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Securities - Narrative (Details) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2024 USD ($) debt_security bonds | Mar. 31, 2023 USD ($) | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($)
Schedule of Investments [Line Items] |  |  |  |
Debt Securities, Available-for-sale, Allowance for Credit Loss | $ 0 |  | $ 0 |
Transferred Securities, Unrealized Loss, Before Tax | 111,500,000 |  | 113,500,000 |
Securities transferred, Unrealized Loss, Net of Tax | 88,100,000 |  | 89,700,000 |
Fair value of securities transferred from AFS to HTM | 0 |  | 0 |
Available-for-sale Securities, Gross Realized Gain (Loss) [Abstract] |  |  |  |
Net gain (loss) on sale of securities AFS | (18,000) | $ (2,146,000) |  |
Gross realized gains on AFS securities | 61,000 | 3,300,000 |  |
Gross realized losses on AFS securities | 79,000 | 1,100,000 |  |
Equity investments | 9,554,000 |  | 9,691,000 |
Debt Securities, Held-to-maturity, Sale or Transfer of Investment [Abstract] |  |  |  |
Sale of HTM portfolio | 0 | 0 |  |
Interest receivable | 39,716,000 |  | 50,489,000 |
FHLB stock with other-than-temporary impairment | 0 |  |  |
Other-than-temporarily impaired equity investments | $ 0 |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | debt_security | 396 |  |  |
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents | $ 469,373,000 | 310,492,000 | 560,510,000 | $ 199,252,000
Less: Net gains recognized during the period on equity investments sold during the period | 0 | 2,416,000 |  |
Debt Securities, Available-for-Sale, Accrued Interest | 9,800,000 |  | 15,400,000 |
Debt Securities, Held-to-Maturity, Accrued Interest | $ 8,400,000 |  | 13,700,000 |
Held-To-Maturity Securities, Number Of Bonds Below Investment Grade | bonds | 3 |  |  |
State and political subdivisions |  |  |  |
Debt Securities, Held-to-maturity, Sale or Transfer of Investment [Abstract] |  |  |  |
Debt Securities, Held-to-Maturity, Excluding Accrued Interest, Credit Loss Expense (Reversal) | $ 0 | 0 |  |
Corporate bonds and other |  |  |  |
Debt Securities, Held-to-maturity, Sale or Transfer of Investment [Abstract] |  |  |  |
Debt Securities, Held-to-Maturity, Excluding Accrued Interest, Credit Loss Expense (Reversal) | 0 | $ 0 |  |
Asset Pledged as Collateral |  |  |  |
Debt Securities, Held-to-maturity, Sale or Transfer of Investment [Abstract] |  |  |  |
Debt Securities | 2,310,000,000 |  | 2,280,000,000 |
Asset Pledged As Collateral, Excess Amount |  |  |  |
Debt Securities, Held-to-maturity, Sale or Transfer of Investment [Abstract] |  |  |  |
Debt Securities | 251,900,000 |  | 213,100,000 |
Financial Asset, Past Due |  |  |  |
Debt Securities, Held-to-maturity, Sale or Transfer of Investment [Abstract] |  |  |  |
Securities held to maturity (estimated fair value of $138,879) | $ 0 |  | $ 0 |

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