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, 2024	Sep. 30, 2023	Sep. 30, 2022
Conversion of Stock [Line Items]
Recovery through conversion rate adjustment	$ (6)	$ 1	$ 0
Series B preferred stock
Conversion of Stock [Line Items]
Reduction in equivalent number of class A common stock (in shares)	5	0	8
Effective price per share (in dollars per share)	$ 272.89	$ 219.12	$ 197.93
Recovery through conversion rate adjustment	$ 161	$ 19	$ 135
Anniversary Releases	$ 1,149	$ 0	$ 1,510
Series C Preferred Stock
Conversion of Stock [Line Items]
Reduction in equivalent number of class A common stock (in shares)	6	0	10
Effective price per share (in dollars per share)	$ 273.24	$ 215.28	$ 197.50
Recovery through conversion rate adjustment	$ 20	$ 11	$ 6
Anniversary Releases	$ 1,569	$ 0	$ 1,982