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	3 Months Ended				6 Months Ended
	Mar. 31, 2025 USD ($) $ / shares shares		Mar. 31, 2024 USD ($) shares		Mar. 31, 2025 USD ($) $ / shares shares		Mar. 31, 2024 USD ($) shares		Dec. 31, 2024 shares	Sep. 30, 2024 USD ($) $ / shares shares	Dec. 31, 2023 shares	Sep. 30, 2023 shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 38,296		$ 39,733		$ 39,137		$ 38,733
VE territory covered losses	3		(36)		(24)		(60)
Recovery through conversion rate adjustment					0		0
Balance as of end of period	38,030		40,485		38,030		40,485
As-converted Value of Preferred Stock	2,839				2,839					$ 2,234
Book Value of Preferred Stock	880				880					1,031
Book Value of Preferred Stock, Total	483				483					491
Less: right to recover for covered losses	(120)				(120)					(104)
As-converted Value of Preferred Stock, Total recovery for covered losses available	2,719				2,719					2,130
Book Value of Preferred of Stock, Total recovery for covered losses available	$ 363				$ 363					$ 387
Preferred stock, shares outstanding (in shares) | shares	5				5					5
Share price (in dollars per share) | $ / shares	$ 350.46				$ 350.46					$ 274.95
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 904		1,615		$ 1,031	[1]	1,698	[2]
Recovery through conversion rate adjustment					(8)		(25)
Balance as of end of period	$ 880	[1]	$ 1,602	[2]	$ 880	[1]	$ 1,602	[2]
Preferred stock, shares outstanding (in shares) | shares	5		5		5		5		5	5	5	5
Right to Recover for Covered Losses
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ (123)		$ (139)		$ (104)		$ (140)
VE territory covered losses	3		(36)		(24)		(60)
Recovery through conversion rate adjustment					8		25
Balance as of end of period	(120)		(175)		(120)		(175)
Series B preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment					5		22
As-converted Value of Preferred Stock	866				866					$ 684
Book Value of Preferred Stock	$ 99				$ 99					$ 104
Preferred stock, shares outstanding (in shares) | shares	2				2					2
Preferred stock, conversion rate	0.9960				0.9960					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 adjustment					(5)		(22)
Balance as of end of period	$ 99		419		99		419
Series C preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment					3		3
As-converted Value of Preferred Stock	1,973				1,973					$ 1,550
Book Value of Preferred Stock	$ 384				$ 384					$ 387
Preferred stock, shares outstanding (in shares) | shares	3				3					3
Preferred stock, conversion rate	1.7830				1.7830					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 adjustment					(3)		(3)
Balance as of end of period	$ 384		$ 798		$ 384		$ 798

[1] As of March 31, 2025 and September 30, 2024, the book value of series A convertible participating preferred stock (series A preferred stock) was $397 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).

[2] As of March 31, 2024 and September 30, 2023, the book value of series A preferred stock was $385 million and $456 million, respectively. See Note 5—U.S. and Europe Retrospective Responsibility Plans for the book value of series B and series C preferred stock.