Summary of major accounting policies - property plant and equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2020	Aug. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment	$ 25,797	$ 24,789
Less: accumulated depreciation and amortization	12,456	11,310
Property and equipment, net	$ 13,342	13,478
Land and land improvements
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	20 years
Property and equipment	$ 3,253	3,507
Buildings and building improvements
Property, Plant and Equipment [Line Items]
Property and equipment	$ 8,023	8,023
Buildings and building improvements | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	3 years
Buildings and building improvements | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	50 years
Fixtures and equipment
Property, Plant and Equipment [Line Items]
Property and equipment	$ 10,290	9,786
Fixtures and equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	3 years
Fixtures and equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	20 years
Capitalized system development costs and software
Property, Plant and Equipment [Line Items]
Property and equipment	$ 3,215	2,770
Capitalized system development costs and software | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	3 years
Capitalized system development costs and software | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	10 years
Finance lease properties
Property, Plant and Equipment [Line Items]
Property and equipment	$ 1,016	$ 703