Investment Securities - Gross Unrealized Losses and Fair Value of Investment Securities (Details) $ in Millions	Mar. 31, 2024 USD ($)
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	$ 6,977
Less Than 12 Months, Unrealized Losses	(29)
12 Months or Greater, Fair Value	54,502
12 Months or Greater, Unrealized Losses	(7,081)
Total, Fair Value	61,479
Total, Unrealized Losses	(7,110)
U.S. Treasury and agencies
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	1,219
Less Than 12 Months, Unrealized Losses	(6)
12 Months or Greater, Fair Value	17,077
12 Months or Greater, Unrealized Losses	(2,209)
Total, Fair Value	18,296
Total, Unrealized Losses	(2,215)
Residential mortgage-backed securities
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	1,580
Less Than 12 Months, Unrealized Losses	(9)
12 Months or Greater, Fair Value	20,617
12 Months or Greater, Unrealized Losses	(2,386)
Total, Fair Value	22,197
Total, Unrealized Losses	(2,395)
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	7,284
12 Months or Greater, Unrealized Losses	(1,412)
Total, Fair Value	7,284
Total, Unrealized Losses	(1,412)
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	(1)
Total, Fair Value	7
Total, Unrealized Losses	(1)
Asset-backed securities
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	2,942
Less Than 12 Months, Unrealized Losses	(5)
12 Months or Greater, Fair Value	1,806
12 Months or Greater, Unrealized Losses	(20)
Total, Fair Value	4,748
Total, Unrealized Losses	(25)
Obligations of state and political subdivisions
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	1,121
Less Than 12 Months, Unrealized Losses	(9)
12 Months or Greater, Fair Value	7,707
12 Months or Greater, Unrealized Losses	(1,053)
Total, Fair Value	8,828
Total, Unrealized Losses	(1,062)
Other
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	115
Less Than 12 Months, Unrealized Losses	0
12 Months or Greater, Fair Value	4
12 Months or Greater, Unrealized Losses	0
Total, Fair Value	119
Total, Unrealized Losses	$ 0