Basis Of Presentation And Summary Of Significant Accounting Policies (Policy)	3 Months Ended
	Mar. 31, 2015
Summary Of Significant Accounting Policies [Abstract]
Nature Of Operations	Nature of Operations Financial Institutions, Inc., (the Parent) 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).
Basis Of Presentation	Basis of Presentation The consolidated financial statements include the accounts of Financial Institutions, Inc. and its subsidiaries. All significant intercompany accounts and transactions have been eliminated in consolidation. The accounting and reporting policies conform to U.S. generally accepted accounting principles (GAAP). Certain information and footnote disclosures normally included in financial statements prepared in conformity with GAAP have been condensed or omitted pursuant to such rules and regulations. However, 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 2014 Annual Report on Form 10-K for the year ended December 31, 2014. 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. Other than the closing of the Companys $40.0 million subordinated note offering, as described in Note 14 Subsequent Event, the Company did not have any material recognizable 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 May 2014, the Financial Accounting Standards Board issued Accounting Standards Update (ASU) No. 2014-09, Revenue from Contracts with Customers (Topic 606). ASU 2014-09 implements a common revenue standard that clarifies the principles for recognizing revenue. The core principle of ASU 2014-09 is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve that core principle, an entity should apply the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies a performance obligation. ASU 2014-09 will be effective for the Company on January 1, 2017. In April 2015, the FASB proposed a one-year deferral of the revenue recognition standards effective date for all entities. The Company is evaluating the potential impact on the Companys financial statements. In June 2014, the FASB issued ASU 2014-12, CompensationStock Compensation (Topic 718). The pronouncement was issued to clarify the accounting for share-based payments when the terms of an award provide that a performance target could be achieved after the requisite service period. ASU 2014-12 will be effective for the Company beginning January 1, 2016, though early adoption is permitted. The adoption of ASU 2014-12 is not expected to have a significant impact on the Companys financial statements. In January 2015, the FASB issued ASU 2015-01, Income Statement - Extraordinary and Unusual Items (Subtopic 225-20) Simplifying Income Statement Presentation by Eliminating the Concept of Extraordinary Items. ASU 2015-01 eliminates from U.S. GAAP the concept of extraordinary items, which, among other things, required an entity to segregate extraordinary items considered to be unusual and infrequent from the results of ordinary operations and show the item separately in the income statement, net of tax, after income from continuing operations. ASU 2015-01 will be effective for the Company beginning January 1, 2016, though early adoption is permitted. ASU 2015-01 is not expected to have a significant impact on the Companys financial statements.