Summary of major accounting policies - property plant and equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2019	Aug. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment	$ 24,789	$ 24,424
Less: accumulated depreciation and amortization	11,310	10,513
Property and equipment, net	$ 13,478	13,911
Land and land improvements
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	20 years
Property and equipment	$ 3,507	3,593
Buildings and building improvements
Property, Plant and Equipment [Line Items]
Property and equipment	$ 8,023	7,874
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	$ 9,786	9,750
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	$ 2,770	2,464
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	8 years
Capital lease properties
Property, Plant and Equipment [Line Items]
Property and equipment	$ 703	$ 743