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Subsequent Event (Details) (Subsequent Event [Member], USD $)	0 Months Ended
	Apr. 15, 2015	Apr. 15, 2015
Principal amount	$ 40,000,000us-gaap_DebtInstrumentFaceAmount	$ 40,000,000us-gaap_DebtInstrumentFaceAmount
Interst rate	6.00%us-gaap_DebtInstrumentInterestRateStatedPercentage	6.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
Number of years at stated rate	10 years
LIBOR [Member]
Spread on variable rate	3.944%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember

X

- Definition

Debt Instrument, Number of Years at Stated rate

+ References

No definition available.

+ Details

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Namespace Prefix:	fisi_
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Balance Type:	na
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X

- 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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Namespace Prefix:	us-gaap_
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X

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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