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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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