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, 2024 USD ($) $ / shares shares		Sep. 30, 2023 USD ($) $ / shares shares		Sep. 30, 2022 USD ($) shares		Sep. 30, 2021 shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 38,733		$ 35,581		$ 37,589
VE territory covered losses incurred	(139)		(136)		(43)
Recovery through conversion rate adjustment	(6)		1		0
Balance as of end of period	39,137		38,733		35,581
As-converted Value of Preferred Stock	2,234		4,311
Book Value of Preferred Stock	1,031		1,698
Book Value of Preferred Stock, Total	491		1,242
Less: right to recover for covered losses	(104)		(140)
As-converted value of Preferred Stock, Total recovery for covered losses available	2,130		4,171
Book value of Preferred of Stock, Total recovery for covered losses available	$ 387		$ 1,102
Preferred stock, shares outstanding (in shares) | shares	5		5
Share price (in dollars per share) | $ / shares	$ 274.95		$ 230.01
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 1,698	[1],[2]	$ 2,324	[1],[3]	3,080	[3]
Recovery through conversion rate adjustment	(181)		(30)		(141)
Balance as of end of period	$ 1,031	[2]	$ 1,698	[1],[2]	$ 2,324	[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	$ (140)		$ (35)		$ (133)
VE territory covered losses incurred	(139)		(136)		(43)
Recovery through conversion rate adjustment	175		31		141
Eighth Anniversary Release	0
Balance as of end of period	(104)		(140)		(35)
Series B Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment	161		19		135
As-converted Value of Preferred Stock	684		1,676
Book Value of Preferred Stock	$ 104		$ 441
Preferred stock, shares outstanding (in shares) | shares	2		2
Preferred stock, conversion rate	1.0030		2.9370
Series B Preferred Stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 441		$ 460
VE territory covered losses incurred	0		0
Recovery through conversion rate adjustment	(161)		(19)
Eighth Anniversary Release	(176)
Balance as of end of period	104		441		460
Series C Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment	20		11		6
As-converted Value of Preferred Stock	1,550		2,635
Book Value of Preferred Stock	$ 387		$ 801
Preferred stock, shares outstanding (in shares) | shares	3		3
Preferred stock, conversion rate	1.7860		3.6290
Series C Preferred Stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 801		$ 812
VE territory covered losses incurred	0		0
Recovery through conversion rate adjustment	(20)		(11)
Eighth Anniversary Release	(394)
Balance as of end of period	$ 387		$ 801		$ 812

[1] As of September 30, 2023 and 2022, the book value of series A preferred stock was $456 million and $1.0 billion, respectively. Refer to 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, 2024 and 2023, the book value of series A convertible participating preferred stock (series A preferred stock) was $540 million and $456 million, respectively. Refer to 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, 2022 and 2021, the book value of series A preferred stock was $1.0 billion and $486 million, respectively. Refer to Note 5—U.S. and Europe Retrospective Responsibility Plans for the book value of series B and C preferred stock.