Retirement benefits - assumptions (Details) - Pension Plan	12 Months Ended
	Aug. 31, 2023	Aug. 31, 2022
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	5.22%	4.20%
Rate of compensation increase	3.73%	3.04%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate	4.20%	1.57%
Expected long-term return on plan assets	4.90%	2.90%
Rate of compensation increase	3.04%	2.80%