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]	12 Months Ended
Dec. 31, 2017 R / $ $ / $	Dec. 31, 2016 R / $ $ / $
Disclosure of nature and extent of risks arising from financial instruments [line items]
Foreign exchange rate	12.58	14.03
Johannesburg Interbank Average Rate and Prime Interest Rates [member]
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average rate	13.33	14.70
Change in Bank Bill Swap Bid Rate [member]
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average rate \| $ / $	0.77	0.75

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]

12 Months Ended

Dec. 31, 2017

R / $

$ / $

Dec. 31, 2016

R / $

$ / $

Disclosure of nature and extent of risks arising from financial instruments [line items]

Foreign exchange rate

12.58

14.03

Johannesburg Interbank Average Rate and Prime Interest Rates [member]