Investment Securities - Gross Unrealized Losses and Fair Value of Investment Securities (Details) $ in Millions	Sep. 30, 2025 USD ($)
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	$ 10,386
Less Than 12 Months, Unrealized Losses	(49)
12 Months or Greater, Fair Value	49,728
12 Months or Greater, Unrealized Losses	(4,998)
Total, Fair Value	60,114
Total, Unrealized Losses	(5,047)
U.S. Treasury and agencies
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	4,398
Less Than 12 Months, Unrealized Losses	(4)
12 Months or Greater, Fair Value	20,288
12 Months or Greater, Unrealized Losses	(1,494)
Total, Fair Value	24,686
Total, Unrealized Losses	(1,498)
Residential mortgage-backed securities
Schedule Of Fair Value And Gross Unrealized Losses [Line Items]
Less Than 12 Months, Fair Value	4,825
Less Than 12 Months, Unrealized Losses	(30)
12 Months or Greater, Fair Value	13,924
12 Months or Greater, Unrealized Losses	(1,459)
Total, Fair Value	18,749
Total, Unrealized Losses	(1,489)
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	7,694
12 Months or Greater, Unrealized Losses	(961)
Total, Fair Value	7,694
Total, Unrealized Losses	(961)
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	291
Less Than 12 Months, Unrealized Losses	0
12 Months or Greater, Fair Value	0
12 Months or Greater, Unrealized Losses	0
Total, Fair Value	291
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	872
Less Than 12 Months, Unrealized Losses	(15)
12 Months or Greater, Fair Value	7,815
12 Months or Greater, Unrealized Losses	(1,084)
Total, Fair Value	8,687
Total, Unrealized Losses	$ (1,099)