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Employee Benefit Plans (Target Allocation, Percentage Of Plan Assets And Weighted Average Expected Rate Of Return) (Details) | 12 Months Ended |
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Dec. 31, 2014 | Dec. 31, 2013
Cash And Cash Equivalents [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target Allocation, Minimum | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember |
Target Allocation, Maximum | 20.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember |
Percentage of Plan Assets | 8.70%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember | 5.50%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember
Weighted Average Expected Long-term Rate of Return | 0.26%fisi_DefinedBenefitPlanWeightedAverageExpectedLongTermRateOfReturn / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashAndCashEquivalentsMember |
Equity Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target Allocation, Minimum | 40.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember |
Target Allocation, Maximum | 60.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember |
Percentage of Plan Assets | 48.20%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember | 50.60%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember
Weighted Average Expected Long-term Rate of Return | 4.05%fisi_DefinedBenefitPlanWeightedAverageExpectedLongTermRateOfReturn / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember |
Fixed Income Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target Allocation, Minimum | 40.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember |
Target Allocation, Maximum | 60.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember |
Percentage of Plan Assets | 43.10%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember | 43.90%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember
Weighted Average Expected Long-term Rate of Return | 1.98%fisi_DefinedBenefitPlanWeightedAverageExpectedLongTermRateOfReturn / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember |
Foreign Exchange And Other Derivative Financial Instruments [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target Allocation, Minimum | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ForeignExchangeAndOtherDerivativeFinancialInstrumentsMember |
Target Allocation, Maximum | 5.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ForeignExchangeAndOtherDerivativeFinancialInstrumentsMember |
Percentage of Plan Assets |  |
Weighted Average Expected Long-term Rate of Return |  |

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- Definition Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis. + 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 (d)(i) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e2446-114920 + Details
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- 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
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