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Shareholders' Equity (Narrative) (Details) (USD $) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013
Shareholders Equity [Line Items] |  |
Capital shares authorized | 50,210,000 fisi_CapitalSharesAuthorized |
Common stock, shares authorized | 50,000,000 us-gaap_CommonStockSharesAuthorized | 50,000,000 us-gaap_CommonStockSharesAuthorized
Common stock, par value | $ 0.01 us-gaap_CommonStockParOrStatedValuePerShare | 0.01 us-gaap_CommonStockParOrStatedValuePerShare
Preferred stock, shares authorized | 210,000 us-gaap_PreferredStockSharesAuthorized |
Preferred stock, par value | $ 100 us-gaap_PreferredStockParOrStatedValuePerShare |
Preferred stock, shares issued | 173,398 us-gaap_PreferredStockSharesIssued | 173,423 us-gaap_PreferredStockSharesIssued
Preferred Class A [Member] |  |
Shareholders Equity [Line Items] |  |
Preferred stock, shares authorized | 10,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember |
Series A 3% Preferred Stock [Member] |  |
Shareholders Equity [Line Items] |  |
Preferred stock, dividend percentage | 3.00%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = fisi_SeriesaThreePercentPreferredStockMember |
Preferred stock, shares issued | 1,492 us-gaap_PreferredStockSharesIssued / us-gaap_StatementClassOfStockAxis = fisi_SeriesaThreePercentPreferredStockMember | 1,496 us-gaap_PreferredStockSharesIssued / us-gaap_StatementClassOfStockAxis = fisi_SeriesaThreePercentPreferredStockMember
Preferred stock, dividend per share | $ 3.00 us-gaap_PreferredStockDividendRatePerDollarAmount / us-gaap_StatementClassOfStockAxis = fisi_SeriesaThreePercentPreferredStockMember |
Preferred Class B [Member] |  |
Shareholders Equity [Line Items] |  |
Preferred stock, shares authorized | 200,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassBMember |
Series B-1 8.48% Preferred Stock [Member] |  |
Shareholders Equity [Line Items] |  |
Preferred stock, shares authorized | 200,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember | 200,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember
Preferred stock, par value | $ 100 us-gaap_PreferredStockParOrStatedValuePerShare / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember | 100 us-gaap_PreferredStockParOrStatedValuePerShare / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember
Preferred stock, dividend percentage | 8.48%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember |
Preferred stock, shares issued | 171,906 us-gaap_PreferredStockSharesIssued / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember | 171,927 us-gaap_PreferredStockSharesIssued / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember
Preferred stock, dividend per share | $ 8.48 us-gaap_PreferredStockDividendRatePerDollarAmount / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember |

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- Definition Face amount or stated value per share of common stock. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
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- Definition The maximum number of common shares permitted to be issued by an entity's charter and bylaws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
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- Definition Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.28) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 + Details
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