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Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
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Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items] |  |
Investment securities available for sale, amortized cost | $ 12,397 | $ 10,691
Investment securities available for sale, gross unrealized gains | 1 |
Investment securities available for sale, gross unrealized losses | 264 | 251
Investment securities available for sale, estimated fair value | 12,134 | 10,440
Amortized cost for held to maturity | 15,078 | 15,330
Gross unrealized gains for held to maturity | 9 | 13
Gross unrealized losses for held to maturity | 1,222 | 1,035
Estimated fair value for held to maturity | 13,865 | 14,308
Equity and other securities, Amortized Cost | 1,279 | 1,125
Equity securities, Gross Unrealized Gains | 8 | 5
Equity securities, Gross Unrealized Losses | 3 | 3
Equity and other securities, Estimated Fair Value | 1,284 | 1,127
Other securities, Amortized cost | 928 | 859
Other securities, Estimated fair value | 928 | 859
Total debt securities Amortized cost | 27,475 | 26,021
Total debt securities Gross unrealized gains | 10 | 13
Total debt securities Gross unrealized losses | 1,486 | 1,286
Total debt securities Estimated fair value | 25,999 | 24,748
Readily marketable securities Amortized cost | 351 | 266
Readily marketable securities Gross unrealized gains | 8 | 5
Readily marketable securities Gross unrealized losses | 3 | 3
Readily marketable securities Estimated fair value | 356 | 268
U.S. Treasury and Federal Agencies [Member] |  |
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items] |  |
Investment securities available for sale, amortized cost | 7,818 | 7,818
Investment securities available for sale, gross unrealized losses | 99 | 113
Investment securities available for sale, estimated fair value | 7,719 | 7,705
Amortized cost for held to maturity | 1,007 | 1,005
Gross unrealized losses for held to maturity | 33 | 31
Estimated fair value for held to maturity | 974 | 974
Obligations of States and Political Subdivisions [Member] |  |
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items] |  |
Amortized cost for held to maturity | 2,466 | 2,501
Gross unrealized losses for held to maturity | 94 | 67
Estimated fair value for held to maturity | 2,372 | 2,434
Commercial [Member] |  |
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items] |  |
Investment securities available for sale, amortized cost | 1,355 | 425
Investment securities available for sale, gross unrealized gains | 1 |
Investment securities available for sale, gross unrealized losses | 13 | 9
Investment securities available for sale, estimated fair value | 1,343 | 416
Amortized cost for held to maturity | 2,035 | 2,033
Gross unrealized losses for held to maturity | 154 | 130
Estimated fair value for held to maturity | 1,881 | 1,903
Residential [Member] |  |
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items] |  |
Investment securities available for sale, amortized cost | 3,053 | 2,272
Investment securities available for sale, gross unrealized losses | 143 | 118
Investment securities available for sale, estimated fair value | 2,910 | 2,154
Amortized cost for held to maturity | 9,527 | 9,747
Gross unrealized gains for held to maturity |  | 4
Gross unrealized losses for held to maturity | 935 | 802
Estimated fair value for held to maturity | 8,592 | 8,949
Other Debt Securities [Member] |  |
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items] |  |
Investment securities available for sale, amortized cost | 171 | 176
Investment securities available for sale, gross unrealized losses | 9 | 11
Investment securities available for sale, estimated fair value | 162 | 165
Amortized cost for held to maturity | 2 | 2
Estimated fair value for held to maturity | 2 | 2
Privately Issued [Member] |  |
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items] |  |
Amortized cost for held to maturity | 41 | 42
Gross unrealized gains for held to maturity | 9 | 9
Gross unrealized losses for held to maturity | 6 | 5
Estimated fair value for held to maturity | $ 44 | $ 46

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