U.S. and Europe Retrospective Responsibility Plan - Changes in Preferred Stock and Right to Recover Covered Losses (Details) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020 USD ($) $ / shares shares	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) $ / shares shares	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($) $ / shares shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 34,585	$ 34,275	$ 34,684	$ 34,006
VE territory covered losses incurred	(9)	(8)	(22)	(170)
Recovery through conversion rate adjustment	5	0	5
Ending balance	35,524	34,995	35,524	34,995
Preferred Stock, As-Converted Value	14,489		14,489		$ 13,058
Preferred Stock Available to Recover Covered Losses, Value	5,298		5,298		5,462
Less: right to recover for covered losses	(24)		(24)		(171)
Preferred Stock, As-Converted Value, Total Recovery for Covered Losses Available	14,465		14,465		12,887
Preferred Stock, Book Value, Total Recovery for Covered Losses Available	$ 5,274		$ 5,274		$ 5,291
Share price (in dollars per share) | $ / shares	$ 193.17		$ 193.17		$ 172.01
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 5,462	5,464	$ 5,462	5,470
Recovery through conversion rate adjustment	(164)	(2)	(164)	(8)
Ending balance	5,298	5,462	5,298	5,462
Right to Recover for Covered Losses
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(184)	(163)	(171)	(7)
VE territory covered losses incurred	(9)	(8)	(22)	(170)
Recovery through conversion rate adjustment	169	2	169	8
Ending balance	(24)	$ (169)	(24)	(169)
UK&I preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment			72	6
Preferred Stock, As-Converted Value	6,121		6,121		$ 5,519
Preferred Stock, Book Value	$ 2,213		$ 2,213		$ 2,285
Preferred stock, shares outstanding (in shares) | shares	2,000,000		2,000,000		2,000,000
Preferred stock, conversion rate	12.775		12.775		12.936
UK&I preferred stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 2,285
VE territory covered losses incurred			0
Recovery through conversion rate adjustment			72
Ending balance	$ 2,213		2,213
Europe preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment			92	$ 2
Preferred Stock, As-Converted Value	8,368		8,368		$ 7,539
Preferred Stock, Book Value	$ 3,085		$ 3,085		$ 3,177
Preferred stock, shares outstanding (in shares) | shares	3,000,000		3,000,000		3,000,000
Preferred stock, conversion rate	13.722		13.722		13.884
Europe preferred stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 3,177
VE territory covered losses incurred			0
Recovery through conversion rate adjustment			92
Ending balance	$ 3,085		$ 3,085