Capital stock (Details) - USD ($)	1 Months Ended	12 Months Ended
	May 31, 2017	Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016	Jun. 30, 2018	Oct. 31, 2017	Jun. 30, 2017	Apr. 30, 2017
Equity, Class of Treasury Stock [Line Items]
Purchase of shares under stock repurchase program (in shares)		72,000,000	59,000,000.0
Value of shares purchased under stock repurchase program		$ 4,900,000,000	$ 4,800,000,000
Stock repurchased during current fiscal year, value		$ 289,000,000	$ 457,000,000	$ 1,000,000,000
Shares of common stock reserved for future issuances (in shares)		33,000,000
April 2017 Stock Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Share repurchase program, authorized maximum amount								$ 1,000,000,000.0
Shares repurchased during period (in shares)	11,800,000.0
June 2017 Stock Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Share repurchase program, authorized maximum amount							$ 5,000,000,000.0
Stock repurchase program, increase in authorized maximum amount						$ 1,000,000,000.0
June 2018 Stock Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Share repurchase program, authorized maximum amount					$ 10,000,000,000.0