New accounting pronouncements - Reconciliation of Effect of New Pension and Postretirement Benefits Accounting Guidance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	May 31, 2019	May 31, 2018	May 31, 2019	May 31, 2018	Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Selling, general and administrative expenses	$ 6,235	$ 6,235	$ 18,834	$ 18,466
Operating income	1,203	1,597	4,120	4,893
Other income (expense)	$ 182	0	$ 227	(122)
As reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Selling, general and administrative expenses		6,231		18,456
Operating income		1,601		4,903
Other income (expense)		(4)		(132)
Accounting standards update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Selling, general and administrative expenses					$ (125)	$ (73)	$ 69
Other income (expense)					$ 125	$ 73	$ (69)
Accounting standards update 2017-07 | Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Selling, general and administrative expenses		4		10
Operating income		(4)		(10)
Other income (expense)		$ 4		$ 10