Stockholders' Equity - Effect of VE Territory Covered Losses Recovery on the Company Repurchasing its Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Conversion of Stock [Line Items]
Recovery through conversion rate adjustment	$ 5
Fourth anniversary release	$ (5)
Series B Preferred Stock
Conversion of Stock [Line Items]
Reduction in equivalent number of as-converted shares of class A common stock (in shares)	16	0	0
Effective price per share (in dollars per share)	$ 194.31	$ 141.32	$ 113.05
Recovery through conversion rate adjustment	$ 72	$ 6	$ 35
Fourth anniversary release	$ 3,084	$ 0	$ 0
Series C Preferred Stock
Conversion of Stock [Line Items]
Reduction in equivalent number of as-converted shares of class A common stock (in shares)	22	0	0
Effective price per share (in dollars per share)	$ 194.33	$ 150.26	$ 112.92
Recovery through conversion rate adjustment	$ 92	$ 2	$ 21
Fourth anniversary release	$ 4,216	$ 0	$ 0