New accounting pronouncements - Reconciliation of Effect of New Pension and Postretirement Benefits Accounting Guidance (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 28, 2019	Feb. 28, 2018	Feb. 28, 2019	Feb. 28, 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	$ 6,320	$ 6,321	$ 12,599	$ 12,231
Operating income	1,517	1,977	2,918	3,296
Other income (expense)	$ 19	12	$ 45	(122)
Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Selling, general and administrative expenses		6,318		12,225
Operating income		1,980		3,302
Other income (expense)		9		(128)
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 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Selling, general and administrative expenses		3		6
Operating income		(3)		(6)
Other income (expense)		$ 3		$ 6