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Commitments and Contingencies (Schedule of Credit Loss (Benefit) for Unfunded Commitments) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Unfunded Loan Commitment [Member] | Provision For Credit Losses [Member]
Commitments And Contingencies [Line Items]
Credit loss (benefit) for unfunded commitments	$ 185	$ 42	$ (179)	$ (527)

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- Definition

Commitments and contingencies.

+ References

No definition available.

+ Details

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Namespace Prefix:	fisi_
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- Definition

Credit loss benefit expense.

+ References

No definition available.

+ Details

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Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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- Details

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Period Type: