Summary of major accounting policies - property plant and equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2021	Aug. 31, 2020
Property, Plant and Equipment [Line Items]
Property and equipment	$ 25,321	$ 24,927
Less: accumulated depreciation and amortization	13,073	12,131
Property and equipment, net	$ 12,247	12,796
Land and land improvements
Property, Plant and Equipment [Line Items]
Estimated useful life	20 years
Property and equipment	$ 2,798	3,157
Buildings and building improvements
Property, Plant and Equipment [Line Items]
Property and equipment	$ 7,569	7,795
Buildings and building improvements | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Buildings and building improvements | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	50 years
Fixtures and equipment
Property, Plant and Equipment [Line Items]
Property and equipment	$ 10,314	9,904
Fixtures and equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Fixtures and equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	20 years
Capitalized system development costs and software
Property, Plant and Equipment [Line Items]
Property and equipment	$ 3,624	3,061
Capitalized system development costs and software | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Capitalized system development costs and software | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	10 years
Finance lease properties
Property, Plant and Equipment [Line Items]
Property and equipment	$ 1,016	$ 1,011