U.S. and Europe Retrospective Responsibility Plan - Changes in Preferred Stock and Right to Recover Covered Losses (Details) $ / shares in Units, $ in Millions				12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance				$ 34,006	$ 32,760	$ 32,912
VE Territory Covered Losses Incurred (Note 14)				(172)	(11)	(209)
Recovery through conversion rate adjustment				(8)
Ending Balance	$ 34,684	$ 34,006	$ 32,912	34,684	34,006	32,760
Preferred Stock Available to Recover Covered Losses, Value, As-converted	13,058	11,403		13,058	11,403
Preferred Stock Available to Recover Covered Losses, Value	5,462	5,470		5,462	5,470
Right to recover for covered losses (Note 5)	(171)	(7)		(171)	(7)
Preferred Stock Available to Recover Covered Losses, Value, As-converted, Net	12,887	11,396		12,887	11,396
Preferred Stock Available to Recover Covered Losses, Value, Net	$ 5,291	$ 5,463		$ 5,291	$ 5,463
Share price (US per share) | $ / shares	$ 172.01			$ 172.01
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares outstanding (in shares) | shares	5,000,000	5,000,000		5,000,000	5,000,000
Series B Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment				$ (6)	$ (35)	(190)
Preferred Stock Available to Recover Covered Losses, Value, As-converted	$ 5,519	$ 4,823		5,519	4,823
Preferred stock	$ 2,285	$ 2,291		$ 2,285	$ 2,291
Preferred stock, shares outstanding (in shares) | shares	2,000,000	2,000,000		2,000,000	2,000,000
Preferred stock, conversion rate	12.936	12.9550	13.952
Series C Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment				$ (2)	$ (21)	(1)
Preferred Stock Available to Recover Covered Losses, Value, As-converted	$ 7,539	$ 6,580		7,539	6,580
Preferred stock	$ 3,177	$ 3,179		$ 3,177	$ 3,179
Preferred stock, shares outstanding (in shares) | shares	3,000,000	3,000,000		3,000,000	3,000,000
Preferred stock, conversion rate	13.884	13.888
Class A common stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share price (US per share) | $ / shares	$ 172.01	$ 150.09		$ 172.01	$ 150.09
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance				$ 5,470	$ 5,526	5,717
Recovery through conversion rate adjustment				8	56	191
Ending Balance	$ 5,462	$ 5,470	$ 5,717	$ 5,462	$ 5,470	5,526
Preferred stock, shares outstanding (in shares) | shares	5,000,000	5,000,000		5,000,000	5,000,000
Preferred Stock | Series B Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance				$ 2,291
Recovery through conversion rate adjustment				(6)
Ending Balance	$ 2,285	$ 2,291		2,285	$ 2,291
Preferred Stock | Series C Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance				3,179
Recovery through conversion rate adjustment				(2)
Ending Balance	3,177	3,179		3,177	3,179
Right to Recover for Covered Losses
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance				(7)	(52)	(34)
VE Territory Covered Losses Incurred (Note 14)				(172)	(11)	(209)
Recovery through conversion rate adjustment				(8)	(56)	(191)
Ending Balance	$ (171)	$ (7)	$ (34)	$ (171)	$ (7)	$ (52)