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Financial risk management and financial instruments - Significant Derivatives Sensitivity Analysis (Details)
R in Millions
 12 Months Ended
Jun. 30, 2019
ZAR (R)
$ / gal
R / $
Jun. 30, 2018
ZAR (R)
$ / bbl
R / $
Ethane swaps | Volatility    
Financial risk management and financial instruments    
Increase in significant input variable (as a percent) 2.00%  
Decrease in significant input variable (as a percent) (2.00%)  
Impact on fair value from increase in significant input variable R 90  
Impact on fair value from decrease in significant input variable R (82)  
Ethane swaps | Commodity price    
Financial risk management and financial instruments    
Increase in price or rate assumption | $ / gal 0.02  
Decrease in price or rate assumption | $ / gal (0.02)  
Impact on fair value from increase in significant input variable R 146  
Impact on fair value from decrease in significant input variable R (146)  
Crude oil options | Volatility    
Financial risk management and financial instruments    
Increase in significant input variable (as a percent)   2.00%
Decrease in significant input variable (as a percent)   (2.00%)
Impact on fair value from increase in significant input variable   R 88
Impact on fair value from decrease in significant input variable   R (80)
Crude oil options | Commodity price    
Financial risk management and financial instruments    
Increase in price or rate assumption | $ / bbl   2
Decrease in price or rate assumption | $ / bbl   (2)
Impact on fair value from increase in significant input variable   R (68)
Impact on fair value from decrease in significant input variable   R 81
Zero-cost collar | Volatility    
Financial risk management and financial instruments    
Increase in significant input variable (as a percent) 2.00% 2.00%
Decrease in significant input variable (as a percent) (2.00%) (2.00%)
Impact on fair value from increase in significant input variable R 115 R 27
Impact on fair value from decrease in significant input variable R (125) R (22)
Zero-cost collar | USD/ZAR spot price    
Financial risk management and financial instruments    
Decrease in price or rate assumption | R / $ (1) (1)
Impact on fair value from decrease in significant input variable R 2,495 R 2,731
Minimum weakening of exchange rate for fair value impact | R / $ 2.55  
Gain (loss) for derivatives settled at spot price between cap and floor R 0  
Derivative cap | R / $ 16.63  
Interest rate swaps | USD Libor curve    
Financial risk management and financial instruments    
Increase in significant input variable (as a percent) 0.50% 0.50%
Decrease in significant input variable (as a percent) (0.50%) (0.50%)
Impact on fair value from increase in significant input variable R 754 R 866
Impact on fair value from decrease in significant input variable R (748) R (865)