Supplementary financial information - summary of quarterly results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Aug. 31, 2018	May 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2017	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
Supplementary Financial Information [Abstract]
Sales	$ 33,442	$ 34,334	$ 33,021	$ 30,740	$ 30,149	$ 30,118	$ 29,446	$ 28,501	$ 131,537	$ 118,214	$ 117,351
Gross profit	7,575	7,780	8,096	7,341	7,340	7,145	7,561	7,116	30,792	29,162	29,874
Net earnings attributable to Walgreens Boots Alliance, Inc.	$ 1,512	$ 1,342	$ 1,349	$ 821	$ 802	$ 1,162	$ 1,060	$ 1,054	$ 5,024	$ 4,078	$ 4,173
Net earnings per common share [Abstract]
Basic (in dollars per share)	$ 1.55	$ 1.35	$ 1.36	$ 0.82	$ 0.76	$ 1.08	$ 0.98	$ 0.97	$ 5.07	$ 3.80	$ 3.85
Diluted (in dollars per share)	1.55	1.35	1.36	0.81	0.76	1.07	0.98	0.97	5.05	3.78	3.82
Cash dividends declared (in dollars per share)	$ 0.440	$ 0.400	$ 0.400	$ 0.400	$ 0.400	$ 0.375	$ 0.375	$ 0.375	$ 1.640	$ 1.525	$ 1.455