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Leases - Future Minimum Payments Under Non-cancellable Operating Leases (Parenthetical) (Details) $ in Millions	Dec. 31, 2021 USD ($)
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