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Borrowings - Summary of Borrowings under Credit Facilities (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year $ 1,845.8    
Repayments (1,014.2) $ (1,604.3) $ (535.9)
Balance at end of the year 1,526.9 1,845.8  
Variable rate with exposure to repricing (six months or less) 533.5 252.0  
Fixed rate with no exposure to repricing 993.4 1,593.8  
Borrowing denominated in currencies 1,526.9 1,845.8  
Total committed undrawn borrowing facilities 1,471.3 1,727.6  
Uncommitted 104.5 116.8  
Total undrawn borrowing facilities 1,471.3 1,727.6  
Undrawn borrowing facilities net. 1,575.8 1,844.4  
Within one year [member]      
Disclosure of detailed information about borrowings [line items]      
Total committed undrawn borrowing facilities 166.5 137.9  
Later Than One Year And Not Later Than Two Years [member]      
Disclosure of detailed information about borrowings [line items]      
Total committed undrawn borrowing facilities 96.2 282.5  
Later Than Two Years And Not Later Than Three Years [member]      
Disclosure of detailed information about borrowings [line items]      
Total committed undrawn borrowing facilities 608.6 600.0  
Later Than Three Years and Not Later Than Five Years [member]      
Disclosure of detailed information about borrowings [line items]      
Total committed undrawn borrowing facilities 600.0 707.1  
US Dollars [member] | Other Currencies Short-term borrowings [member]      
Disclosure of detailed information about borrowings [line items]      
Borrowing denominated in currencies 1,326.9 1,677.3  
Australia, Dollars [member] | Other Currencies Short-term borrowings [member]      
Disclosure of detailed information about borrowings [line items]      
Borrowing denominated in currencies 200.0 168.5  
Rand [member]      
Disclosure of detailed information about borrowings [line items]      
Borrowing denominated in currencies 0.0 0.0  
US $1 Billion Notes [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 601.4    
Balance at end of the year 0.0 601.4  
US $1 Billion Notes [member] | US Dollars [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 601.4 849.4  
Buy-back of US$250 million notes 0.0 (255.0)  
Loss on buy-back of notes 0.0 5.0  
Unwinding of transaction costs 0.9 2.0  
Repayments (602.3) 0.0  
Balance at end of the year 0.0 601.4 849.4
US$500 million 5-year notes issue [member] | US Dollars [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 496.3 0.0  
Loans advanced 0.0 500.0  
transaction costs 0.0 (4.1)  
Unwinding of transaction costs 0.7 0.4  
Balance at end of the year 497.0 496.3 0.0
US$500 million 10-year notes issue (the 10-year notes) [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 496.1    
Balance at end of the year 496.4 496.1  
US$500 million 10-year notes issue (the 10-year notes) [member] | US Dollars [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 496.1 0.0  
Loans advanced 0.0 500.0  
transaction costs 0.0 (4.1)  
Unwinding of transaction costs 0.3 0.2  
Balance at end of the year 496.4 496.1 0.0
US$150 million revolving senior secured credit facility - old [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 83.5    
Balance at end of the year 83.5 83.5  
US$150 million revolving senior secured credit facility - old [member] | US Dollars [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 83.5 83.5  
Balance at end of the year 83.5 83.5 83.5
US$100 million revolving credit facility [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 0.0    
Balance at end of the year 0.0 0.0  
US$100 million revolving credit facility [member] | US Dollars [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
R 500 million Standard Bank revolving credit facility [member] | Rand [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 0.0 13.7  
Loans advanced 0.0 21.2  
Repayments 0.0 (35.1)  
Translation adjustment 0.0 0.2  
Balance at end of the year 0.0 0.0 13.7
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 0.0 472.0  
Loans advanced 0.0 434.4  
Repayments 0.0 (906.4)  
Balance at end of the year 0.0 0.0 472.0
US$1,200 million revolving credit facilities [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 0.0 0.0  
Loans advanced 413.0 0.0  
Repayments (163.0) 0.0  
Balance at end of the year 250.0 0.0 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    
Balance at end of the year 0.0 0.0  
R 500 million Absa Bank revolving credit facility [member] | Rand [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 0.0 34.2  
Repayments 0.0 (34.5)  
Translation adjustment 0.0 0.3  
Balance at end of the year 0.0 0.0 34.2
Short-term Rand Uncommitted Credit Facilities [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 0.0    
Balance at end of the year 0.0 0.0  
Short-term Rand Uncommitted Credit Facilities [member] | Rand [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 0.0 92.5  
Loans advanced 0.0 90.6  
Repayments 0.0 (184.7)  
Translation adjustment 0.0 1.6  
Balance at end of the year 0.0 0.0 92.5
AU $ 500 Million Syndicated Revolving Credit Facility Old [Member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 168.5    
Balance at end of the year 0.0 168.5  
AU $ 500 Million Syndicated Revolving Credit Facility Old [Member] | Australia, Dollars [member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 168.5 316.5  
Loans advanced 85.8 0.0  
Repayments (248.9) (143.6)  
Translation adjustment (5.4) (4.4)  
Balance at end of the year 0.0 168.5 $ 316.5
AU $ 500 Million Syndicated Revolving Credit Facility New [Member]      
Disclosure of detailed information about borrowings [line items]      
Balance at beginning of the year 0.0    
Loans advanced 191.0 0.0  
Translation adjustment 9.0 0.0  
Balance at end of the year $ 200.0 $ 0.0