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Equity method investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	May 31, 2019	May 31, 2018	May 31, 2019	May 31, 2018	Aug. 31, 2018
Schedule of Equity Method Investments [Line Items]
Equity earnings (losses)	$ (5)	$ 15	$ 19	$ 42
AmerisourceBergen
Schedule of Equity Method Investments [Line Items]
Outstanding shares owned (in shares)	56,854,867		56,854,867		56,854,867
Ownership percentage	27.00%		27.00%		26.00%
Period of reporting lag			2 months
Equity investment, exceeded its proportionate share of net assets	$ 4,400		$ 4,400
AmerisourceBergen \| Level 1
Schedule of Equity Method Investments [Line Items]
Fair market value of equity investment	4,400		4,400
Other Investments
Schedule of Equity Method Investments [Line Items]
Equity earnings (losses)	$ (5)	$ 15	$ 19	$ 42