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Commitments And Contingencies (Narrative) (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
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Commitments And Contingencies [Abstract] |  |
Forward sales commitments | $ 598.0 | $ 566.0

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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