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Post-retirement benefit obligations - Sensitivity Analysis (Details) - Healthcare - ZAR (R)
R in Millions
Jun. 30, 2019
Jun. 30, 2018
Sensitivity analysis    
Cost inflation as premium over CPI 2.00%  
South Africa | Healthcare cost inflation    
Sensitivity analysis    
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 518 R 582
Increase (decrease) in net obligation due to decrease in actuarial assumption R (417) R (475)
South Africa | Discount rate    
Sensitivity analysis    
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 (417) R (452)
Increase (decrease) in net obligation due to decrease in actuarial assumption R 507 R 562
United States of America | Healthcare cost inflation    
Sensitivity analysis    
Increase in actuarial assumption (as a percent) 1.00% 1.00%
Decrease in actuarial assumption (as a percent) 1.00% 1.00%
United States of America | Discount rate    
Sensitivity analysis    
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 (23) R (21)
Increase (decrease) in net obligation due to decrease in actuarial assumption R 28 R 25