Debt Instrument, Number of Years at Stated rate

No definition available.

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

- Definition

Debt issuance cost amortization date.

+ References

No definition available.

+ Details

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Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Short-term and long-term borrowings.

+ References

No definition available.

+ Details

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Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Date when the debt instrument is scheduled to be fully repaid, in YYYY-MM-DD format.

+ References

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Balance Type:	na
Period Type:	duration

- Definition

Description of the payment terms of the debt instrument (for example, whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment.

+ References

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of advance from Federal Home Loan Bank (FHLBank) maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

The cash outflow paid to third parties in connection with debt origination, which will be amortized over the remaining maturity period of the associated long-term debt.

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+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

+ References

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Balance Type:	debit
Period Type:	duration

- Definition

Weighted average interest rate of short-term debt outstanding calculated at point in time.

+ References

+ Details

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Period Type:	instant

- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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Borrowings (Narrative) (Details) - USD ($)			12 Months Ended
Borrowings (Narrative) (Details) - USD ($)	Oct. 07, 2021	Apr. 15, 2015	Dec. 31, 2021	Dec. 31, 2020	Oct. 07, 2020
Short Term And Long Term Borrowings [Line Items]
Short-term FHLB borrowings			$ 30,000,000	$ 5,300,000
Short-term borrowings, weighted average rate			0.34%	1.70%
Line of credit drawn amount			$ 0	$ 0
Debt issuance costs				779,000
2020 Notes [Member]
Short Term And Long Term Borrowings [Line Items]
Debt issuance cost amortization date			Oct. 15, 2025
2020 Notes [Member] \| Private Placement [Member]
Short Term And Long Term Borrowings [Line Items]
Debt instrument, aggregate principal amount					$ 35,000,000.0
Debt instrument, maturity date	Oct. 15, 2030
Debt instrument, payment terms			The 2020 Notes have a maturity date of October 15, 2030 and bear interest, payable semi-annually, at the rate of 4.375% per annum, until October 15, 2025. Commencing on that date, the interest rate will reset quarterly to an interest rate per annum equal to the then current three-month secured overnight financing rate (“SOFR”) plus 4.265%, payable quarterly until maturity.
Debt instrument, interest rate	4.375%
Debt issuance costs	$ 779,000
Proceeds from issuance of debt, net	$ 34,200,000
2020 Notes [Member] \| Private Placement [Member] \| Secured Overnight Financing Rate (“SOFR”) [Member]
Short Term And Long Term Borrowings [Line Items]
Debt instrument, percentage of basis spread on variable rate	4.265%
2015 Notes [Member]
Short Term And Long Term Borrowings [Line Items]
Debt instrument, aggregate principal amount		$ 40,000,000.0
Debt instrument, interest rate		6.00%
Debt issuance costs			$ 1,100,000
Proceeds from issuance of debt, net			$ 38,900,000
Number of years at stated rate			10 years
Debt issuance cost amortization date			Apr. 15, 2025
2015 Notes [Member] \| LIBOR [Member]
Short Term And Long Term Borrowings [Line Items]
Debt instrument, percentage of basis spread on variable rate		3.944%
Commercial Bank [Member]
Short Term And Long Term Borrowings [Line Items]
Revolving line of credit allowing borrowings			$ 20,000,000.0
Federal Home Loan Borrowings Short Term Advances [Member]
Short Term And Long Term Borrowings [Line Items]
Short-term FHLB borrowings			$ 30,000,000	$ 5,300,000