Exit and disposal activities - Restructuring costs and reserves (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	May 31, 2018	May 31, 2018
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 171	$ 592
Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	114	229
Real estate costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	15	264
Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	42	99
Retail Pharmacy USA
Restructuring Cost and Reserve [Line Items]
Restructuring costs	129	517
Retail Pharmacy USA | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	96	207
Retail Pharmacy USA | Real estate costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	15	264
Retail Pharmacy USA | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	18	46
Retail Pharmacy International
Restructuring Cost and Reserve [Line Items]
Restructuring costs	26	51
Retail Pharmacy International | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	18	21
Retail Pharmacy International | Real estate costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	0	0
Retail Pharmacy International | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	8	30
Pharmaceutical Wholesale
Restructuring Cost and Reserve [Line Items]
Restructuring costs	16	24
Pharmaceutical Wholesale | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	0	1
Pharmaceutical Wholesale | Real estate costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	0	0
Pharmaceutical Wholesale | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	16	23
Store Optimization Program
Restructuring Cost and Reserve [Line Items]
Restructuring costs	24	24
Store Optimization Program | Lease obligations and other real estate costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	2	2
Store Optimization Program | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 22	$ 22