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Regulatory Matters (Narrative) (Details) (USD $) | Dec. 31, 2014 | Dec. 31, 2013
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Preferred Equity [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier 1 capital | $ 17,300,000 us-gaap_TierOneRiskBasedCapital / us-gaap_StatementEquityComponentsAxis = us-gaap_PreferredStockMember |
Federal Reserve Bank Of New York [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Reserve requirement |  | $ 1,000,000 us-gaap_RestrictedCashAndCashEquivalents / dei_LegalEntityAxis = fisi_FederalReserveBankOfNewYorkMember

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- Definition The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. This element is for unclassified presentations; for classified presentations there is a separate and distinct element. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 1 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Tier 1 Risk Based Capital as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=7881181&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958568-112826 + Details
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