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Borrowings - Summary of Borrowings under Credit Facilities (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year $ 1,781.5    
Loans advanced 691.7 $ 779.7 $ 1,298.7
Repayments (431.9) (695.5) (1,413.2)
Balance at end of the year 2,011.6 1,781.5  
Variable rate with exposure to repricing (six months or less) 1,162.2 933.6  
Undrawn borrowing facilities committed 986.7 1,305.1  
Uncommitted 26.5 17.1  
Total undrawn borrowing facilities 1,013.2 1,322.2  
Within one year [member]      
Disclosure of detailed information about borrowings [line items]      
Undrawn borrowing facilities committed 0.0 39.7  
Later Than One Year And Not Later Than Two Years [member]      
Disclosure of detailed information about borrowings [line items]      
Undrawn borrowing facilities committed 93.0 0.0  
Later Than Two Years And Not Later Than Three Years [member]      
Disclosure of detailed information about borrowings [line items]      
Undrawn borrowing facilities committed 791.2 715.4  
Later Than Three Years and Not Later Than Five Years [member]      
Disclosure of detailed information about borrowings [line items]      
Undrawn borrowing facilities committed 102.5 550.0  
US Dollars [member]      
Disclosure of detailed information about borrowings [line items]      
Borrowing denominated in currencies 1,560.9 1,356.4  
Australia, Dollars [member]      
Disclosure of detailed information about borrowings [line items]      
Borrowing denominated in currencies 316.5 231.5  
Rand [member]      
Disclosure of detailed information about borrowings [line items]      
Borrowing denominated in currencies 134.2 193.6  
US $150 Million Revolving Senior Secured Credit Facility [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 0.0 82.0  
Loans advanced 0.0 0.0  
Repayments 0.0 (82.0)  
Balance at end of the year 0.0 0.0 82.0
US$150 million revolving senior secured credit facility - new [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 83.5 0.0  
Loans advanced 0.0 83.5  
Balance at end of the year 83.5 83.5 0.0
US$70 million revolving senior secured credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 0.0 45.0  
Repayments 0.0 (45.0)  
Balance at end of the year 0.0 0.0 45.0
US$100 million revolving senior secured credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 45.0 0.0  
Loans advanced 0.0 45.0  
Balance at end of the year 45.0 45.0 0.0
US$1,290 Million Term Loan and Revolving Credit Facilities [Member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 380.0 658.5  
Loans advanced 382.6 73.5  
Repayments (179.6) (352.0)  
Balance at end of the year 583.0 380.0 658.5
R 1,500 Million Nedbank Revolving Credit Facility [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 79.5 0.0  
Loans advanced 20.7 78.5  
Repayments (107.7) 0.0  
Translation adjustment 7.5 1.0  
Balance at end of the year 0.0 79.5 0.0
US $1 Billion Notes [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 847.9 846.4  
Unwinding of transaction costs 1.5 1.5  
Balance at end of the year 849.4 847.9 846.4
Fixed rate with no exposure to repricing (US$1 billion notes issue) 849.4 847.9  
A $500 million syndicated revolving credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 231.5 0.0  
Loans advanced 119.9 236.6  
Translation adjustment (34.9) (5.1)  
Balance at end of the year 316.5 231.5 0.0
Short-term Rand Uncommitted Credit Facilities [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 114.1 61.0  
Loans advanced 118.7 262.6  
Repayments (144.6) (216.5)  
Translation adjustment (1.9) 7.0  
Balance at end of the year 86.3 114.1 $ 61.0
R 500 million Standard Bank revolving credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 0.0    
Loans advanced (13.7) 0.0  
Translation adjustment 0.0 0.0  
Balance at end of the year 13.7 0.0  
R 500 million Absa Bank revolving credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 0.0    
Loans advanced (36.1) 0.0  
Translation adjustment (1.9) 0.0  
Balance at end of the year $ 34.2 $ 0.0