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Equity method investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Feb. 28, 2022	Nov. 30, 2021	Feb. 28, 2021	Sep. 30, 2020	Feb. 28, 2022	Feb. 28, 2021	Aug. 31, 2021
Schedule of Equity Method Investments [Line Items]
Equity earnings (losses)					$ 226	$ (1,253)
Gain on sale of equity method investment					0	191
Impairment of equity method investments and investments in equity securities					$ 190	0
AmerisourceBergen
Schedule of Equity Method Investments [Line Items]
Outstanding shares owned (in shares)	58,854,867				58,854,867		58,854,867
Ownership percentage	28.20%				28.20%		28.20%
Period of reporting lag					2 months
Equity earnings (losses)					$ 202	(1,300)
Equity investment, exceeded its proportionate share of net assets	$ 4,300				4,300
AmerisourceBergen \| Opiod Litigation
Schedule of Equity Method Investments [Line Items]
Equity earnings (losses)				$ (5,600)
AmerisourceBergen \| Level 1
Schedule of Equity Method Investments [Line Items]
Fair market value of equity investment	8,400				8,400		$ 7,200
Other Investments
Schedule of Equity Method Investments [Line Items]
Equity earnings (losses)	31		$ 13		$ 24	$ 29
VillageMD
Schedule of Equity Method Investments [Line Items]
Gain on sale of equity method investment		$ 2,200
Investments in China
Schedule of Equity Method Investments [Line Items]
Impairment of equity method investments and investments in equity securities	124
Option Care Health
Schedule of Equity Method Investments [Line Items]
Gain on sale of equity method investment	$ 191