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, 2017 USD ($) $ / shares shares		Sep. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Jun. 21, 2016 USD ($)
Class of Stock [Line Items]
Beginning Balance				$ 32,912	$ 29,842
VE Territory Covered Losses Incurred (Note 14)				(209)	(34)
Ending Balance	$ 32,760		$ 32,912	32,760	32,912
Preferred Stock Available to Recover Covered Losses, Value, As-converted	8,048		6,504	8,048	6,504
Preferred Stock Available to Recover Covered Losses, Value	5,526		5,717	5,526	5,717
Right to recover for covered losses	(52)		(34)	(52)	(34)	$ (25)
Preferred Stock Available to Recover Covered Losses, Value, As-converted, Net	7,996		6,470	7,996	6,470
Preferred Stock Available to Recover Covered Losses, Value, Net	$ 5,474		$ 5,683	$ 5,474	$ 5,683
Closing Stock Price | $ / shares	$ 105.24			$ 105.24
Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding | shares	5,000,000		5,000,000	5,000,000	5,000,000
Series B Preferred Stock
Class of Stock [Line Items]
Recovery through conversion rate adjustment				$ 190
Preferred Stock Available to Recover Covered Losses, Value, As-converted	$ 3,414		$ 2,862	3,414	$ 2,862
Preferred stock	$ 2,326		$ 2,516	$ 2,326	$ 2,516
Preferred stock, shares outstanding | shares	2,000,000		2,000,000	2,000,000	2,000,000
Preferred stock, conversion rate	13.0770	[1]	13.952
Series C Preferred Stock
Class of Stock [Line Items]
Recovery through conversion rate adjustment				$ 1
Preferred Stock Available to Recover Covered Losses, Value, As-converted	$ 4,634		$ 3,642	4,634	$ 3,642
Preferred stock	$ 3,200		$ 3,201	$ 3,200	$ 3,201
Preferred stock, shares outstanding | shares	3,000,000		3,000,000	3,000,000	3,000,000
Preferred stock, conversion rate	13.9480		13.952
Class A common stock
Class of Stock [Line Items]
Closing Stock Price | $ / shares	$ 105.24		$ 82.70	$ 105.24	$ 82.70
Right to Recover for Covered Losses
Class of Stock [Line Items]
Beginning Balance				$ (34)	$ 0
VE Territory Covered Losses Incurred (Note 14)				(209)	(34)
Recovery through conversion rate adjustment				191
Ending Balance	$ (52)		$ (34)	(52)	(34)
Preferred Stock
Class of Stock [Line Items]
Beginning Balance				5,717	0
Recovery through conversion rate adjustment				(191)
Ending Balance	5,526		5,717	5,526	5,717
Preferred Stock | Series B Preferred Stock
Class of Stock [Line Items]
Beginning Balance				2,516
Recovery through conversion rate adjustment				(190)
Ending Balance	2,326		2,516	2,326	2,516
Preferred Stock | Series C Preferred Stock
Class of Stock [Line Items]
Beginning Balance				3,201
Recovery through conversion rate adjustment				(1)
Ending Balance	$ 3,200		$ 3,201	$ 3,200	$ 3,201

[1] The class B to class A common stock conversion rate is presented on a rounded basis. Conversion calculations for dividend payments are based on a conversion rate rounded to the tenth decimal.