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Securities - Additional Information (Details) | 12 Months Ended
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Dec. 31, 2017 USD ($) Security | Dec. 31, 2016 USD ($) Security | Dec. 31, 2015 USD ($)
Schedule Of Investments [Line Items] |  |  |
Trading securities | $ 0 | $ 0 |
Net gains or (losses) reclassified from accumulated other comprehensive income (loss) into earnings | 8,000 | 3,200,000 | $ 3,100,000
Carrying amount of securities pledged as collateral | $ 262,500,000 | $ 178,600,000 |
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% |
Federal Home Loan Bank Advances |  |  |
Schedule Of Investments [Line Items] |  |  |
Carrying amount of securities pledged as collateral | $ 128,400,000 | $ 58,700,000 |
Public Fund Deposits |  |  |
Schedule Of Investments [Line Items] |  |  |
Carrying amount of securities pledged as collateral | 99,300,000 | 93,200,000 |
Customer Repurchase Agreements |  |  |
Schedule Of Investments [Line Items] |  |  |
Carrying amount of securities pledged as collateral | $ 34,800,000 | $ 19,900,000 |
Available-for-sale Securities |  |  |
Schedule Of Investments [Line Items] |  |  |
Investment securities with unrealized losses | Security | 101 | 88 |
Held-to-maturity Securities |  |  |
Schedule Of Investments [Line Items] |  |  |
Investment securities with unrealized losses | Security | 28 | 42 |

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