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Equity method investments - additional information (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Equity earnings	$ 143	$ 81	$ 339
Equity investment, noncontrolling interest	$ 204	$ 176
AmerisourceBergen
Schedule of Equity Method Investments [Line Items]
Outstanding shares owned (in shares)	56,854,867	56,854,867
Percentage of outstanding common shares owned	26.00%	24.00%
Equity investment, exceeded its proportionate share of net assets	$ 4,400
Ownership percentage	26.00%	24.00%
Others
Schedule of Equity Method Investments [Line Items]
Equity earnings	$ 8	$ 44	$ 24
Alliance Boots
Schedule of Equity Method Investments [Line Items]
Ownership percentage		45.00%	45.00%	45.00%
Level 1 \| AmerisourceBergen
Schedule of Equity Method Investments [Line Items]
Fair value of assets	$ 4,600