Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Apr. 23, 2021	Jan. 31, 2021	Jan. 31, 2020
Stockholders Equity Note [Line Items]
Share repurchase program						$ 8,000	$ 9,500
January 2020 share repurchase program remaining authorized funds	$ 10,000		$ 10,000
Cash dividends declared and paid, at a quarterly amount per Class A share	$ 701	$ 668	$ 1,404	$ 1,339
Subsequent Event | Class A common stock
Stockholders Equity Note [Line Items]
Quarterly cash dividend (in dollars per share)					$ 0.32