Retirement benefits - components of net periodic pension costs (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service costs	$ 5	$ 5	$ 4
Interest costs	193	174	308
Expected returns on plan assets/other	(209)	(146)	(249)
Total net periodic pension (income) cost	$ (11)	$ 33	$ 63