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				9 Months Ended
	Jun. 30, 2024 USD ($) $ / shares shares		Jun. 30, 2023 USD ($) shares		Jun. 30, 2024 USD ($) $ / shares shares		Jun. 30, 2023 USD ($) shares		Mar. 31, 2024 shares	Sep. 30, 2023 USD ($) $ / shares shares	Mar. 31, 2023 shares	Sep. 30, 2022 shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 40,485		$ 38,565		$ 38,733		$ 35,581
VE territory covered losses incurred	(21)		(6)		(81)		(21)
Recovery through conversion rate adjustment	(6)		0		(6)		1
Balance as of end of period	39,729		38,981		39,729		38,981
As-converted Value of Preferred Stock	4,744				4,744					$ 4,311
Book Value of Preferred Stock	1,425				1,425					1,698
Book Value of Preferred Stock, Total	1,061				1,061					1,242
Less: right to recover for covered losses	(46)				(46)					(140)
As-converted Value of Preferred Stock, Total recovery for covered losses available	4,698				4,698					4,171
Book Value of Preferred of Stock, Total recovery for covered losses available	$ 1,015				$ 1,015					$ 1,102
Preferred stock, shares outstanding (in shares) | shares	5				5					5
Share price (in dollars per share) | $ / shares	$ 262.47				$ 262.47					$ 230.01
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 1,602		1,885		$ 1,698	[1]	2,324	[2]
Recovery through conversion rate adjustment	(156)		(16)		(181)		(30)
Balance as of end of period	$ 1,425	[1]	$ 1,786	[2]	$ 1,425	[1]	$ 1,786	[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	$ (175)		$ (35)		$ (140)		$ (35)
VE territory covered losses incurred	(21)		(6)		(81)		(21)
Recovery through conversion rate adjustment	150		16		175		31
Balance as of end of period	(46)		(25)		(46)		(25)
Series B preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment					161		19
As-converted Value of Preferred Stock	1,757				1,757					$ 1,676
Book Value of Preferred Stock	$ 280				$ 280					$ 441
Preferred stock, shares outstanding (in shares) | shares	2				2					2
Preferred stock, conversion rate	2.6980				2.6980					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)
Balance as of end of period	$ 280		441		280		441
Series C preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment					20		11
As-converted Value of Preferred Stock	2,987				2,987					$ 2,635
Book Value of Preferred Stock	$ 781				$ 781					$ 801
Preferred stock, shares outstanding (in shares) | shares	3				3					3
Preferred stock, conversion rate	3.6050				3.6050					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)
Balance as of end of period	$ 781		$ 801		$ 781		$ 801

[1] As of June 30, 2024 and September 30, 2023, the book value of series A preferred stock was $364 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 June 30, 2023 and September 30, 2022, the book value of series A preferred stock was $544 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.