Segment reporting - geographic data (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Aug. 31, 2018	May 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2017	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales	$ 33,442	$ 34,334	$ 33,021	$ 30,740	$ 30,149	$ 30,118	$ 29,446	$ 28,501	$ 131,537	$ 118,214	$ 117,351
Long-Lived Assets	13,911				13,642				13,911	13,642
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									2,254	2,136	2,675
Long-Lived Assets	199				180				199	180
Reportable Geographical Components | United States of America
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									98,392	87,302	83,802
Long-Lived Assets	10,678				10,344				10,678	10,344
Reportable Geographical Components | United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									13,297	12,552	14,081
Long-Lived Assets	2,458				2,502				2,458	2,502
Reportable Geographical Components | Europe (excluding the United Kingdom)
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									17,594	16,224	$ 16,793
Long-Lived Assets	$ 576				$ 616				$ 576	$ 616