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, 2023	Sep. 30, 2022	Sep. 30, 2021
Conversion of Stock [Line Items]
Recovery through conversion rate adjustment	$ 1	$ 0	$ (2)
Series B preferred stock
Conversion of Stock [Line Items]
Reduction in equivalent number of class A common stock (in shares)	0	8	0
Effective price per share (in dollars per share)	$ 219.12	$ 197.93	$ 220.84
Recovery through conversion rate adjustment	$ 19	$ 135	$ 35
Sixth Anniversary Release	$ 0	$ 1,510	$ 0
Series C
Conversion of Stock [Line Items]
Reduction in equivalent number of class A common stock (in shares)	0	10	0
Effective price per share (in dollars per share)	$ 215.28	$ 197.50	$ 220.71
Recovery through conversion rate adjustment	$ 11	$ 6	$ 20
Sixth Anniversary Release	$ 0	$ 1,982	$ 0