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		9 Months Ended
	Jun. 30, 2021 USD ($) $ / shares shares	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) $ / shares shares	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) $ / shares shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 37,696	$ 34,585	$ 36,210	$ 34,684
VE territory covered losses incurred	(21)	(9)	(38)	(22)
Recovery through conversion rate adjustment	(2)	5	(2)	5
Ending balance	37,790	35,524	37,790	35,524
As-converted Value of Preferred Stock	8,710		8,710		$ 7,499
Book Value of Preferred Stock, Total	2,594		2,594		2,649
Less: right to recover for covered losses	(24)		(24)		(39)
Preferred Stock, As-Converted Value, Total recovery for covered losses available	8,686		8,686		7,460
Book Value of Preferred of Stock, Total recovery for covered losses available	$ 2,570		$ 2,570		$ 2,610
Share price (in dollars per share) | $ / shares	$ 233.82		$ 233.82		$ 199.97
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 3,347	5,462	$ 5,086	5,462
Recovery through conversion rate adjustment	(40)	(164)	(55)	(164)
Ending balance	3,132	5,298	3,132	5,298
Right to Recover for Covered Losses
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(41)	(184)	(39)	(171)
VE territory covered losses incurred	(21)	(9)	(38)	(22)
Recovery through conversion rate adjustment	38	169	53	169
Ending balance	(24)	$ (24)	(24)	(24)
UK&I preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment			35	72
As-converted Value of Preferred Stock	3,666		3,666		$ 3,168
Book Value of Preferred Stock	$ 1,071		$ 1,071		$ 1,106
Preferred stock, shares outstanding (in shares) | shares	2		2		2
Preferred stock, conversion rate	6.3210		6.3210		6.387
UK&I preferred stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 1,106
VE territory covered losses incurred			0
Recovery through conversion rate adjustment			(35)
Ending balance	$ 1,071		1,071
Europe preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment			20	$ 92
As-converted Value of Preferred Stock	5,044		5,044		$ 4,331
Book Value of Preferred Stock	$ 1,523		$ 1,523		$ 1,543
Preferred stock, shares outstanding (in shares) | shares	3		3		3
Preferred stock, conversion rate	6.834		6.834		6.861
Europe preferred stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 1,543
VE territory covered losses incurred			0
Recovery through conversion rate adjustment			(20)
Ending balance	$ 1,523		$ 1,523