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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	50,210,000fisi_CapitalSharesAuthorized
Common stock, shares authorized	50,000,000us-gaap_CommonStockSharesAuthorized	50,000,000us-gaap_CommonStockSharesAuthorized
Common stock, par value	$ 0.01us-gaap_CommonStockParOrStatedValuePerShare	0.01us-gaap_CommonStockParOrStatedValuePerShare
Preferred stock, shares authorized	210,000us-gaap_PreferredStockSharesAuthorized
Preferred stock, par value	$ 100us-gaap_PreferredStockParOrStatedValuePerShare
Preferred stock, shares issued	173,398us-gaap_PreferredStockSharesIssued	173,423us-gaap_PreferredStockSharesIssued
Preferred Class A [Member]
Shareholders Equity [Line Items]
Preferred stock, shares authorized	10,000us-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	1,492us-gaap_PreferredStockSharesIssued / us-gaap_StatementClassOfStockAxis = fisi_SeriesaThreePercentPreferredStockMember	1,496us-gaap_PreferredStockSharesIssued / us-gaap_StatementClassOfStockAxis = fisi_SeriesaThreePercentPreferredStockMember
Preferred stock, dividend per share	$ 3.00us-gaap_PreferredStockDividendRatePerDollarAmount / us-gaap_StatementClassOfStockAxis = fisi_SeriesaThreePercentPreferredStockMember
Preferred Class B [Member]
Shareholders Equity [Line Items]
Preferred stock, shares authorized	200,000us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassBMember
Series B-1 8.48% Preferred Stock [Member]
Shareholders Equity [Line Items]
Preferred stock, shares authorized	200,000us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember	200,000us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember
Preferred stock, par value	$ 100us-gaap_PreferredStockParOrStatedValuePerShare / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember	100us-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	171,906us-gaap_PreferredStockSharesIssued / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember	171,927us-gaap_PreferredStockSharesIssued / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember
Preferred stock, dividend per share	$ 8.48us-gaap_PreferredStockDividendRatePerDollarAmount / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember

X

- Definition

Capital Shares Authorized

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- Definition

Face amount or stated value per share of common stock.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

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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.

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