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Investment Securities - Amortized Cost and Fair Value of Securities Available-for-Sale (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | --- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | $ 3,163,880 | $ 3,410,272 |  |
Allowance for Credit Losses | 0 | (2,195) | $ (2,195) | $ (2,525)
Net Carrying Amount | 3,163,880 | 3,408,077 |  |
Gross Unrealized Gains | 5,622 | 3,166 |  |
Gross Unrealized (Losses) | (245,006) | (338,604) |  |
Estimated Fair Value | 2,924,496 | 3,072,639 |  |
U.S. government-sponsored enterprises |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 254,741 | 297,698 |  |
Allowance for Credit Losses | 0 | 0 |  |
Net Carrying Amount | 254,741 | 297,698 |  |
Gross Unrealized Gains | 968 | 1,164 |  |
Gross Unrealized (Losses) | (8,394) | (14,072) |  |
Estimated Fair Value | 247,315 | 284,790 |  |
U.S. government-sponsored mortgage-backed securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 1,378,237 | 1,527,463 |  |
Allowance for Credit Losses | 0 | 0 |  |
Net Carrying Amount | 1,378,237 | 1,527,463 |  |
Gross Unrealized Gains | 1,360 | 760 |  |
Gross Unrealized (Losses) | (138,541) | (203,539) |  |
Estimated Fair Value | 1,241,056 | 1,324,684 |  |
Private mortgage-backed securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 155,155 | 184,643 |  |
Allowance for Credit Losses | 0 | 0 |  |
Net Carrying Amount | 155,155 | 184,643 |  |
Gross Unrealized Gains | 163 | 0 |  |
Gross Unrealized (Losses) | (7,417) | (13,249) |  |
Estimated Fair Value | 147,901 | 171,394 |  |
Non-government-sponsored asset backed securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 171,752 | 228,751 |  |
Allowance for Credit Losses | 0 |  |  |
Net Carrying Amount | 171,752 | 228,751 |  |
Gross Unrealized Gains | 348 | 331 |  |
Gross Unrealized (Losses) | (1,067) | (3,434) |  |
Estimated Fair Value | 171,033 | 225,648 |  |
State and political subdivisions |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 959,533 | 956,055 |  |
Allowance for Credit Losses | 0 | 0 |  |
Net Carrying Amount | 959,533 | 956,055 |  |
Gross Unrealized Gains | 1,015 | 335 |  |
Gross Unrealized (Losses) | (78,219) | (86,029) |  |
Estimated Fair Value | 882,329 | 870,361 |  |
Other securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 244,462 | 215,662 |  |
Allowance for Credit Losses | 0 | (2,195) |  |
Net Carrying Amount | 244,462 | 213,467 |  |
Gross Unrealized Gains | 1,768 | 576 |  |
Gross Unrealized (Losses) | (11,368) | (18,281) |  |
Estimated Fair Value | $ 234,862 | $ 195,762 |  |

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 + Details
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