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Commitments and Contingencies (Schedule of Credit Loss for Unfunded Commitments) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Unfunded Loan Commitment [Member] | Provision For Credit Losses [Member] |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |
Credit loss for unfunded commitments | $ 670 | $ 713 | $ 491 | $ 186

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- Definition Commitments and contingencies. + References No definition available. + Details
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Namespace Prefix: | fisi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Credit loss benefit expense. + References No definition available. + Details
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Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |