Exit and disposal activities - Restructuring costs and reserves (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	19 Months Ended
	May 31, 2019	May 31, 2019	May 31, 2019
Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 56	$ 235
Transformational cost management program | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	24	86
Transformational cost management program | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	32	149
Transformational cost management program | Retail Pharmacy USA
Restructuring Cost and Reserve [Line Items]
Restructuring costs	13	29
Transformational cost management program | Retail Pharmacy USA | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	7	24
Transformational cost management program | Retail Pharmacy USA | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	5	5
Transformational cost management program | Retail Pharmacy International
Restructuring Cost and Reserve [Line Items]
Restructuring costs	21	88
Transformational cost management program | Retail Pharmacy International | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	5	39
Transformational cost management program | Retail Pharmacy International | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	16	48
Transformational cost management program | Pharmaceutical Wholesale
Restructuring Cost and Reserve [Line Items]
Restructuring costs	22	119
Transformational cost management program | Pharmaceutical Wholesale | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	11	22
Transformational cost management program | Pharmaceutical Wholesale | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	11	96
Store optimization program
Restructuring Cost and Reserve [Line Items]
Restructuring costs	49	99	$ 200
Store optimization program | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	5	54	136
Store optimization program | Lease obligations and other real estate costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 44	$ 45	$ 64