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Goodwill and other intangible assets - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	May 31, 2020 USD ($) segment	May 31, 2019 USD ($)	May 31, 2020 USD ($) segment	May 31, 2019 USD ($)	Aug. 31, 2019 USD ($)	Jun. 01, 2019
Goodwill [Line Items]
Number of reporting segments included in impairment analysis \| segment	2		2
Goodwill impairment			$ (1,675)
Goodwill	$ 14,948		14,948		$ 16,560
Indefinite-lived intangible assets	6,883		6,883		$ 7,110
Impairment of definite-lived intangible assets	32
Amortization of intangible assets	112	$ 142	348	$ 415
Boots Reporting Unit
Goodwill [Line Items]
Goodwill impairment	(1,700)
Intangible asset impairment	300
Goodwill	1,000		1,000
Indefinite-lived intangible assets	$ 6,700		$ 6,700
Boots Reporting Unit \| Minimum
Goodwill [Line Items]
Reporting unit fair value in excess of carrying amount (as a percent)	2.50%		2.50%
Boots Reporting Unit \| Maximum
Goodwill [Line Items]
Reporting unit fair value in excess of carrying amount (as a percent)	26.50%		26.50%
Retail Pharmacy International
Goodwill [Line Items]
Reporting unit fair value in excess of carrying amount (as a percent)	6.00%		6.00%			16.00%
Goodwill	$ 500		$ 500