Capital stock (Details) - USD ($) shares in Millions	1 Months Ended	12 Months Ended
	May 31, 2017	Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2015	Jun. 30, 2017	Apr. 30, 2017
Equity, Class of Treasury Stock [Line Items]
Share repurchase program, authorized maximum amount					$ 5,000,000,000.0	$ 1,000,000,000.0
Stock repurchased during current fiscal year, value	$ 1,000,000,000	$ 457,000,000	$ 1,000,000,000	$ 500,000,000
Shares of common stock reserved for future issuances (in shares)		39.0
2014 Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Purchase of shares under stock repurchase program (in shares)		59.0	1.3
Value of shares purchased under stock repurchase program		$ 4,763,000,000	$ 110,000,000