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Retirement benefits - assumptions (Details) - Pension Plan
12 Months Ended
Aug. 31, 2018
Aug. 31, 2017
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]    
Discount rate 2.67% 2.41%
Rate of compensation increase 2.68% 2.83%
Weighted-average assumptions used to determine net periodic benefit cost    
Discount rate 2.12% 2.16%
Expected long-term return on plan assets 2.27% 1.69%
Rate of compensation increase 2.64% 2.44%