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Commitments and Contingencies (Schedule of Credit Loss for Unfunded Commitments) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Unfunded Loan Commitment [Member] | Provision For Credit Losses [Member]
Commitments And Contingencies [Line Items]
Credit loss for unfunded commitments	$ 670	$ 713	$ 491	$ 186

X

- Definition

Commitments and contingencies.

+ References

No definition available.

+ Details

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Namespace Prefix:	fisi_
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Balance Type:	na
Period Type:	duration

X

- 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:

X

- Details

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