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Leases - Future Minimum Payments Under Non-cancellable Operating Leases (Parenthetical) (Details) $ in Millions | Dec. 31, 2021 USD ($)
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Leases [Abstract] |
Lessee Operating Lease Lease Not Yet Commenced | $ 13.8

X
- Definition Lessee operating lease lease not yet commenced. + References No definition available. + Details
Name: | fisi_LesseeOperatingLeaseLeaseNotYetCommenced
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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