Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Apr. 21, 2020	Jan. 31, 2020	Jan. 31, 2019
Stockholders Equity Note [Line Items]
Share repurchase program						$ 9,500	$ 8,500
January 2020 share repurchase program remaining authorized funds	$ 8,100		$ 8,100
Cash dividends declared and paid, at a quarterly amount per Class A share	$ 668	$ 569	$ 1,339	$ 1,141
Subsequent Event | Class A common stock
Stockholders Equity Note [Line Items]
Quarterly cash dividend (in dollars per share)					$ 0.30