Capital stock (Details) - USD ($)	12 Months Ended
	Aug. 31, 2020	Aug. 31, 2019	Aug. 31, 2018	Apr. 30, 2017
Equity, Class of Treasury Stock [Line Items]
Cost of stock repurchased	$ 1,589,000,000	$ 4,160,000,000	$ 5,228,000,000
Stock repurchased during current fiscal year, value	$ 103,000,000	$ 339,000,000	$ 289,000,000
Shares of common stock reserved for future issuances (in shares)	16,000,000
June 2018 Stock Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Share repurchase program, authorized maximum amount				$ 10,000,000,000.0
Shares repurchased during period (in shares)	30,000,000	57,000,000
Cost of stock repurchased	$ 1,500,000,000	$ 3,800,000,000