Goodwill and other intangible assets - Schedule of Finite-Lived Intangible Assets by Major Class (Details) - USD ($) $ in Millions	May 31, 2018	Aug. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross amortizable intangible assets	$ 5,836	$ 3,928
Accumulated amortization	1,544	1,341
Total amortizable intangible assets, net	4,292	2,587
Indefinite lived intangible assets	7,819	7,569
Total intangible assets, net	12,111	10,156
Trade names and trademarks
Finite-Lived Intangible Assets [Line Items]
Indefinite lived intangible assets	5,696	5,514
Pharmacy licenses
Finite-Lived Intangible Assets [Line Items]
Indefinite lived intangible assets	2,123	2,055
Customer relationships and loyalty card holders
Finite-Lived Intangible Assets [Line Items]
Gross amortizable intangible assets	4,331	2,510
Accumulated amortization	930	780
Favorable lease interests and non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross amortizable intangible assets	621	523
Accumulated amortization	353	355
Trade names and trademarks
Finite-Lived Intangible Assets [Line Items]
Gross amortizable intangible assets	495	504
Accumulated amortization	190	155
Purchasing and payer contracts
Finite-Lived Intangible Assets [Line Items]
Gross amortizable intangible assets	389	391
Accumulated amortization	$ 71	$ 51