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Securities (Tables) | 9 Months Ended
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Sep. 30, 2025
Investments, Debt and Equity Securities [Abstract] |
Debt Securities, Available-for-sale | The amortized cost and fair value of securities available-for-sale and held-to-maturity at September 30, 2025 and December 31, 2024 are summarized as follows (in thousands):
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
September 30, 2025: |  |  |  |  |  |  |
Securities available-for-sale: |  |  |  |  |  |  |
U.S. Treasury securities | $ | 1,606,437 |  |  | $ | 46 |  |  | $ | 12,925 |  |  | $ | 1,593,558 |
U.S. Government agency securities | 219,925 |  |  | 18 |  |  | 16,127 |  |  | 203,816 |
Mortgage-backed securities | 2,684,056 |  |  | 12,604 |  |  | 78,470 |  |  | 2,618,190 |
State and municipal securities | 1,774,299 |  |  | 3,994 |  |  | 87,755 |  |  | 1,690,538 |
Asset-backed securities | 195 |  |  | — |  |  | — |  |  | 195 |
Corporate notes and other | 314,816 |  |  | 1,976 |  |  | 11,283 |  |  | 305,509 |
| $ | 6,599,728 |  |  | $ | 18,638 |  |  | $ | 206,560 |  |  | $ | 6,411,806 |
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
September 30, 2025: |  |  |  |  |  |  |
Securities held-to-maturity: |  |  |  |  |  |  |
U.S. Treasury securities | $ | 19,912 |  |  | $ | — |  |  | $ | 448 |  |  | $ | 19,464 |
U.S. Government agency securities | 255,676 |  |  | — |  |  | 6,440 |  |  | 249,236 |
Mortgage-backed securities | 352,571 |  |  | 530 |  |  | 20,455 |  |  | 332,646 |
State and municipal securities | 1,828,121 |  |  | 778 |  |  | 181,161 |  |  | 1,647,738 |
Asset-backed securities | 120,815 |  |  | 22 |  |  | 2,862 |  |  | 117,975 |
Corporate notes and other | 69,414 |  |  | — |  |  | 4,405 |  |  | 65,009 |
| $ | 2,646,509 |  |  | $ | 1,330 |  |  | $ | 215,771 |  |  | $ | 2,432,068 |
Allowance for credit losses - securities held-to-maturity | (1,707) |  |  |  |  |  |  |
Securities held-to-maturity, net of allowance for credit losses | $ | 2,644,802 |  |  |  |  |  |  |
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
December 31, 2024: |  |  |  |  |  |  |
Securities available-for-sale: |  |  |  |  |  |  |
U.S. Treasury securities | $ | 1,419,727 |  |  | $ | 28 |  |  | $ | 14,277 |  |  | $ | 1,405,478 |
U.S. Government agency securities | 238,636 |  |  | 2 |  |  | 24,572 |  |  | 214,066 |
Mortgage-backed securities | 2,068,950 |  |  | 976 |  |  | 107,567 |  |  | 1,962,359 |
State and municipal securities | 1,537,910 |  |  | 15,382 |  |  | 46,735 |  |  | 1,506,557 |
Asset-backed securities | 45,280 |  |  | 85 |  |  | 27 |  |  | 45,338 |
Corporate notes and other | 476,900 |  |  | 107 |  |  | 28,436 |  |  | 448,571 |
| $ | 5,787,403 |  |  | $ | 16,580 |  |  | 221,614 |  |  | $ | 5,582,369 |
Securities held-to-maturity: |  |  |  |  |  |  |
U.S Treasury securities | $ | 19,841 |  |  | $ | — |  |  | $ | 954 |  |  | $ | 18,887 |
U.S. Government agency securities | 315,286 |  |  | — |  |  | 13,719 |  |  | 301,567 |
Mortgage-backed securities | 366,029 |  |  | 6 |  |  | 34,573 |  |  | 331,462 |
State and municipal securities | 1,854,942 |  |  | 1,956 |  |  | 178,744 |  |  | 1,678,154 |
Asset-backed securities | 161,957 |  |  | 41 |  |  | 6,920 |  |  | 155,078 |
Corporate notes | 82,551 |  |  | — |  |  | 7,447 |  |  | 75,104 |
| $ | 2,800,606 |  |  | $ | 2,003 |  |  | $ | 242,357 |  |  | $ | 2,560,252 |
Allowance for credit losses - securities held-to-maturity | (1,707) |  |  |  |  |  |  |
Securities held-to-maturity, net of allowance for credit losses | $ | 2,798,899 |  |  |  |  |  |  |
