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Loans (Summary of Collateral Dependent Loans) (Details) $ in Thousands | Mar. 31, 2020 USD ($)
--- | ---
Financing Receivable, Impaired [Line Items] |
Collateral dependent loans on business assets | $ 2,974
Collateral dependent loans on real propery | 9,989
Collateral dependent loans | 12,963
Commercial Business [Member] |
Financing Receivable, Impaired [Line Items] |
Collateral dependent loans on business assets | 2,974
Collateral dependent loans on real propery | 4,571
Collateral dependent loans | 7,545
Commercial Mortgage [Member] |
Financing Receivable, Impaired [Line Items] |
Collateral dependent loans on real propery | 5,418
Collateral dependent loans | $ 5,418

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- Definition Collateral dependent loans. + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Collateral dependent loans on business assets. + References No definition available. + Details
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Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Collateral dependent loans on real property. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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Data Type: | na
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |