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, 2022 USD ($) $ / shares shares	Mar. 31, 2021 USD ($)	Mar. 31, 2022 USD ($) $ / shares shares	Mar. 31, 2021 USD ($)	Sep. 30, 2021 USD ($) $ / shares shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 36,194	$ 37,679	$ 37,589	$ 36,210
VE territory covered losses incurred	(9)	(7)	(16)	(17)
Recovery through conversion rate adjustment			0	0
Ending balance	35,989	37,696	35,989	37,696
As-converted Value of Preferred Stock	8,231		8,231		$ 8,299
Book Value of Preferred Stock, Total	2,565		2,565		2,594
Less: right to recover for covered losses	(120)		(120)		(133)
Preferred Stock, As-Converted Value, Total recovery for covered losses available	8,111		8,111		8,166
Book Value of Preferred of Stock, Total recovery for covered losses available	$ 2,445		$ 2,445		$ 2,461
Share price (in dollars per share) | $ / shares	$ 221.77		$ 221.77		$ 222.75
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 2,995	3,683	$ 3,080	5,086
Recovery through conversion rate adjustment			(29)	(15)
Ending balance	2,987	3,347	2,987	3,347
Right to Recover for Covered Losses
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(111)	(34)	(133)	(39)
VE territory covered losses incurred	(9)	(7)	(16)	(17)
Recovery through conversion rate adjustment			29	15
Ending balance	(120)	(41)	(120)	(41)
Series B preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment			26	9
As-converted Value of Preferred Stock	3,450		3,450		$ 3,493
Book Value of Preferred Stock	$ 1,045		$ 1,045		$ 1,071
Preferred stock, shares outstanding (in shares) | shares	2		2		2
Preferred stock, conversion rate	6.2710		6.2710		6.3210
Series B preferred stock | Preferred Stock
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			(26)	(9)
Ending balance	$ 1,045	1,097	1,045	1,097
Series C preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment			3	6
As-converted Value of Preferred Stock	4,781		4,781		$ 4,806
Book Value of Preferred Stock	$ 1,520		$ 1,520		$ 1,523
Preferred stock, shares outstanding (in shares) | shares	3		3		3
Preferred stock, conversion rate	6.8290		6.8290		6.834
Series C preferred stock | Preferred Stock
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			(3)	(6)
Ending balance	$ 1,520	$ 1,537	$ 1,520	$ 1,537