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Consolidated Statements Of Changes In Shareholders' Equity (Parenthetical) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Common stock dividends per share, declared	$ 0.20us-gaap_CommonStockDividendsPerShareDeclared	$ 0.19us-gaap_CommonStockDividendsPerShareDeclared
Series A 3% Preferred Stock [Member]
Preferred stock dividends per share, declared	$ 0.75us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementClassOfStockAxis = fisi_Series3PreferredStockMember	$ 0.75us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementClassOfStockAxis = fisi_Series3PreferredStockMember
Preferred stock, dividend percentage	3.00%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = fisi_Series3PreferredStockMember	3.00%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = fisi_Series3PreferredStockMember
Series B-1 8.48% Preferred Stock [Member]
Preferred stock dividends per share, declared	$ 2.12us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember	$ 2.12us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember
Preferred stock, dividend percentage	8.48%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember	8.48%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember

X

- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

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



-Publisher SEC



-Name Regulation S-X (SX)



-Number 210



-Section 04



-Article 3







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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 505



-SubTopic 10



-Section S99



-Paragraph 1



-Subparagraph (SX 210.3-04)



-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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- Definition

The percentage rate used to calculate dividend payments on preferred stock.

+ References

No definition available.

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- Definition

Aggregate dividends declared during the period for each share of preferred stock outstanding.

+ References

No definition available.

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