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Risk Management Activities - Market Risk - Interest Rate - Additional Information (Detail) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
 [1]
Disclosure Of Financial Risk Management [line items]      
Borrowings $ 1,845.8 $ 1,906.8 [1] $ 1,782.4
Change in London Interbank Offered Rate [member] | Interest Bearing Borrowings [Member] | Floating Interest Rate [member]      
Disclosure Of Financial Risk Management [line items]      
Borrowings exposed to interest rate fluctuations 83.5 600.5  
Johannesburg Interbank Average Rate and Prime Interest Rates [member] | Interest Bearing Borrowings [Member] | Floating Interest Rate [member]      
Disclosure Of Financial Risk Management [line items]      
Borrowings exposed to interest rate fluctuations 0.0 47.9  
Change in South African Prime Interest Rate [Member] | Interest Bearing Borrowings [Member] | Floating Interest Rate [member]      
Disclosure Of Financial Risk Management [line items]      
Borrowings exposed to interest rate fluctuations 0.0 92.5  
Change in Bank Bill Swap Bid Rate [member] | Interest Bearing Borrowings [Member] | Floating Interest Rate [member]      
Disclosure Of Financial Risk Management [line items]      
Borrowings exposed to interest rate fluctuations 168.5 316.5  
Sensitivity to interest rates [member] | Interest Bearing Borrowings [Member]      
Disclosure Of Financial Risk Management [line items]      
Borrowings exposed to interest rate fluctuations $ 252.0 $ 1,057.4  
[1] Refer note 42.