U.S. and Europe Retrospective Responsibility Plans - Preferred Stock (Details) $ / shares in Units, shares in Millions, $ in Millions	Jun. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares
Class of Stock [Line Items]
As-converted value of preferred stock	$ 13,175	$ 11,403
Book Value of Preferred stock	5,462	5,470
Right to recover for covered losses	(169)	(7)
Total recovery for covered losses available, as converted	13,006	11,396
Total recovery for covered losses available, book value	$ 5,293	$ 5,463
Share Price (in dollars per share) | $ / shares	$ 173.55	$ 150.09
U.K.& I preferred stock
Class of Stock [Line Items]
As-converted value of preferred stock	$ 5,569	$ 4,823
Book Value of Preferred stock	$ 2,285	$ 2,291
Preferred stock, shares outstanding | shares	2	2
Preferred stock, conversion ratio	12.9360	12.9550
Europe preferred stock
Class of Stock [Line Items]
As-converted value of preferred stock	$ 7,606	$ 6,580
Book Value of Preferred stock	$ 3,177	$ 3,179
Preferred stock, shares outstanding | shares	3	3
Preferred stock, conversion ratio	13.884	13.8880