbank Revolving Credit Facility [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate JIBAR plus 2.50    
R 1,500 million Nedbank revolving credit facility - new [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 0.0 0.0  
Name of borrower GFIJVH/GFO    
Commitment fee 0.90%    
Maturity date 8 May 2023    
R 1,500 million Nedbank revolving credit facility - new [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate JIBAR plus 2.80    
R 500 million Standard Bank revolving credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 13.7 0.0  
Name of borrower GFIJVH/GFO    
Commitment fee 1.05%    
Maturity date 31 March 2020    
R 500 million Standard Bank revolving credit facility [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate JIBAR plus 2.75    
R 500 million Absa Bank revolving credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 34.2 0.0  
Name of borrower GFIJVH/GFO    
Commitment fee 0.893%    
Maturity date 31 March 2020    
R 500 million Absa Bank revolving credit facility [member] | Fixed Interest Rate [member]      
Disclosure of detailed information about borrowings [line items]      
Nominal interest rate JIBAR plus 2.55    
Short-term Rand Uncommitted Credit Facilities [member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 86.3 114.1 61.0
Commitment fee 0.00%    
US$1,290 Million Term Loan and Revolving Credit Facilities [Member]      
Disclosure of detailed information about borrowings [line items]      
Total borrowings $ 583.0 $ 380.0 $ 658.5