XML 490 R192.htm IDEA: XBRL DOCUMENT v3.20.1
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, 2019
USD ($)
R / $
Dec. 31, 2018
USD ($)
R / $
Jan. 01, 2018
USD ($)
Dec. 31, 2017
USD ($)
Disclosure of maturity analysis for non-derivative financial liabilities [line items]        
Funding from environmental trust funds $ 69.5 $ 60.8 [1] $ 55.5 $ 55.5
Notional amount 1,200.0 1,290.0    
South Africa and Ghana [member]        
Disclosure of maturity analysis for non-derivative financial liabilities [line items]        
Funding from environmental trust funds 69.5 $ 60.8    
US $1 Billion Notes [member]        
Disclosure of maturity analysis for non-derivative financial liabilities [line items]        
Notional amount 1,000.0      
US$500 million 5-year notes issue (the 5-year notes) [member]        
Disclosure of maturity analysis for non-derivative financial liabilities [line items]        
Notional amount 500.0      
US$500 million 10-year notes issue (the 10-year notes) [member]        
Disclosure of maturity analysis for non-derivative financial liabilities [line items]        
Notional amount $ 500.0      
Closing foreign exchange rate [member]        
Disclosure of maturity analysis for non-derivative financial liabilities [line items]        
Foreign exchange rate | R / $ 14.00 14.63    
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 1.7625% Spot LIBOR (one month fix) rate adjusted by specific facility agreement 2.50625%    
Borrowings adjustment to interest rate 1.7625% 2.50625%    
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 0.92% Spot Bank Bill Swap Bid Rate (BBSY) (one month fix) rate adjusted by specific facility agreement 2.02%    
Borrowings adjustment to interest rate 0.92% 2.02%    
Liquidity Risk [Member] | ZAR Borrowings [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%    
Borrowings adjustment to interest rate   6.942%    
Average bank overnight borrowing rate on uncommitted credit facilities   8.10%    
[1] Refer note 42.