U.S. and Europe Retrospective Responsibility Plan - Changes in Preferred Stock and Right to Recover Covered Losses (Details) $ / shares in Units, shares in Millions, $ in Millions	6 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares
Class of Stock [Line Items]
Preferred Stock, As-Converted Value	$ 12,232	$ 13,058
Preferred Stock, Book Value, Total	5,462	5,462
Less: right to recover for covered losses	(184)	(171)
Preferred Stock, As-Converted Value, Total Recovery for Covered Losses Available	12,048	12,887
Preferred Stock, Book Value, Total Recovery for Covered Losses Available	$ 5,278	$ 5,291
Share price (in dollars per share) | $ / shares	$ 161.12	$ 172.01
UK&I preferred stock
Class of Stock [Line Items]
Preferred Stock, As-Converted Value	$ 5,170	$ 5,519
Preferred Stock, Book Value	$ 2,285	$ 2,285
Preferred stock, shares outstanding (in shares) | shares	2	2
Preferred stock, conversion rate	12.936	12.936
Europe preferred stock
Class of Stock [Line Items]
Preferred Stock, As-Converted Value	$ 7,062	$ 7,539
Preferred Stock, Book Value	$ 3,177	$ 3,177
Preferred stock, shares outstanding (in shares) | shares	3	3
Preferred stock, conversion rate	13.884	13.884