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Equity method investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Feb. 28, 2018	Feb. 28, 2017	Feb. 28, 2018	Feb. 28, 2017	Aug. 31, 2017
Schedule of Equity Method Investments [Line Items]
Equity earnings (losses)	$ 14	$ 16	$ 27	$ 28
AmerisourceBergen
Schedule of Equity Method Investments [Line Items]
Outstanding shares owned (in shares)	56,854,867		56,854,867		56,854,867
Ownership percentage	26.00%		26.00%		26.00%
Period of reporting lag			2 months
Equity investment, exceeded its proportionate share of net assets	$ 4,300		$ 4,300
AmerisourceBergen \| Level 1
Schedule of Equity Method Investments [Line Items]
Fair market value of equity investment	5,400		5,400
Other Investments
Schedule of Equity Method Investments [Line Items]
Equity earnings (losses)	$ 14	$ 16	27	$ 28
Other income (expense) \| Guangzhou Pharmaceuticals Corporation
Schedule of Equity Method Investments [Line Items]
Equity method investments impairment			$ 170