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Risk Management Activities - Market Risk - Interest Rate - Additional Information (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Financial Risk Management [line items]    
Borrowings $ 2,011.6 $ 1,781.5
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 711.5 508.5
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 47.9 79.5
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 86.3 114.1
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 316.5 231.5
Sensitivity to interest rates [member] | Interest Bearing Borrowings [Member]    
Disclosure Of Financial Risk Management [line items]    
Borrowings exposed to interest rate fluctuations $ 1,162.2 $ 933.6