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Securities - Additional Information (Details) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) Security | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) Security | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($)
Schedule Of Investments [Line Items] |  |  |  |  |
Trading securities | $ 0 |  | $ 0 |  | $ 0
Net gains reclassified from accumulated other comprehensive income earnings | 1,000,000.0 | $ 178,000 | 2,400,000 | $ 1,200,000 |
Investment securities posted as collateral | 994,100,000 |  | 994,100,000 |  | 552,400,000
Carrying amount of securities pledged as collateral | $ 407,900,000 |  | $ 407,900,000 |  | $ 301,100,000
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 | 346,400,000 |  | 346,400,000 |  | 240,400,000
Customer Repurchase Agreements |  |  |  |  |
Schedule Of Investments [Line Items] |  |  |  |  |
Carrying amount of securities pledged as collateral | $ 56,400,000 |  | $ 56,400,000 |  | $ 55,700,000
Available-for-sale Securities |  |  |  |  |
Schedule Of Investments [Line Items] |  |  |  |  |
Investment securities with unrealized losses | Security | 41 |  | 41 |  |
Held-to-maturity Securities |  |  |  |  |
Schedule Of Investments [Line Items] |  |  |  |  |
Investment securities with unrealized losses | Security | 0 |  | 0 |  |

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