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Post-retirement benefit obligations - Sensitivity analysis (Details) - Pension - ZAR (R)
R in Millions
Jun. 30, 2018
Jun. 30, 2017
South Africa | Salary increase    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
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 14 R 15
Increase (decrease) in net obligation due to decrease in actuarial assumption R (13) R (14)
South Africa | Discount rate    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
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 (1,447) R (1,552)
Increase (decrease) in net obligation due to decrease in actuarial assumption R 1,981 R 2,494
South Africa | Pension increase    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
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 2,035 R 2,538
Increase (decrease) in net obligation due to decrease in actuarial assumption R (1,523) R (1,622)
Foreign Countries | Salary increase    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
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 454 R 416
Increase (decrease) in net obligation due to decrease in actuarial assumption R (368) R (353)
Foreign Countries | Discount rate    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
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 (1,634) R (1,507)
Increase (decrease) in net obligation due to decrease in actuarial assumption R 2,174 R 1,989
Foreign Countries | Pension increase    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
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 1,071 R 956
Increase (decrease) in net obligation due to decrease in actuarial assumption R (851) R (731)