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Subsequent Event (Narrative) (Details) - Subsequent Event [Member] - Maximum [Member] $ in Millions | 1 Months Ended
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Mar. 31, 2024 USD ($)
Subsequent Event [Line Items] |
Deposit activity, the Bank's potential exposure for fraudulent activity | $ 18.9
Deposit activity, the Bank's potential net income impact for fraudulent activity | $ (14.1)

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- Definition With respect to the deposit activity, the Bank's potential exposure for fraudulent activity. + 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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- Definition Potential net income impact for fraudulent activity with deposit transaction. + 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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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 830 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481674/830-30-50-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 855 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483399/855-10-50-2 + Details
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Balance Type: | na
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: |
Data Type: | na
Balance Type: |
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