Supplementary financial information - summary of quarterly results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Aug. 31, 2019	May 31, 2019	Feb. 28, 2019	Nov. 30, 2018	Aug. 31, 2018	May 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2019	Aug. 31, 2018	Aug. 31, 2017
Supplementary Financial Information [Abstract]
Sales	$ 33,954	$ 34,591	$ 34,528	$ 33,793	$ 33,442	$ 34,334	$ 33,021	$ 30,740	$ 136,866	$ 131,537	$ 118,214
Gross profit	7,228	7,453	7,754	7,641	7,575	7,780	8,096	7,341	30,076	30,792	29,162
Net earnings attributable to Walgreens Boots Alliance, Inc.	$ 677	$ 1,025	$ 1,156	$ 1,123	$ 1,512	$ 1,342	$ 1,349	$ 821	$ 3,982	$ 5,024	$ 4,078
Net earnings per common share [Abstract]
Basic (in dollars per share)	$ 0.75	$ 1.13	$ 1.25	$ 1.18	$ 1.55	$ 1.35	$ 1.36	$ 0.82	$ 4.32	$ 5.07	$ 3.80
Diluted (in dollars per share)	0.75	1.13	1.24	1.18	1.55	1.35	1.36	0.81	4.31	5.05	$ 3.78
Cash dividends declared per common share (in dollars per share)	$ 0.458	$ 0.440	$ 0.440	$ 0.440	$ 0.440	$ 0.400	$ 0.400	$ 0.400	$ 1.778	$ 1.640