Exit and disposal activities - Restructuring costs and reserves (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Feb. 28, 2018	Feb. 28, 2018
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 340	$ 421
Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	67	115
Real estate costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	240	249
Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	33	57
Retail Pharmacy USA
Restructuring Cost and Reserve [Line Items]
Restructuring costs	316	388
Retail Pharmacy USA | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	65	111
Retail Pharmacy USA | Real estate costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	240	249
Retail Pharmacy USA | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	11	28
Retail Pharmacy International
Restructuring Cost and Reserve [Line Items]
Restructuring costs	19	25
Retail Pharmacy International | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	1	3
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	18	22
Pharmaceutical Wholesale
Restructuring Cost and Reserve [Line Items]
Restructuring costs	5	8
Pharmaceutical Wholesale | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	1	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	$ 4	$ 7