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, 2022 USD ($) $ / shares shares		Dec. 31, 2021 USD ($) shares		Sep. 30, 2022 USD ($) $ / shares shares	Sep. 30, 2021 shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 35,581		$ 37,589
VE territory covered losses incurred	(8)		(7)
Recovery through conversion rate adjustment	1		0
Ending balance	36,924		36,194
As-converted value of Preferred Stock	3,907				$ 3,353
Book Value of Preferred Stock	1,981				2,324
Book Value of Preferred Stock, Total	1,258				1,272
Less: right to recover for covered losses	(28)				(35)
As-converted value of Preferred Stock, Total recovery for covered losses available	3,879				3,318
Book value of Preferred of Stock, Total recovery for covered losses available	$ 1,230				$ 1,237
Share price (in dollars per share) | $ / shares	$ 207.76				$ 177.65
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 2,324	[1]	3,080	[2]
Recovery through conversion rate adjustment	(14)		(29)
Ending balance	$ 1,981	[1]	$ 2,995	[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]
Beginning balance	$ (35)		$ (133)
VE territory covered losses incurred	(8)		(7)
Recovery through conversion rate adjustment	15		29
Ending balance	(28)		(111)
Series B preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment	7		26
As-converted value of Preferred Stock	1,524				$ 1,309
Book Value of Preferred Stock	$ 453				$ 460
Preferred stock, shares outstanding (in shares) | shares	2				2
Preferred stock, conversion rate	2.9580				2.9710
Series B preferred stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 460		1,071
VE territory covered losses incurred	0		0
Recovery through conversion rate adjustment	(7)		(26)
Ending balance	453		1,045
Series C preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment	7		3
As-converted value of Preferred Stock	2,383				$ 2,044
Book Value of Preferred Stock	$ 805				$ 812
Preferred stock, shares outstanding (in shares) | shares	3				3
Preferred stock, conversion rate	3.6340				3.6450
Series C preferred stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 812		1,523
VE territory covered losses incurred	0		0
Recovery through conversion rate adjustment	(7)		(3)
Ending balance	$ 805		$ 1,520

[1] 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 4—U.S. and Europe Retrospective Responsibility Plans for the book value of series B and series C preferred stock.

[2] As of December 31, 2021 and September 30, 2021, the book value of series A preferred stock was $430 million and $486 million, respectively. Refer to Note 4—U.S. and Europe Retrospective Responsibility Plans for the book value of series B and series C preferred stock.