U.S. and Europe Retrospective Responsibility Plans - Schedule of Changes in Preferred Stock and Right to Recover Covered Losses (Details) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2025 USD ($) $ / shares shares		Sep. 30, 2024 USD ($) $ / shares shares		Sep. 30, 2023 USD ($) shares		Sep. 30, 2022 shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 39,137		$ 38,733		$ 35,581
VE territory covered losses	(28)		(139)		(136)
Recovery through conversion rate adjustments	0		(6)		1
Balance as of end of period	37,909		39,137		38,733
As-converted Value of Preferred Stock	1,389		2,234
Book value of preferred stock	745		1,031
Book Value of Preferred Stock, Total	232		491
Less: right to recover for covered losses	(124)		(104)
As-converted value of Preferred Stock, Total recovery for covered losses available	1,265		2,130
Book value of Preferred of Stock, Total recovery for covered losses available	$ 108		$ 387
Preferred stock, shares outstanding (in shares) | shares	5		5
Share price (in dollars per share) | $ / shares	$ 341.38		$ 274.95
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 1,031	[1],[2]	$ 1,698	[1],[3]	2,324	[3]
Recovery through conversion rate adjustments	(8)		(181)		(30)
Balance as of end of period	$ 745	[2]	$ 1,031	[1],[2]	$ 1,698	[1],[3]
Preferred stock, shares outstanding (in shares) | shares	5		5		5		5
Right to Recover for Covered Losses
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ (104)		$ (140)		$ (35)
VE territory covered losses	(28)		(139)		(136)
Recovery through conversion rate adjustments	8		175		31
Anniversary Release	0		0
Balance as of end of period	(124)		(104)		(140)
Series B Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustments	5		161		19
As-converted Value of Preferred Stock	566		684
Book value of preferred stock	$ 67		$ 104
Preferred stock, shares outstanding (in shares) | shares	2		2
Preferred stock, conversion rate	0.6690		1.0030
Series B Preferred Stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 104		$ 441
VE territory covered losses	0		0
Recovery through conversion rate adjustments	(5)		(161)
Anniversary Release	(32)		(176)
Balance as of end of period	67		104		441
Series C Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustments	3		20		11
As-converted Value of Preferred Stock	823		1,550
Book value of preferred stock	$ 165		$ 387
Preferred stock, shares outstanding (in shares) | shares	3		3
Preferred stock, conversion rate	0.7640		1.7860
Series C Preferred Stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 387		$ 801
VE territory covered losses	0		0
Recovery through conversion rate adjustments	(3)		(20)
Anniversary Release	(219)		(394)
Balance as of end of period	$ 165		$ 387		$ 801

[1] As of September 30, 2024 and 2023, the book value of series A preferred stock was $540 million and $456 million, respectively. See Note 5—U.S. and Europe Retrospective Responsibility Plans for the book value of series B and C preferred stock.

[2] As of September 30, 2025 and 2024, the book value of series A convertible participating preferred stock (series A preferred stock) was $513 million and $540 million, respectively. See Note 5—U.S. and Europe Retrospective Responsibility Plans for the book value of series B convertible participating preferred stock (series B preferred stock) and series C convertible participating preferred stock (series C preferred stock).

[3] As of September 30, 2023 and 2022, the book value of series A preferred stock was $456 million and $1.0 billion, respectively. See Note 5—U.S. and Europe Retrospective Responsibility Plans for the book value of series B and C preferred stock.