Amortized Cost and Fair Value of Debt Securities by Contractual Maturity | The amortized cost and fair value of debt securities as of September 30, 2025 by contractual maturity is shown below. Actual maturities may differ from contractual maturities of mortgage- and asset-backed securities since the mortgages and assets underlying the securities may be called or prepaid with or without penalty. Therefore, these securities are not included in the maturity categories in the following summary (in thousands):
| Available-for-sale |  | Held-to-maturity
September 30, 2025: | Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value
Due in one year or less | $ | 254,717 |  |  | $ | 254,763 |  |  | $ | 19,912 |  |  | $ | 19,464 |
Due in one year to five years | 153,534 |  |  | 148,861 |  |  | 301,156 |  |  | 292,411 |
Due in five years to ten years | 343,759 |  |  | 329,620 |  |  | 69,337 |  |  | 64,814 |
Due after ten years | 3,163,467 |  |  | 3,060,177 |  |  | 1,782,718 |  |  | 1,604,758 |
Mortgage-backed securities | 2,684,056 |  |  | 2,618,190 |  |  | 352,571 |  |  | 332,646 |
Asset-backed securities | 195 |  |  | 195 |  |  | 120,815 |  |  | 117,975 |
| $ | 6,599,728 |  |  | $ | 6,411,806 |  |  | $ | 2,646,509 |  |  | $ | 2,432,068 |
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value | At September 30, 2025 and December 31, 2024, the following available-for-sale securities had unrealized losses. The table below classifies these investments according to the term of the unrealized losses of less than twelve months or twelve months or longer (in thousands):
| Investments with an Unrealized Loss of less than 12 months |  | Investments with an Unrealized Loss of 12 months or longer |  | Total Investments with an Unrealized Loss
| Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses
At September 30, 2025 |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities | $ | 226,406 |  |  | $ | 3,684 |  |  | $ | 1,111,559 |  |  | $ | 9,241 |  |  | $ | 1,337,965 |  |  | $ | 12,925 |
U.S. Government agency securities | 1,251 |  |  | 4 |  |  | 198,385 |  |  | 16,123 |  |  | 199,636 |  |  | 16,127 |
Mortgage-backed securities | 530,173 |  |  | 1,955 |  |  | 916,298 |  |  | 76,515 |  |  | 1,446,471 |  |  | 78,470 |
State and municipal securities | 411,231 |  |  | 20,562 |  |  | 991,290 |  |  | 67,193 |  |  | 1,402,521 |  |  | 87,755 |
Asset-backed securities | 195 |  |  | — |  |  | — |  |  | — |  |  | 195 |  |  | — |
Corporate notes | 17,033 |  |  | 39 |  |  | 146,695 |  |  | 11,244 |  |  | 163,728 |  |  | 11,283 |
Total temporarily-impaired securities | $ | 1,186,289 |  |  | $ | 26,244 |  |  | $ | 3,364,227 |  |  | $ | 180,316 |  |  | $ | 4,550,516 |  |  | $ | 206,560 |
At December 31, 2024 |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities | $ | 1,123,453 |  |  | $ | 12,223 |  |  | $ | 177,930 |  |  | $ | 2,054 |  |  | $ | 1,301,383 |  |  | $ | 14,277 |
U.S. Government agency securities | 2,347 |  |  | 18 |  |  | 210,127 |  |  | 24,554 |  |  | 212,474 |  |  | 24,572 |
Mortgage-backed securities | 990,956 |  |  | 9,211 |  |  | 607,659 |  |  | 98,356 |  |  | 1,598,615 |  |  | 107,567 |
State and municipal securities | 446,241 |  |  | 5,590 |  |  | 348,807 |  |  | 41,145 |  |  | 795,048 |  |  | 46,735 |
Asset-backed securities | 30,369 |  |  | 27 |  |  | — |  |  | — |  |  | 30,369 |  |  | 27 |
Corporate notes | 131,073 |  |  | 1,215 |  |  | 274,601 |  |  | 27,221 |  |  | 405,674 |  |  | 28,436 |
Total temporarily-impaired securities | $ | 2,724,439 |  |  | $ | 28,284 |  |  | $ | 1,619,124 |  |  | $ | 193,330 |  |  | $ | 4,343,563 |  |  | $ | 221,614 |

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