Investment Securities - Gross Unrealized Losses and Fair Value of Investment Securities (Details) $ in Millions	Mar. 31, 2026 USD ($)
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	$ 19,274
Less Than 12 Months, Unrealized Losses	(126)
12 Months or Greater, Fair Value	42,238
12 Months or Greater, Unrealized Losses	(4,697)
Total, Fair Value	61,512
Total, Unrealized Losses	(4,823)
U.S. Treasury and agencies
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	7,344
Less Than 12 Months, Unrealized Losses	(14)
12 Months or Greater, Fair Value	14,937
12 Months or Greater, Unrealized Losses	(1,348)
Total, Fair Value	22,281
Total, Unrealized Losses	(1,362)
Residential mortgage-backed securities
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	8,131
Less Than 12 Months, Unrealized Losses	(77)
12 Months or Greater, Fair Value	12,845
12 Months or Greater, Unrealized Losses	(1,340)
Total, Fair Value	20,976
Total, Unrealized Losses	(1,417)
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	7,711
12 Months or Greater, Unrealized Losses	(966)
Total, Fair Value	7,711
Total, Unrealized Losses	(966)
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	1,974
Less Than 12 Months, Unrealized Losses	(5)
12 Months or Greater, Fair Value	0
12 Months or Greater, Unrealized Losses	0
Total, Fair Value	1,974
Total, Unrealized Losses	(5)
Obligations of state and political subdivisions
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	1,825
Less Than 12 Months, Unrealized Losses	(30)
12 Months or Greater, Fair Value	6,738
12 Months or Greater, Unrealized Losses	(1,043)
Total, Fair Value	8,563
Total, Unrealized Losses	$ (1,073)