Supplementary financial information - summary of quarterly results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Aug. 31, 2021	May 31, 2021	Feb. 28, 2021	Nov. 30, 2020	Aug. 31, 2020	May 31, 2020	Feb. 29, 2020	Nov. 30, 2019	Aug. 31, 2021	Aug. 31, 2020	Aug. 31, 2019
Supplementary Financial Information [Abstract]
Sales	$ 34,262	$ 34,030	$ 32,779	$ 31,438	$ 30,371	$ 30,364	$ 31,336	$ 29,912	$ 132,509	$ 121,982	$ 120,074
Gross profit	7,503	7,153	6,781	6,630	6,324	5,959	7,017	6,777	28,067	26,078	28,159
Net earnings attributable to Walgreens Boots Alliance, Inc.:
Continuing operations	358	1,105	922	(391)	337	(1,794)	867	769	1,994	180	3,816
Discontinued operations	268	92	104	83	36	86	79	76	548	277	166
Net earnings attributable to Walgreens Boots Alliance, Inc.	$ 627	$ 1,197	$ 1,026	$ (308)	$ 373	$ (1,708)	$ 946	$ 845	$ 2,542	$ 456	$ 3,982
Basic earnings (loss) per common share:
Continuing operations (in dollars per share)	$ 0.41	$ 1.28	$ 1.07	$ (0.45)	$ 0.39	$ (2.05)	$ 0.98	$ 0.86	$ 2.31	$ 0.20	$ 4.14
Discontinued operations (in dollars per share)	0.31	0.11	0.12	0.10	0.04	0.10	0.09	0.08	0.63	0.31	0.18
Total (in dollars per share)	0.72	1.38	1.19	(0.36)	0.43	(1.95)	1.07	0.95	2.94	0.52	4.32
Diluted earnings (loss) per common share:
Continuing operations (in dollars per share)	0.41	1.27	1.06	(0.45)	0.39	(2.05)	0.98	0.86	2.30	0.20	4.13
Discontinued operations (in dollars per share)	0.31	0.11	0.12	0.10	0.04	0.10	0.09	0.08	0.63	0.31	0.18
Total (in dollars per share)	0.72	1.38	1.19	(0.36)	0.43	(1.95)	1.07	0.95	2.93	0.52	$ 4.31
Cash dividends declared per common share (in dollars per share)	$ 0.4775	$ 0.4675	$ 0.4675	$ 0.4675	$ 0.4675	$ 0.4575	$ 0.4575	$ 0.4575	$ 1.8800	$ 1.8400