Basis Of Presentation And Summary Of Significant Accounting Policies (Policy)	6 Months Ended
	Jun. 30, 2016
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. The Company provides diversified financial services through its subsidiaries, Five Star Bank, Scott Danahy Naylon, LLC ("Scott Danahy Naylon") and Courier Capital, LLC ("Courier Capital"). 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 Bank has also expanded its indirect lending network to include relationships with franchised automobile dealers in the Capital District of New York and Northern and Central Pennsylvania. Scott Danahy Naylon provides a broad range of insurance services to personal and business clients across 44 states. Acquired on January 5, 2016, Courier Capital provides customized investment management, investment consulting and retirement plan services to individuals, businesses, institutions, foundations and retirement plans across nine states.
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 year's presentation. These consolidated financial statements should be read in conjunction with the Company's 2015 Annual Report on Form 10-K for the year ended December 31, 2015. The results of operations for any interim periods are not necessarily indicative of the results which may be expected for the entire year.
Reclassifications	Reclassifications Certain reclassifications of previously reported amounts have been made to conform to the current year presentation. Such reclassifications did not impact net income or shareholders' equity as previously reported.
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 there were no 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, and assumptions used in the defined benefit pension plan accounting.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board ("FASB") 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. The effective date was recently deferred for one year to the interim and annual periods beginning on or after December 15, 2017. Early adoption is permitted as of the original effective date – interim and annual periods beginning on or after December 15, 2016. The Company continues to assess the potential impact of ASU 2014-09 on its accounting and disclosures. In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments - Overall (Subtopic 825-10) - Recognition and Measurement of Financial Assets and Financial Liabilities. ASU 2016-01 is intended to improve the recognition and measurement of financial instruments by requiring equity investments to be measured at fair value with changes in fair value recognized in net income; requiring public business entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes; requiring separate presentation of financial assets and financial liabilities by measurement category and form of financial asset on the balance sheet or the accompanying notes to the financial statements; eliminating the requirement for public business entities to disclose the method(s) and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured and amortized at cost on the balance sheet; and requiring a reporting organization to present separately in other comprehensive income the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the organization has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. ASU 2016-01 is effective for annual periods and interim periods within those annual periods, beginning after December 15, 2017. The amendments should be applied by means of a cumulative-effect adjustment to the balance sheet as of the beginning of the fiscal year of adoption. The amendments related to equity securities without readily determinable fair values (including disclosure requirements) should be applied prospectively to equity investments that exist as of the date of adoption. The Company is assessing the impact of ASU 2016-01 on its accounting and disclosures. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842). ASU 2016-02 establishes a right of use model that requires a lessee to record a right of use asset and a lease liability for all leases with terms longer than 12 months. Leases will be classified as either finance or operating, with classification affecting the pattern of expense recognition in the income statement. For lessors, the guidance modifies the classification criteria and the accounting for sales-type and direct financing leases. A lease will be treated as sale if it transfers all of the risks and rewards, as well as control of the underlying asset, to the lessee. If risks and rewards are conveyed without the transfer of control, the lease is treated as a financing. If the lessor doesn't convey risks and rewards or control, an operating lease results. The amendments are effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years for public business entities. Entities are required to use a modified retrospective approach for leases that exist or are entered into after the beginning of the earliest comparative period in the financial statements, with certain practical expedients available. Early adoption is permitted. The Company is assessing the impact of ASU 2016-02 on its accounting and disclosures. In March 2016, the FASB issued ASU No. 2016-09, Compensation - Stock Compensation (Topic 718) - Improvements to Employee Share-Based Payment Accounting. ASU 2016-09 requires all income tax effects of awards to be recognized in the income statement when the awards vest or are settled. It also allows an employer to repurchase more of an employee's shares than it can today for tax withholding purposes without triggering liability accounting and to make a policy election for forfeitures as they occur. The guidance is effective for public business entities for fiscal years beginning after December 15, 2016, and interim periods within those years. Early adoption is permitted. The Company is assessing the impact of ASU 2016-09 on its accounting and disclosures. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326) – Measurement of Credit Losses on Financial Instruments. ASU 2016-13 amends guidance on reporting credit losses for financial assets held at amortized cost basis and available for sale debt securities. Topic 326 eliminates the probable initial recognition threshold in current GAAP and, instead, requires an entity to reflect its current estimate of all expected credit losses based on historical experience, current conditions and reasonable and supportable forecasts. The allowance for credit losses is a valuation account that is deducted from the amortized cost basis of the financial assets to present the net amount expected to be collected. The guidance is effective for public business entities for fiscal years beginning after December 15, 2019, and interim periods within those years. Early adoption is permitted beginning after December 15, 2018. The Company is assessing the impact of ASU 2016-13 on its accounting and disclosures.