Retirement benefits - classes of securities (Details) - Pension Plan	12 Months Ended
	Aug. 31, 2017	Aug. 31, 2016
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of defined benefit pension plan assets investment of fair market value	10.60%	8.90%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of defined benefit pension plan assets investment of fair market value	83.30%	78.80%
Real estate
Defined Benefit Plan Disclosure [Line Items]
Percentage of defined benefit pension plan assets investment of fair market value	5.30%	4.30%
Other
Defined Benefit Plan Disclosure [Line Items]
Percentage of defined benefit pension plan assets investment of fair market value	0.80%	8.00%