Risk Management Activities - Schedule of Contractually Due Undiscounted Cash Flows Resulting from Maturities of All Financial Liabilities, Including Interest Payments (Parenthetical) (Detail) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) R / $	Dec. 31, 2017 USD ($) R / $	Dec. 31, 2016 USD ($) R / $
Disclosure of maturity analysis for non-derivative financial liabilities [line items]
Funding from environmental trust funds \| $	$ 60.8	$ 55.5	$ 44.5
Closing foreign exchange rate [member]
Disclosure of maturity analysis for non-derivative financial liabilities [line items]
Foreign exchange rate \| R / $	14.63	12.58
Liquidity Risk [Member] \| South Africa and Ghana [member]
Disclosure of maturity analysis for non-derivative financial liabilities [line items]
Funding from environmental trust funds \| $	$ 60.8	$ 55.5
Liquidity Risk [Member] \| US dollar borrowings [member]
Disclosure of maturity analysis for non-derivative financial liabilities [line items]
Borrowings interest rate description	Spot LIBOR (one month fix) rate adjusted by specific facility agreement 2.50625%	Spot LIBOR (one month fix) rate adjusted by specific facility agreement 1.5638%
Borrowings adjustment to interest rate	2.50625%	1.5638%
Liquidity Risk [Member] \| Australian Dollar borrowings [member]
Disclosure of maturity analysis for non-derivative financial liabilities [line items]
Borrowings interest rate description	Spot Bank Bill Swap Bid Rate (BBSY) (one month fix) rate adjusted by specific facility agreement 2.02%	Spot Bank Bill Swap Bid Rate (BBSY) (one month fix) rate adjusted by specific facility agreement 1.76%
Borrowings adjustment to interest rate	2.02%	1.76%
Liquidity Risk [Member] \| Rand borrowing [member] \| Uncommitted credit facility [member]
Disclosure of maturity analysis for non-derivative financial liabilities [line items]
Borrowings interest rate description	Spot JIBAR (one month fix) rate adjusted by specific facility agreement 6.942%	Spot JIBAR (one month fix) rate adjusted by specific facility agreement 6.908%
Borrowings adjustment to interest rate	6.942%	6.908%
Average bank overnight borrowing rate on uncommitted credit facilities	8.10%	8.30%
Liquidity Risk [Member] \| Closing foreign exchange rate [member]
Disclosure of maturity analysis for non-derivative financial liabilities [line items]
Foreign exchange rate \| R / $	14.63	12.58	14.03

Risk Management Activities - Schedule of Contractually Due Undiscounted Cash Flows Resulting from Maturities of All Financial Liabilities, Including Interest Payments (Parenthetical) (Detail)
$ in Millions

12 Months Ended

Dec. 31, 2018

USD ($)

R / $

Dec. 31, 2017

USD ($)

R / $

Dec. 31, 2016

USD ($)

R / $

Disclosure of maturity analysis for non-derivative financial liabilities [line items]

Funding from environmental trust funds | $