Summary of major accounting policies - reconciliation of effect of new pension and postretirement benefits accounting guidance (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2019	Aug. 31, 2018	Aug. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Selling, general and administrative expenses	$ 25,242	$ 24,694	$ 23,813
Operating income	4,998	6,289	5,484
Other income	$ 233	302	62
As reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Selling, general and administrative expenses		24,569	23,740
Operating income		6,414	5,557
Other income		177	(11)
Accounting standards update 2017-07 | Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Selling, general and administrative expenses		125	73
Operating income		(125)	(73)
Other income		$ 125	$ 73