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Securities - Additional Information (Details) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2021 USD ($) Security | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) Security | Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($)
Schedule Of Investments [Line Items] |  |  |  |  |
Trading securities | $ 0 |  | $ 0 |  |
Net gains reclassified from accumulated other comprehensive income earnings | 130,000 | $ 1,000,000.0 | 1,500,000 | $ 2,400,000 |
Investment securities posted as collateral | 361,000,000.0 |  | 361,000,000.0 |  | $ 731,800,000
Carrying amount of securities pledged as collateral | $ 361,000,000.0 |  | $ 361,000,000.0 |  | $ 323,900,000
Minimum |  |  |  |  |
Schedule Of Investments [Line Items] |  |  |  |  |
Percentage of shareholders equity for which securities holdings exceeds for no issuer other than U.S. Government and agencies | 10.00% |  | 10.00% |  | 10.00%
Federal Home Loan Bank Advances |  |  |  |  |
Schedule Of Investments [Line Items] |  |  |  |  |
Carrying amount of securities pledged as collateral | $ 0 |  | $ 0 |  | $ 0
Public Fund Deposits |  |  |  |  |
Schedule Of Investments [Line Items] |  |  |  |  |
Carrying amount of securities pledged as collateral | 311,300,000 |  | 311,300,000 |  | 245,100,000
Customer Repurchase Agreements |  |  |  |  |
Schedule Of Investments [Line Items] |  |  |  |  |
Carrying amount of securities pledged as collateral | $ 38,700,000 |  | $ 38,700,000 |  | $ 64,100,000
Available-for-sale Securities |  |  |  |  |
Schedule Of Investments [Line Items] |  |  |  |  |
Investment securities with unrealized losses | Security | 97 |  | 97 |  |
Held-to-maturity Securities |  |  |  |  |
Schedule Of Investments [Line Items] |  |  |  |  |
Investment securities with unrealized losses | Security | 0 |  | 0 |  |

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