Investment Securities - Investment Ratings of All Privately Issued Mortgage-Backed Securities and Other Debt Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturityTable [Line Items]
Amortized Cost	$ 14,195	$ 15,330
Estimated Fair Value	12,955	14,308
State and political subdivisions
ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturityTable [Line Items]
Amortized Cost	2,335	2,501
Estimated Fair Value	2,218	2,434
State and political subdivisions \| A or Better
ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturityTable [Line Items]
Estimated Fair Value	2,212
State and political subdivisions \| Not Rated
ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturityTable [Line Items]
Estimated Fair Value	6
Privately issued
ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturityTable [Line Items]
Amortized Cost	37	42
Estimated Fair Value	46	46
Privately issued \| A or Better
ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturityTable [Line Items]
Estimated Fair Value	0
Privately issued \| Not Rated
ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturityTable [Line Items]
Estimated Fair Value	46
Other
ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturityTable [Line Items]
Amortized Cost	1	2
Estimated Fair Value	1	$ 2
Amortized Cost	3
Estimated Fair Value	3
Other \| A or Better
ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturityTable [Line Items]
Estimated Fair Value	2
Other \| Not Rated
ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturityTable [Line Items]
Estimated Fair Value	$ 1

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