Investment Securities - Gross Unrealized Losses and Fair Value of Investment Securities (Details) $ in Millions	Sep. 30, 2024 USD ($)
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	$ 13,303
Less Than 12 Months, Unrealized Losses	(27)
12 Months or Greater, Fair Value	51,085
12 Months or Greater, Unrealized Losses	(5,300)
Total, Fair Value	64,388
Total, Unrealized Losses	(5,327)
U.S. Treasury and agencies
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	11,842
Less Than 12 Months, Unrealized Losses	(23)
12 Months or Greater, Fair Value	17,500
12 Months or Greater, Unrealized Losses	(1,780)
Total, Fair Value	29,342
Total, Unrealized Losses	(1,803)
Residential mortgage-backed securities
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	712
Less Than 12 Months, Unrealized Losses	(2)
12 Months or Greater, Fair Value	16,836
12 Months or Greater, Unrealized Losses	(1,573)
Total, Fair Value	17,548
Total, Unrealized Losses	(1,575)
Commercial mortgage-backed securities | Agency
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	0
Less Than 12 Months, Unrealized Losses	0
12 Months or Greater, Fair Value	7,605
12 Months or Greater, Unrealized Losses	(1,076)
Total, Fair Value	7,605
Total, Unrealized Losses	(1,076)
Commercial mortgage-backed securities | Non-agency
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	0
Less Than 12 Months, Unrealized Losses	0
12 Months or Greater, Fair Value	7
12 Months or Greater, Unrealized Losses	0
Total, Fair Value	7
Total, Unrealized Losses	0
Asset-backed securities
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	160
Less Than 12 Months, Unrealized Losses	0
12 Months or Greater, Fair Value	1,361
12 Months or Greater, Unrealized Losses	0
Total, Fair Value	1,521
Total, Unrealized Losses	0
Obligations of state and political subdivisions
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	589
Less Than 12 Months, Unrealized Losses	(2)
12 Months or Greater, Fair Value	7,772
12 Months or Greater, Unrealized Losses	(871)
Total, Fair Value	8,361
Total, Unrealized Losses	(873)
Other
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	0
Less Than 12 Months, Unrealized Losses	0
12 Months or Greater, Fair Value	4
12 Months or Greater, Unrealized Losses	0
Total, Fair Value	4
Total, Unrealized Losses	$ 0