Stockholders' Equity - Schedule of 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, 2025	Sep. 30, 2024	Sep. 30, 2023
Conversion of Stock [Line Items]
Recovery through conversion rate adjustments	$ 0	$ (6)	$ 1
Series B preferred stock
Conversion of Stock [Line Items]
Reduction in equivalent number of class A common stock (in shares)	1	5	0
Effective price per share (in dollars per share)	$ 352.58	$ 272.89	$ 219.12
Recovery through conversion rate adjustments	$ 5	$ 161	$ 19
Anniversary Releases	$ 287	$ 1,149	$ 0
Series C Preferred Stock
Conversion of Stock [Line Items]
Reduction in equivalent number of class A common stock (in shares)	3	6	0
Effective price per share (in dollars per share)	$ 353.32	$ 273.24	$ 215.28
Recovery through conversion rate adjustments	$ 3	$ 20	$ 11
Anniversary Releases	$ 1,137	$ 1,569	$ 0