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
	Dec. 31, 2023 USD ($) $ / shares shares		Dec. 31, 2022 USD ($) shares		Sep. 30, 2023 USD ($) $ / shares shares	Sep. 30, 2022 shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 38,733		$ 35,581
VE territory covered losses incurred	(24)		(8)
Recovery through conversion rate adjustment	0		1
Balance as of end of period	39,733		36,924
As-converted Value of Preferred Stock	4,854				$ 4,311
Book Value of Preferred Stock	1,615				1,698
Book Value of Preferred Stock, Total	1,217				1,242
Less: right to recover for covered losses	(139)				(140)
As-converted Value of Preferred Stock, Total recovery for covered losses available	4,715				4,171
Book Value of Preferred of Stock, Total recovery for covered losses available	$ 1,078				$ 1,102
Share price (in dollars per share) | $ / shares	$ 260.35				$ 230.01
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 1,698	[1]	2,324	[2]
Recovery through conversion rate adjustment	(25)		(14)
Balance as of end of period	$ 1,615	[1]	$ 1,981	[2]
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)
VE territory covered losses incurred	(24)		(8)
Recovery through conversion rate adjustment	25		15
Balance as of end of period	(139)		(28)
Series B preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment	22		7
As-converted Value of Preferred Stock	1,875				$ 1,676
Book Value of Preferred Stock	$ 419				$ 441
Preferred stock, shares outstanding (in shares) | shares	2				2
Preferred stock, conversion rate	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
Series C preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment	3		7
As-converted Value of Preferred Stock	2,979				$ 2,635
Book Value of Preferred Stock	$ 798				$ 801
Preferred stock, shares outstanding (in shares) | shares	3				3
Preferred stock, conversion rate	3.6250				3.629
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

[1] As of December 31, 2023 and September 30, 2023, the book value of series A preferred stock was $398 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 December 31, 2022 and September 30, 2022, the book value of series A preferred stock was $723 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.