Basis Of Presentation And Summary Of Significant Accounting Policies (Policy)	9 Months Ended
	Sep. 30, 2014
Basis of Presentation And Summary Of Significant Accounting Policies [Abstract]
Nature Of Operations	Nature of Operations Financial Institutions, Inc., (the Company) is a financial holding company organized in 1931 under the laws of New York State (New York or NYS). The Company offers a broad array of deposit, lending and other financial services to individuals, municipalities and businesses in Western and Central New York through its wholly-owned New York chartered banking subsidiary, Five Star Bank (the Bank). The Company has also expanded its indirect lending network to include relationships with franchised automobile dealers in the Capital District of New York and Northern Pennsylvania. On August 1, 2014, the Company acquired Scott Danahy Naylon Co., Inc., a full service insurance agency located in Amherst, New York. As a result of the acquisition the Company now provides insurance and risk consulting services through its wholly-owned insurance subsidiary, Scott Danahy Naylon, LLC (SDN). The consolidated financial statements include the accounts of the Company, the Bank and SDN. All significant intercompany balances and transactions have been eliminated in the consolidation. References to the Company mean the consolidated reporting entity.
Basis Of Presentation	Basis of Presentation The unaudited interim consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information and pursuant to the requirements for reporting on Form 10-Q and Article 10 of Regulation S-X of the Securities Exchange Act of 1934. Accordingly, certain disclosures accompanying annual consolidated financial statements are omitted. In the opinion of management, the accompanying consolidated financial statements reflect all adjustments of a normal and recurring nature necessary for a fair presentation of the consolidated statements of financial condition, income, comprehensive income, changes in shareholders equity and cash flows for the periods indicated, and contain adequate disclosure to make the information presented not misleading. Prior years consolidated financial statements are re-classified whenever necessary to conform to the current years presentation. These consolidated financial statements should be read in conjunction with the Companys 2013 Annual Report on Form 10-K. The results of operations for any interim periods are not necessarily indicative of the results which may be expected for the entire year.
Subsequent Events	Subsequent Events The Company has evaluated events and transactions for potential recognition or disclosure through the day the financial statements were issued and determined that there were no subsequent events.
Use Of Estimates	Use of Estimates The preparation of these financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates. Material estimates relate to the determination of the allowance for loan losses, the carrying value of goodwill and deferred tax assets, the valuation and other than temporary impairment (OTTI) considerations related to the securities portfolio, and assumptions used in the defined benefit pension plan accounting.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In January 2014, the Financial Accounting Standards Board issued Accounting Standards Update (ASU) No. 2014-04, Reclassification of Residential Real Estate Collateralized Consumer Mortgage Loans upon Foreclosure. The objective of this guidance is to clarify when an in substance repossession or foreclosure occurs, that is, when a creditor should be considered to have received physical possession of residential real estate property collateralizing a consumer mortgage loan such that the loan receivable should be derecognized and the real estate property recognized. ASU No. 2014-04 states that an in substance repossession or foreclosure occurs, and a creditor is considered to have received physical possession of residential real estate property collateralizing a consumer mortgage loan, upon either (1) the creditor obtaining legal title to the residential real estate property upon completion of a foreclosure or (2) the borrower conveying all interest in the residential real estate property to the creditor to satisfy that loan through completion of a deed in lieu of foreclosure or through a similar legal agreement. Additionally, ASU No. 2014-04 requires interim and annual disclosure of both (1) the amount of foreclosed residential real estate property held by the creditor and (2) the recorded investment in consumer mortgage loans collateralized by residential real estate property that are in the process of foreclosure according to local requirements of the applicable jurisdiction. ASU No. 2014-04 is effective for interim and annual reporting periods beginning after December 15, 2014. The adoption of ASU No. 2014-04 is not expected to have a material impact on the Company's consolidated statements of income and condition.