Goodwill and other intangible assets - Schedule of Finite-Lived Intangible Assets by Major Class (Details) - USD ($) $ in Millions	Feb. 28, 2018	Aug. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross amortizable intangible assets	$ 5,553	$ 3,928
Accumulated amortization	1,457	1,341
Total amortizable intangible assets, net	4,096	2,587
Indefinite lived intangible assets	8,124	7,569
Total intangible assets, net	12,220	10,156
Trade names and trademarks
Finite-Lived Intangible Assets [Line Items]
Indefinite lived intangible assets	5,919	5,514
Pharmacy licenses
Finite-Lived Intangible Assets [Line Items]
Indefinite lived intangible assets	2,205	2,055
Customer relationships and loyalty card holders
Finite-Lived Intangible Assets [Line Items]
Gross amortizable intangible assets	4,036	2,510
Accumulated amortization	873	780
Favorable lease interests and non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross amortizable intangible assets	618	523
Accumulated amortization	338	355
Trade names and trademarks
Finite-Lived Intangible Assets [Line Items]
Gross amortizable intangible assets	509	504
Accumulated amortization	182	155
Purchasing and payer contracts
Finite-Lived Intangible Assets [Line Items]
Gross amortizable intangible assets	390	391
Accumulated amortization	$ 64	$ 51