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, 2021 USD ($) $ / shares shares	Mar. 31, 2020 USD ($)	Mar. 31, 2021 USD ($) $ / shares shares	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($) $ / shares shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 37,679	$ 35,270	$ 36,210	$ 34,684
VE territory covered losses incurred	(7)	(9)	(17)	(13)
Ending balance	37,696	34,585	37,696	34,585
As-converted Value of Preferred Stock	7,925		7,925		$ 7,499
Book Value of Preferred Stock, Total	2,634		2,634		2,649
Less: right to recover for covered losses	(41)		(41)		(39)
Preferred Stock, As-Converted Value, Total recovery for covered losses available	7,884		7,884		7,460
Book Value of Preferred of Stock, Total recovery for covered losses available	$ 2,593		$ 2,593		$ 2,610
Share price (in dollars per share) | $ / shares	$ 211.73		$ 211.73		$ 199.97
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 3,683	5,462	$ 5,086	5,462
Recovery through conversion rate adjustment			15
Ending balance	3,347	5,462	3,347	5,462
Right to Recover for Covered Losses
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(34)	(175)	(39)	(171)
VE territory covered losses incurred	(7)	(9)	(17)	(13)
Recovery through conversion rate adjustment			(15)
Ending balance	(41)	$ (184)	(41)	$ (184)
UK&I preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment			(9)
As-converted Value of Preferred Stock	3,344		3,344		$ 3,168
Book Value of Preferred Stock	$ 1,097		$ 1,097		$ 1,106
Preferred stock, shares outstanding (in shares) | shares	2		2		2
Preferred stock, conversion rate	6.368		6.368		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			(9)
Ending balance	$ 1,097		1,097
Europe preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment			(6)
As-converted Value of Preferred Stock	4,581		4,581		$ 4,331
Book Value of Preferred Stock	$ 1,537		$ 1,537		$ 1,543
Preferred stock, shares outstanding (in shares) | shares	3		3		3
Preferred stock, conversion rate	6.853		6.853		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			(6)
Ending balance	$ 1,537		$ 1,537