Retirement benefits - components of net periodic pension costs (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2019	Aug. 31, 2018	Aug. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service costs	$ 4	$ 5	$ 5
Interest costs	196	193	174
Expected returns on plan assets/other	(246)	(209)	(146)
Total net periodic pension (income) cost	$ (46)	$ (11)	$ 33