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v3.22.2

Leases - Future Minimum Payments Under Non-cancellable Operating Leases (Parenthetical) (Details) $ in Millions | Jun. 30, 2022 USD ($)
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Leases [Abstract] |
Lessee Operating Lease Lease Not Yet Commenced | $ 14.5

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

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