Segment reporting - geographic data (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Aug. 31, 2019	May 31, 2019	Feb. 28, 2019	Nov. 30, 2018	Aug. 31, 2018	May 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2019	Aug. 31, 2018	Aug. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales	$ 33,954	$ 34,591	$ 34,528	$ 33,793	$ 33,442	$ 34,334	$ 33,021	$ 30,740	$ 136,866	$ 131,537	$ 118,214
Total long-lived assets	13,478				13,911				13,478	13,911
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									2,597	2,254	2,136
Total long-lived assets	197				199				197	199
Reportable Geographical Components | United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									104,532	98,392	87,302
Total long-lived assets	10,598				10,678				10,598	10,678
Reportable Geographical Components | United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									12,729	13,297	12,552
Total long-lived assets	2,162				2,458				2,162	2,458
Reportable Geographical Components | Europe (excluding the United Kingdom)
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									17,009	17,594	$ 16,224
Total long-lived assets	$ 521				$ 576				$ 521	$ 576