Stockholders' Equity - Schedule of As-Converted Class B Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Equity [Abstract]
Reduction in equivalent number of as-converted shares of class A common stock (in shares)	0	2	5
Effective price per share (in dollars per share)	$ 0	$ 174.73	$ 132.32
Deposits under the U.S. retrospective responsibility plan	$ 0	$ 300	$ 600