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Investment securities (Tables) | 12 Months Ended
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Dec. 31, 2024
Investments, Debt and Equity Securities [Abstract] |
Amortized Cost and Estimated Fair Value of Investment Securities | The amortized cost and estimated fair value of investment securities were as follows:
(Dollars in millions) | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Value
December 31, 2024 |  |  |  |  |  |  |
Investment securities available for sale: |  |  |  |  |  |  |
U.S. Treasury | $ | 7,945 |  |  | $ | 13 |  |  | $ | 27 |  |  | $ | 7,931 |
Mortgage-backed securities: |  |  |  |  |  |  |
Government issued or guaranteed: |  |  |  |  |  |  |
Commercial | 3,739 |  |  | 8 |  |  | 45 |  |  | 3,702 |
Residential | 7,368 |  |  | 13 |  |  | 167 |  |  | 7,214 |
Other | 2 |  |  | — |  |  | — |  |  | 2 |
| 19,054 |  |  | 34 |  |  | 239 |  |  | 18,849 |
Investment securities held to maturity: |  |  |  |  |  |  |
U.S. Treasury | 1,015 |  |  | — |  |  | 14 |  |  | 1,001 |
Mortgage-backed securities: |  |  |  |  |  |  |
Government issued or guaranteed: |  |  |  |  |  |  |
Commercial | 2,034 |  |  | — |  |  | 157 |  |  | 1,877 |
Residential | 8,773 |  |  | — |  |  | 961 |  |  | 7,812 |
Privately issued | 37 |  |  | 9 |  |  | — |  |  | 46 |
State and political subdivisions | 2,335 |  |  | — |  |  | 117 |  |  | 2,218 |
Other | 1 |  |  | — |  |  | — |  |  | 1 |
| 14,195 |  |  | 9 |  |  | 1,249 |  |  | 12,955 |
Total debt securities | $ | 33,249 |  |  | $ | 43 |  |  | $ | 1,488 |  |  | $ | 31,804 |
Equity and other securities: |  |  |  |  |  |  |
Readily marketable equity — at fair value | $ | 235 |  |  | $ | 3 |  |  | $ | 3 |  |  | $ | 235 |
Other — at cost | 772 |  |  | — |  |  | — |  |  | 772 |
Total equity and other securities | $ | 1,007 |  |  | $ | 3 |  |  | $ | 3 |  |  | $ | 1,007 |
December 31, 2023 |  |  |  |  |  |  |
Investment securities available for sale: |  |  |  |  |  |  |
U.S. Treasury | $ | 7,818 |  |  | $ | — |  |  | $ | 113 |  |  | $ | 7,705 |
Mortgage-backed securities: |  |  |  |  |  |  |
Government issued or guaranteed: |  |  |  |  |  |  |
Commercial | 425 |  |  | — |  |  | 9 |  |  | 416 |
Residential | 2,272 |  |  | — |  |  | 118 |  |  | 2,154 |
Other | 176 |  |  | — |  |  | 11 |  |  | 165 |
| 10,691 |  |  | — |  |  | 251 |  |  | 10,440 |
Investment securities held to maturity: |  |  |  |  |  |  |
U.S. Treasury | 1,005 |  |  | — |  |  | 31 |  |  | 974 |
Mortgage-backed securities: |  |  |  |  |  |  |
Government issued or guaranteed: |  |  |  |  |  |  |
Commercial | 2,033 |  |  | — |  |  | 130 |  |  | 1,903 |
Residential | 9,747 |  |  | 4 |  |  | 802 |  |  | 8,949 |
Privately issued | 42 |  |  | 9 |  |  | 5 |  |  | 46 |
State and political subdivisions | 2,501 |  |  | — |  |  | 67 |  |  | 2,434 |
Other | 2 |  |  | — |  |  | — |  |  | 2 |
| 15,330 |  |  | 13 |  |  | 1,035 |  |  | 14,308 |
Total debt securities | $ | 26,021 |  |  | $ | 13 |  |  | $ | 1,286 |  |  | $ | 24,748 |
Equity and other securities: |  |  |  |  |  |  |
Readily marketable equity — at fair value | $ | 266 |  |  | $ | 5 |  |  | $ | 3 |  |  | $ | 268 |
Other — at cost | 859 |  |  | — |  |  | — |  |  | 859 |
Total equity and other securities | $ | 1,125 |  |  | $ | 5 |  |  | $ | 3 |  |  | $ | 1,127 |
Debt Securities, Held-to-Maturity, Credit Quality Indicator | As of December 31, 2024, the latest available investment ratings of obligations of states and political subdivisions, privately issued mortgage-backed securities and other debt securities were as follows:
|  |  |  |  | Average Credit Rating of Fair Value Amount
(Dollars in millions) | Amortized Cost |  | Estimated Fair Value |  | A or Better |  |  |  |  |  |  |  | Not Rated
State and political subdivisions | $ | 2,335 |  |  | $ | 2,218 |  |  | $ | 2,212 |  |  |  |  |  |  |  |  | $ | 6 |
Privately issued mortgage-backed securities | 37 |  |  | 46 |  |  | — |  |  |  |  |  |  |  |  | 46 |
Other | 3 |  |  | 3 |  |  | 2 |  |  |  |  |  |  |  |  | 1 |
Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity | At December 31, 2024, the amortized cost and estimated fair value of debt securities by contractual maturity were as follows:
(Dollars in millions) | Amortized Cost |  | Estimated Fair Value
Debt securities available for sale: |  |  |
Due in one year or less | $ | 2,940 |  |  | $ | 2,929 |
Due after one year through five years | 5,007 |  |  | 5,004 |
Due after five years through ten years | — |  |  | — |
Due after ten years | — |  |  | — |
| 7,947 |  |  | 7,933 |
Mortgage-backed securities | 11,107 |  |  | 10,916 |
| $ | 19,054 |  |  | $ | 18,849 |
Debt securities held to maturity: |  |  |
Due in one year or less | $ | 602 |  |  | $ | 602 |
Due after one year through five years | 635 |  |  | 618 |
Due after five years through ten years | 1,491 |  |  | 1,432 |
Due after ten years | 623 |  |  | 568 |
| 3,351 |  |  | 3,220 |
Mortgage-backed securities | 10,844 |  |  | 9,735 |
| $ | 14,195 |  |  | $ | 12,955 |
Schedule of Unrealized Loss on Investments | A summary of investment securities that as of December 31, 2024 and 2023 had been in a continuous unrealized loss position for less than twelve months and those that had been in a continuous unrealized loss position for twelve months or longer follows:
| Less Than 12 Months |  | 12 Months or More
(Dollars in millions) | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses
December 31, 2024 |  |  |  |  |  |  |
Investment securities available for sale: |  |  |  |  |  |  |
U.S. Treasury | $ | 1,971 |  |  | $ | 9 |  |  | $ | 2,554 |  |  | $ | 18 |
Mortgage-backed securities: |  |  |  |  |  |  |
Government issued or guaranteed: |  |  |  |  |  |  |
Commercial | 2,566 |  |  | 45 |  |  | 64 |  |  | — |
Residential | 4,429 |  |  | 53 |  |  | 1,623 |  |  | 114 |
Other | — |  |  | — |  |  | 2 |  |  | — |
| 8,966 |  |  | 107 |  |  | 4,243 |  |  | 132 |
Investment securities held to maturity: |  |  |  |  |  |  |
U.S. Treasury | 50 |  |  | — |  |  | 951 |  |  | 14 |
Mortgage-backed securities: |  |  |  |  |  |  |
Government issued or guaranteed: |  |  |  |  |  |  |
Commercial | — |  |  | — |  |  | 1,877 |  |  | 157 |
Residential | 996 |  |  | 19 |  |  | 6,811 |  |  | 942 |
Privately issued | — |  |  | — |  |  | — |  |  | — |
State and political subdivisions | 39 |  |  | 1 |  |  | 2,131 |  |  | 116 |
| 1,085 |  |  | 20 |  |  | 11,770 |  |  | 1,229 |
Total | $ | 10,051 |  |  | $ | 127 |  |  | $ | 16,013 |  |  | $ | 1,361 |
December 31, 2023 |  |  |  |  |  |  |
Investment securities available for sale: |  |  |  |  |  |  |
U.S. Treasury | $ | 229 |  |  | $ | 1 |  |  | $ | 7,474 |  |  | $ | 112 |
Mortgage-backed securities: |  |  |  |  |  |  |
Government issued or guaranteed: |  |  |  |  |  |  |
Commercial | 74 |  |  | 1 |  |  | 330 |  |  | 8 |
Residential | 151 |  |  | 2 |  |  | 1,959 |  |  | 116 |
Other | 6 |  |  | — |  |  | 154 |  |  | 11 |
| 460 |  |  | 4 |  |  | 9,917 |  |  | 247 |
Investment securities held to maturity: |  |  |  |  |  |  |
U.S. Treasury | 50 |  |  | — |  |  | 924 |  |  | 31 |
Mortgage-backed securities: |  |  |  |  |  |  |
Government issued or guaranteed: |  |  |  |  |  |  |
Commercial | 328 |  |  | 9 |  |  | 1,575 |  |  | 121 |
Residential | 955 |  |  | 11 |  |  | 7,139 |  |  | 791 |
Privately issued | — |  |  | — |  |  | 34 |  |  | 5 |
State and political subdivisions | 218 |  |  | 3 |  |  | 2,172 |  |  | 64 |
| 1,551 |  |  | 23 |  |  | 11,844 |  |  | 1,012 |
Total | $ | 2,011 |  |  | $ | 27 |  |  | $ | 21,761 |  |  | $ | 1,259 |

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- Definition For all investments in an unrealized loss position, including those for which other-than-temporary impairments have not been recognized in earnings (including investments for which a portion of an other-than-temporary impairment has been recognized in other comprehensive income), a tabular disclosure of the aggregate related fair value of investments with unrealized losses and the aggregate amount of unrealized losses (that is, the amount by which amortized cost basis exceeds fair value). + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 + Details
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