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Loans Held For Sale And Loan Servicing Rights (Activity In Capitalized Mortgage Serving Assets) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Loans Held For Sale And Loan Servicing Rights [Abstract]
Mortgage servicing assets, beginning of year	$ 1,479fisi_MortgageServicingAssetsBeforeValuationAllowance	$ 1,637fisi_MortgageServicingAssetsBeforeValuationAllowance	$ 1,609fisi_MortgageServicingAssetsBeforeValuationAllowance
Originations	172us-gaap_ServicingAssetAtFairValueAdditions	277us-gaap_ServicingAssetAtFairValueAdditions	554us-gaap_ServicingAssetAtFairValueAdditions
Amortization	(316)us-gaap_AmortizationOfMortgageServicingRightsMSRs	(435)us-gaap_AmortizationOfMortgageServicingRightsMSRs	(526)us-gaap_AmortizationOfMortgageServicingRightsMSRs
Mortgage servicing assets, end of year	1,335fisi_MortgageServicingAssetsBeforeValuationAllowance	1,479fisi_MortgageServicingAssetsBeforeValuationAllowance	1,637fisi_MortgageServicingAssetsBeforeValuationAllowance
Valuation allowance	(6)fisi_MortgageServicingAssetsValuationAllowance	(3)fisi_MortgageServicingAssetsValuationAllowance	(168)fisi_MortgageServicingAssetsValuationAllowance
Mortgage servicing assets, net, end of year	$ 1,329fisi_MortgageServicingAssetsNetOfValuationAllowance	$ 1,476fisi_MortgageServicingAssetsNetOfValuationAllowance	$ 1,469fisi_MortgageServicingAssetsNetOfValuationAllowance

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- Definition

Mortgage Servicing Assets, Before Valuation Allowance

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Mortgage Servicing Assets Net Of Valuation Allowance

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Mortgage Servicing Assets, Valuation Allowance

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- Definition

The periodic amortization (in proportion to and over the period of estimated net servicing income or loss) of capitalized servicing rights, which contractually entitle the servicer to receive fees and ancillary revenues for performing billing, collection, disbursement and recordkeeping services in connection with a mortgage portfolio. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 50

-Section 50

-Paragraph 4

-Subparagraph (a)(4)

-URI http://asc.fasb.org/extlink&oid=7882072&loc=d3e122739-111746

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- Definition

The value of new servicing assets, subsequently measured at fair value, acquired or created during the current period through purchases or from transfers of financial assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 50

-Section 50

-Paragraph 3

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=7882072&loc=d3e122625-111746

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