Equity method investments - narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2020	Aug. 31, 2021	Aug. 31, 2020	Aug. 31, 2019
Schedule of Equity Method Investments [Line Items]
Loss from equity method investment		$ 1,139
Equity earnings (loss)		(498)	$ 382	$ 187
Gain on sale of equity method investment		$ 321	$ 0	0
AmerisourceBergen
Schedule of Equity Method Investments [Line Items]
Outstanding shares owned (in shares)		58,854,867	56,854,867
Percentage of outstanding common shares owned		28.50%	27.90%
Equity investment, exceeded its proportionate share of net assets		$ 4,400
AmerisourceBergen | Opiod Litigation
Schedule of Equity Method Investments [Line Items]
Equity earnings (loss)	$ (5,600)
AmerisourceBergen | Level 1
Schedule of Equity Method Investments [Line Items]
Fair value of assets		7,200	$ 5,500
Others
Schedule of Equity Method Investments [Line Items]
Equity earnings (loss)		627	$ 31	$ 8
Option Care Health
Schedule of Equity Method Investments [Line Items]
Equity earnings (loss)		576
Gain on sale of equity method investment		290
Option Care Health | HC Group Holdings I, LLC
Schedule of Equity Method Investments [Line Items]
Gain on sale of equity method investment		1,200
VillageMD
Schedule of Equity Method Investments [Line Items]
Consideration transferred to acquire equity method investments		750
Payments to acquire equity method investments		250
Investment in convertible debt		$ 500