U.S. and Europe Retrospective Responsibility Plans - 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, 2024 USD ($) $ / shares shares		Mar. 31, 2023 USD ($) shares		Mar. 31, 2024 USD ($) $ / shares shares		Mar. 31, 2023 USD ($) shares		Dec. 31, 2023 shares	Sep. 30, 2023 USD ($) $ / shares shares	Dec. 31, 2022 shares	Sep. 30, 2022 shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 39,733		$ 36,924		$ 38,733		$ 35,581
VE territory covered losses incurred	(36)		(7)		(60)		(15)
Recovery through conversion rate adjustment					0		1
Balance as of end of period	40,485		38,565		40,485		38,565
As-converted Value of Preferred Stock	5,204				5,204					$ 4,311
Book Value of Preferred Stock	1,602				1,602					1,698
Book Value of Preferred Stock, Total	1,217				1,217					1,242
Less: right to recover for covered losses	(175)				(175)					(140)
As-converted Value of Preferred Stock, Total recovery for covered losses available	5,029				5,029					4,171
Book Value of Preferred of Stock, Total recovery for covered losses available	$ 1,042				$ 1,042					$ 1,102
Preferred stock, shares outstanding (in shares) | shares	5				5					5
Share price (in dollars per share) | $ / shares	$ 279.08				$ 279.08					$ 230.01
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 1,615		1,981		$ 1,698	[1]	2,324	[2]
Recovery through conversion rate adjustment					(25)		(14)
Balance as of end of period	$ 1,602	[1]	$ 1,885	[2]	$ 1,602	[1]	$ 1,885	[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	$ (139)		$ (28)		$ (140)		$ (35)
VE territory covered losses incurred	(36)		(7)		(60)		(15)
Recovery through conversion rate adjustment					25		15
Balance as of end of period	(175)		(35)		(175)		(35)
Series B preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment					22		7
As-converted Value of Preferred Stock	2,010				2,010					$ 1,676
Book Value of Preferred Stock	$ 419				$ 419					$ 441
Preferred stock, shares outstanding (in shares) | shares	2				2					2
Preferred stock, conversion rate	2.9030				2.9030					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					(22)		(7)
Balance as of end of period	$ 419		453		419		453
Series C preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment					3		7
As-converted Value of Preferred Stock	3,194				3,194					$ 2,635
Book Value of Preferred Stock	$ 798				$ 798					$ 801
Preferred stock, shares outstanding (in shares) | shares	3				3					3
Preferred stock, conversion rate	3.6250				3.6250					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					(3)		(7)
Balance as of end of period	$ 798		$ 805		$ 798		$ 805

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

[2] As of March 31, 2023 and September 30, 2022, the book value of series A preferred stock was $627 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 series C preferred stock.