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Commitments And Contingencies (Narrative) (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
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Commitments And Contingencies Disclosure [Abstract] |  |
Forward sales commitments | $ 8,700 | $ 566

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- Definition Commitments to sell residential real estate loans. + References No definition available. + Details
Name: | fisi_ForwardSalesCommitments
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration