XML 137 R101.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Employee Benefit Plans (Narrative) (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items] |  |  |
Accumulated benefit obligation | $ 54,900,000 us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation | $ 44,000,000 us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation |
Defined Benefit Plan, Contributions by Employer | 8,000,000 us-gaap_DefinedBenefitPlanContributionsByEmployer |  |
Number of consecutive years used for compensation calculation | 5 years |  |
Investment strategy, percentage in long-term growth | 97.00%fisi_InvestmentStrategyPercentageInLongTermGrowth |  |
Investment strategy, percentage in near-term benefit payments | 3.00%fisi_InvestmentStrategyPercentageInNearTermBenefitPayments |  |
Maximum percentage of securities purchasing allowed of the portfolio at any time | 5.00%fisi_MaximumPercentageOfSecuritiesPurchasingAllowedOfPortfolioAtAnyTime |  |
Maximum market value purchase percentage allowed of any one issuer | 8.00%fisi_MaximumMarketValuePurchasePercentageAllowedOfAnyOneIssuer |  |
Expense included in salaries and employee benefits | 1,100,000 us-gaap_DefinedContributionPlanCostRecognized | 1,100,000 us-gaap_DefinedContributionPlanCostRecognized | 1,000,000 us-gaap_DefinedContributionPlanCostRecognized
Postretirement Benefit Plan, Accrued liabilities | 124,000 us-gaap_PensionAndOtherPostretirementAndPostemploymentBenefitPlansLiabilitiesCurrentAndNoncurrent | 93,000 us-gaap_PensionAndOtherPostretirementAndPostemploymentBenefitPlansLiabilitiesCurrentAndNoncurrent |
Estimated future net loss for the plan that will be amortized from accumulated other comprehensive income | 943,000 us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss |  |
Estimated future prior service credit for the plan that will be amortized from accumulated other comprehensive income | 47,000 us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit |  |
Maximum percent of participant's contribution match | 4.50%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent |  |
Effective February Two Thousand Twelve [Member] | Securities Less Than Aquality [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Maximum percentage of securities purchasing allowed of the portfolio at any time | 13.00%fisi_MaximumPercentageOfSecuritiesPurchasingAllowedOfPortfolioAtAnyTime / us-gaap_InternalCreditAssessmentAxis = fisi_SecuritiesLessThanAqualityMember / us-gaap_StatementScenarioAxis = fisi_EffectiveFebruaryTwoThousandTwelveMember |  |
Effective February Two Thousand Twelve [Member] | Commercial Mortgage Back Securities And Assets Backed Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Maximum percentage of securities purchasing allowed of the portfolio at any time | 10.00%fisi_MaximumPercentageOfSecuritiesPurchasingAllowedOfPortfolioAtAnyTime / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialMortgageBackSecuritiesAndAssetsBackedSecuritiesMember / us-gaap_StatementScenarioAxis = fisi_EffectiveFebruaryTwoThousandTwelveMember |  |
Street Bank & Trust Co. Short Term Investment Fund [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage of Plan Assets |  | 5.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_StreetBankTrustCoShortTermInvestmentFundMember |
Portfolio Concentration Commingled Trust Fund1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage of Plan Assets | 13.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_PortfolioConcentrationCommingledTrustFund1Member |  |
Portfolio Concentration Commingled Trust Fund2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage of Plan Assets | 9.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_PortfolioConcentrationCommingledTrustFund2Member |  |
Portfolio Concentration Short Term Investment Fund [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage of Plan Assets | 8.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_PortfolioConcentrationShortTermInvestmentFundMember |  |
Investment Firm One [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Portfolio management, controlled percentage | 57.00%fisi_PortfolioManagementControlledPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fisi_InvestmentFirmOneMember | 58.00%fisi_PortfolioManagementControlledPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fisi_InvestmentFirmOneMember |
Investment Firm Two [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Portfolio management, controlled percentage | 39.00%fisi_PortfolioManagementControlledPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fisi_InvestmentFirmTwoMember | 41.00%fisi_PortfolioManagementControlledPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fisi_InvestmentFirmTwoMember |
Defined Benefit Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Portfolio management, controlled percentage | 4.00%fisi_PortfolioManagementControlledPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 1.00%fisi_PortfolioManagementControlledPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
First 3% Of Participant's Contribution [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage employer matches of the employee's percentage contribution matched | 100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fisi_FirstThreePercentOfParticipantSContributionMember |  |
Percentage for which the employer contributes a matching contribution | 3.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fisi_FirstThreePercentOfParticipantSContributionMember |  |
Next 3% Of Participant's Contribution [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage employer matches of the employee's percentage contribution matched | 50.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fisi_NextThreePercentOfParticipantSContributionMember |  |
Percentage for which the employer contributes a matching contribution | 3.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fisi_NextThreePercentOfParticipantSContributionMember |  |
SERP [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Unfunded pension liability | 2,200,000 us-gaap_DefinedBenefitPensionPlanCurrentAndNoncurrentLiabilities / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember | 2,100,000 us-gaap_DefinedBenefitPensionPlanCurrentAndNoncurrentLiabilities / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember |
Pension expense | $ 295,000 us-gaap_PensionExpense / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember | $ 95,000 us-gaap_PensionExpense / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember | $ 1,300,000 us-gaap_PensionExpense / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember

X
- Definition Investment Strategy, Percentage In Long Term Growth + References No definition available. + Details
Name: | fisi_InvestmentStrategyPercentageInLongTermGrowth
Namespace Prefix: | fisi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Investment Strategy, Percentage In Near Term Benefit Payments + References No definition available. + Details
Name: | fisi_InvestmentStrategyPercentageInNearTermBenefitPayments
Namespace Prefix: | fisi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum Market Value Purchase Percentage Allowed Of Any One Issuer + References No definition available. + Details
Name: | fisi_MaximumMarketValuePurchasePercentageAllowedOfAnyOneIssuer
Namespace Prefix: | fisi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum Percentage Of Securities Purchasing Allowed Of The Portfolio At Any Time + References No definition available. + Details
Name: | fisi_MaximumPercentageOfSecuritiesPurchasingAllowedOfPortfolioAtAnyTime
Namespace Prefix: | fisi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of Years Used for Compensation Calculation + References No definition available. + Details
Name: | fisi_NumberOfYearsUsedForCompensationCalculation
Namespace Prefix: | fisi_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Portfolio Management, Controlled Percentage + References No definition available. + Details
Name: | fisi_PortfolioManagementControlledPercentage
Namespace Prefix: | fisi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition This represents the entire liability recognized in the balance sheet that is associated with the defined benefit pension plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.15) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.15) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e2417-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e2410-114920 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=21915240&loc=d3e1703-114919 + Details
Name: | us-gaap_DefinedBenefitPensionPlanCurrentAndNoncurrentLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503844 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The increase in the fair value of plan assets from contributions made by the employer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount included in accumulated other comprehensive income (AOCI) related to gain (loss) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount included in accumulated other comprehensive income (AOCI) related to prior service cost (credit) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition This represents the entire liability recognized in the balance sheet that is associated with the defined benefit pension plans and other postretirement and postemployment benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.15) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.15) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e2417-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e2410-114920 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=21915240&loc=d3e1703-114919 + Details
Name: | us-gaap_PensionAndOtherPostretirementAndPostemploymentBenefitPlansLiabilitiesCurrentAndNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount of pension benefit costs recognized during the period for (1) defined benefit plans and (2) defined contribution plans. For defined benefit plans, pension expense includes the following components: service cost, interest cost, expected return on plan assets, gain (loss) on plan assets, prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. For defined contribution plans, the pension expense generally equals the firm's contribution to employees' accounts (if the firm contributes) during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
Name: | us-gaap_PensionExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=fisi_PortfolioConcentrationCommingledTrustFund1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=fisi_PortfolioConcentrationCommingledTrustFund2Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=fisi_PortfolioConcentrationShortTermInvestmentFundMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=fisi_StreetBankTrustCoShortTermInvestmentFundMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=fisi_FirstThreePercentOfParticipantSContributionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=fisi_InvestmentFirmOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=fisi_InvestmentFirmTwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=fisi_NextThreePercentOfParticipantSContributionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementScenarioAxis=fisi_EffectiveFebruaryTwoThousandTwelveMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |