Investment Securities - Gross Unrealized Losses and Fair Value of Investment Securities (Details) $ in Millions	Mar. 31, 2025 USD ($)
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	$ 21,181
Less Than 12 Months, Unrealized Losses	(169)
12 Months or Greater, Fair Value	49,016
12 Months or Greater, Unrealized Losses	(6,008)
Total, Fair Value	70,197
Total, Unrealized Losses	(6,177)
U.S. Treasury and agencies
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	6,408
Less Than 12 Months, Unrealized Losses	(5)
12 Months or Greater, Fair Value	17,376
12 Months or Greater, Unrealized Losses	(1,777)
Total, Fair Value	23,784
Total, Unrealized Losses	(1,782)
Residential mortgage-backed securities
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	11,691
Less Than 12 Months, Unrealized Losses	(131)
12 Months or Greater, Fair Value	15,511
12 Months or Greater, Unrealized Losses	(1,781)
Total, Fair Value	27,202
Total, Unrealized Losses	(1,912)
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	7,503
12 Months or Greater, Unrealized Losses	(1,144)
Total, Fair Value	7,503
Total, Unrealized Losses	(1,144)
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	1,719
Less Than 12 Months, Unrealized Losses	(4)
12 Months or Greater, Fair Value	983
12 Months or Greater, Unrealized Losses	(2)
Total, Fair Value	2,702
Total, Unrealized Losses	(6)
Obligations of state and political subdivisions
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	1,363
Less Than 12 Months, Unrealized Losses	(29)
12 Months or Greater, Fair Value	7,632
12 Months or Greater, Unrealized Losses	(1,304)
Total, Fair Value	8,995
Total, Unrealized Losses	(1,333)
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