Summary of major accounting policies - property plant and equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment	$ 24,424	$ 22,960
Less: accumulated depreciation and amortization	10,513	9,318
Property and equipment, net	13,911	13,642
Depreciation	$ 1,400	1,300	$ 1,300
Land and land improvements
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	20 years
Property and equipment	$ 3,593	3,470
Buildings and building improvements
Property, Plant and Equipment [Line Items]
Property and equipment	$ 7,874	7,431
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	$ 9,750	9,209
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	$ 2,464	2,105
Internally developed software amortization	254	245	$ 238
Unamortized capitalized software costs	$ 1,500	895
Capitalized system development costs and software | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	3 years
Useful life of capitalized software costs	3 years
Capitalized system development costs and software | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	8 years
Useful life of capitalized software costs	8 years
Capital lease properties
Property, Plant and Equipment [Line Items]
Property and equipment	$ 743	$ 745