XML 457 R195.htm IDEA: XBRL DOCUMENT v3.21.1
Risk Management Activities - Summary of Effect of Change in Finance Expense on Group's Profit or Loss had LIBOR and Prime Differed as Indicated (Parenthetical) (Detail) - Closing foreign exchange rate [member] - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Change in Bank Bill Swap Bid Rate [member]    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Average rate 0.69 0.70
Johannesburg Interbank Average Rate and Prime Interest Rates [member]    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Average rate 16.38 14.46