Summary of major accounting policies - property plant and equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment	$ 22,960	$ 22,935
Less: accumulated depreciation and amortization	9,318	8,600
Property and equipment, net	13,642	14,335
Depreciation	$ 1,300	1,300	$ 1,300
Land and land improvements
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	20 years
Property and equipment	$ 3,470	3,738
Buildings and building improvements
Property, Plant and Equipment [Line Items]
Property and equipment	$ 7,431	7,557
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	$ 9,209	9,064
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	$ 2,105	1,787
Internally developed software amortization	245	238	$ 178
Unamortized capitalized software costs	$ 900	900
Capitalized system development costs and software | Minimum
Property, Plant and Equipment [Line Items]
Useful life of capitalized software costs	3 years
Capitalized system development costs and software | Maximum
Property, Plant and Equipment [Line Items]
Useful life of capitalized software costs	8 years
Capital lease properties
Property, Plant and Equipment [Line Items]
Property and equipment	$ 745	$ 789