Supplementary financial information - summary of quarterly results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Aug. 31, 2020	May 31, 2020	Feb. 29, 2020	Nov. 30, 2019	Aug. 31, 2019	May 31, 2019	Feb. 28, 2019	Nov. 30, 2018	Aug. 31, 2020	Aug. 31, 2019	Aug. 31, 2018
Supplementary Financial Information [Abstract]
Sales	$ 34,746	$ 34,631	$ 35,820	$ 34,339	$ 33,954	$ 34,591	$ 34,528	$ 33,793	$ 139,537	$ 136,866	$ 131,537
Gross profit	6,803	6,438	7,513	7,263	7,228	7,453	7,754	7,641	28,017	30,076	30,792
Net earnings attributable to Walgreens Boots Alliance, Inc.	$ 373	$ (1,708)	$ 946	$ 845	$ 677	$ 1,025	$ 1,156	$ 1,123	$ 456	$ 3,982	$ 5,024
Net earnings per common share [Abstract]
Basic (in dollars per share)	$ 0.43	$ (1.95)	$ 1.07	$ 0.95	$ 0.75	$ 1.13	$ 1.25	$ 1.18	$ 0.52	$ 4.32	$ 5.07
Diluted (in dollars per share)	0.43	(1.95)	1.07	0.95	0.75	1.13	1.24	1.18	0.52	4.31	$ 5.05
Cash dividends declared per common share (in dollars per share)	$ 0.4675	$ 0.4575	$ 0.4575	$ 0.4575	$ 0.4580	$ 0.4400	$ 0.4400	$ 0.4400	$ 1.8400	$ 1.7780