Supplementary financial information - summary of quarterly results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Aug. 31, 2017	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2016	May 31, 2016	Feb. 29, 2016	Nov. 30, 2015	Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2015
Supplementary Financial Information [Abstract]
Sales	$ 30,149	$ 30,118	$ 29,446	$ 28,501	$ 28,636	$ 29,498	$ 30,184	$ 29,033	$ 118,214	$ 117,351	$ 103,444
Gross profit	7,340	7,145	7,561	7,116	7,155	7,433	7,867	7,419	29,162	29,874	26,753
Net earnings attributable to Walgreens Boots Alliance, Inc.	$ 802	$ 1,162	$ 1,060	$ 1,054	$ 1,030	$ 1,103	$ 930	$ 1,110	$ 4,078	$ 4,173	$ 4,220
Net earnings per common share [Abstract]
Basic (in dollars per share)	$ 0.76	$ 1.08	$ 0.98	$ 0.97	$ 0.95	$ 1.02	$ 0.86	$ 1.02	$ 3.80	$ 3.85	$ 4.05
Diluted (in dollars per share)	0.76	1.07	0.98	0.97	0.95	1.01	0.85	1.01	3.78	3.82	4.00
Cash dividends declared (in dollars per share)	$ 0.400	$ 0.375	$ 0.375	$ 0.375	$ 0.375	$ 0.360	$ 0.360	$ 0.360	$ 1.525	$ 1.455	$ 1.373