Exit and disposal activities - Restructuring Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Nov. 30, 2019	Nov. 30, 2018
Transformational Cost Management Program
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	$ 64	$ 28
Transformational Cost Management Program | Reportable Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	49	0
Transformational Cost Management Program | Reportable Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	9	27
Transformational Cost Management Program | Reportable Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	6	1
Transformational Cost Management Program | Lease obligations and other real estate costs
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	1	4
Transformational Cost Management Program | Lease obligations and other real estate costs | Reportable Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	1	0
Transformational Cost Management Program | Lease obligations and other real estate costs | Reportable Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	0	4
Transformational Cost Management Program | Lease obligations and other real estate costs | Reportable Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	0	0
Transformational Cost Management Program | Asset impairment
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	11	8
Transformational Cost Management Program | Asset impairment | Reportable Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	8	0
Transformational Cost Management Program | Asset impairment | Reportable Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	3	8
Transformational Cost Management Program | Asset impairment | Reportable Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	0	0
Transformational Cost Management Program | Employee severance and business transition costs
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	40	13
Transformational Cost Management Program | Employee severance and business transition costs | Reportable Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	34	0
Transformational Cost Management Program | Employee severance and business transition costs | Reportable Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	1	13
Transformational Cost Management Program | Employee severance and business transition costs | Reportable Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	5	0
Transformational Cost Management Program | Information technology transformation and other exit costs
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	12	2
Transformational Cost Management Program | Information technology transformation and other exit costs | Reportable Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	7	0
Transformational Cost Management Program | Information technology transformation and other exit costs | Reportable Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	4	1
Transformational Cost Management Program | Information technology transformation and other exit costs | Reportable Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	1	1
Store Optimization Program
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	9	20
Store Optimization Program | Lease obligations and other real estate costs
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	3	(7)
Store Optimization Program | Employee severance and other exit costs
Restructuring Reserve Disclosures [Abstract]
Total pre-tax exit and disposal charges	$ 6	$ 27