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Post-retirement benefit obligations - Sensitivity analysis (Details) - Pension - ZAR (R)
R in Millions
Jun. 30, 2019
Jun. 30, 2018
South Africa | Salary increase    
Disclosure of sensitivity analysis for actuarial assumptions    
Increase in actuarial assumption (as a percent) 1.00% 1.00%
Decrease in actuarial assumption (as a percent) 1.00% 1.00%
Increase (decrease) in net obligation due to increase in actuarial assumption R 12 R 14
Increase (decrease) in net obligation due to decrease in actuarial assumption R (12) R (13)
South Africa | Discount rate    
Disclosure of sensitivity analysis for actuarial assumptions    
Increase in actuarial assumption (as a percent) 1.00% 1.00%
Decrease in actuarial assumption (as a percent) 1.00% 1.00%
Increase (decrease) in net obligation due to increase in actuarial assumption R (1,654) R (1,447)
Increase (decrease) in net obligation due to decrease in actuarial assumption R 2,463 R 1,981
South Africa | Pension increase    
Disclosure of sensitivity analysis for actuarial assumptions    
Increase in actuarial assumption (as a percent) 1.00% 1.00%
Decrease in actuarial assumption (as a percent) 1.00% 1.00%
Increase (decrease) in net obligation due to increase in actuarial assumption R 2,541 R 2,035
Increase (decrease) in net obligation due to decrease in actuarial assumption R (1,727) R (1,523)
Foreign Countries | Salary increase    
Disclosure of sensitivity analysis for actuarial assumptions    
Increase in actuarial assumption (as a percent) 1.00% 1.00%
Decrease in actuarial assumption (as a percent) 1.00% 1.00%
Increase (decrease) in net obligation due to increase in actuarial assumption R 494 R 454
Increase (decrease) in net obligation due to decrease in actuarial assumption R (430) R (368)
Foreign Countries | Discount rate    
Disclosure of sensitivity analysis for actuarial assumptions    
Increase in actuarial assumption (as a percent) 1.00% 1.00%
Decrease in actuarial assumption (as a percent) 1.00% 1.00%
Increase (decrease) in net obligation due to increase in actuarial assumption R (1,806) R (1,634)
Increase (decrease) in net obligation due to decrease in actuarial assumption R 2,370 R 2,174
Foreign Countries | Pension increase    
Disclosure of sensitivity analysis for actuarial assumptions    
Increase in actuarial assumption (as a percent) 1.00% 1.00%
Decrease in actuarial assumption (as a percent) 1.00% 1.00%
Increase (decrease) in net obligation due to increase in actuarial assumption R 1,142 R 1,071
Increase (decrease) in net obligation due to decrease in actuarial assumption R (934) R (851)