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Consolidated Statements Of Changes In Shareholders' Equity (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common stock dividends per share, declared	$ 0.77us-gaap_CommonStockDividendsPerShareDeclared	$ 0.74us-gaap_CommonStockDividendsPerShareDeclared	$ 0.57us-gaap_CommonStockDividendsPerShareDeclared
Series A 3% Preferred Stock [Member]
Preferred stock dividends per share, declared	$ 3.00us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementEquityComponentsAxis = fisi_Series3PreferredStockMember	$ 3.00us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementEquityComponentsAxis = fisi_Series3PreferredStockMember	$ 3.00us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementEquityComponentsAxis = fisi_Series3PreferredStockMember
Series B-1 8.48% Preferred Stock [Member]
Preferred stock dividends per share, declared	$ 8.48us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementEquityComponentsAxis = us-gaap_SeriesBPreferredStockMember	$ 8.48us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementEquityComponentsAxis = us-gaap_SeriesBPreferredStockMember	$ 8.48us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementEquityComponentsAxis = 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

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

+ References

No definition available.

+ Details

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