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	12 Months Ended
	Sep. 30, 2022 USD ($) $ / shares shares		Sep. 30, 2021 USD ($) $ / shares shares		Sep. 30, 2020 USD ($) shares		Sep. 30, 2019 shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 37,589		$ 36,210		$ 34,684
VE territory covered losses incurred	(43)		(147)		(37)
Recovery through conversion rate adjustment	0		(2)		5
Ending Balance	35,581		37,589		36,210
As-converted Value of Preferred Stock	3,353		8,299
Book Value of Preferred Stock	2,324		3,080
Book Value of Preferred Stock, Total	1,272		2,594
Less: right to recover for covered losses	(35)		(133)
As-converted value of Preferred Stock, Total recovery for covered losses available	3,318		8,166
Book value of Preferred of Stock, Total recovery for covered losses available	$ 1,237		2,461
Share price (in dollars per share) | $ / shares	$ 177.65
Series B
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment	$ 135		35		72
As-converted Value of Preferred Stock	1,309		3,493
Book Value of Preferred Stock	$ 460		$ 1,071
Preferred stock, shares outstanding (in shares) | shares	2		2
Preferred stock, conversion rate	2.9710		6.3210
Series C
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment	$ 6		$ 20		92
As-converted Value of Preferred Stock	2,044		4,806
Book Value of Preferred Stock	$ 812		$ 1,523
Preferred stock, shares outstanding (in shares) | shares	3		3
Preferred stock, conversion rate	3.6450		6.8340
Class A common stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share price (in dollars per share) | $ / shares	$ 177.65		$ 222.75
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 3,080	[1],[2]	$ 5,086	[1]	5,462
Recovery through conversion rate adjustment	(141)		(55)		(164)
Ending Balance	$ 2,324	[2]	$ 3,080	[1],[2]	$ 5,086	[1]
Preferred stock, shares outstanding (in shares) | shares	5		5		5		5
Preferred Stock | Series B
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 1,071		$ 1,106
VE territory covered losses incurred	0		0
Recovery through conversion rate adjustment	(135)		(35)
Sixth Anniversary Release	(476)
Ending Balance	460		1,071		$ 1,106
Preferred Stock | Series C
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	1,523		1,543
VE territory covered losses incurred	0		0
Recovery through conversion rate adjustment	(6)		(20)
Sixth Anniversary Release	(705)
Ending Balance	812		1,523		1,543
Right to Recover for Covered Losses
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(133)		(39)		(171)
VE territory covered losses incurred	(43)		(147)		(37)
Recovery through conversion rate adjustment	141		53		169
Sixth Anniversary Release	0
Ending Balance	$ (35)		$ (133)		$ (39)

[1] As of September 30, 2021 and 2020, the book value of series A preferred stock was $486 million and $2.4 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.

[2] As of September 30, 2022 and 2021, the book value of series A preferred stock was $1.0 billion and $486 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.