XML 74 R63.htm IDEA: XBRL DOCUMENT
v3.20.2

Derivative Instruments and Hedging Activities (Narrative) (Details) | 6 Months Ended |
--- | --- | ---
Jun. 30, 2020 USD ($) InterestRateDerivative | Dec. 31, 2019 USD ($)
Derivative [Line Items] |  |
Estimated to be reclassified as an increase to interest expense during next twelve months | $ 438,000 |
Derivatives Designated as Hedging Instruments [Member] |  |
Derivative [Line Items] |  |
Notional amount | $ 150,000,000 | $ 100,000,000
Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge of Interest Rate Risk [Member] |  |
Derivative [Line Items] |  |
Number of months hedging exposure to variability in future cash flows for forecasted transactions | 60 months |
Number of outstanding forward starting interest rate Derivative | InterestRateDerivative | 1 |
Notional amount | $ 50,000,000.0 |
Derivative effective date | 2022-04 |

X
- Definition Derivative effective date. + References No definition available. + Details
Name: | fisi_DerivativeEffectiveDate
Namespace Prefix: | fisi_
Data Type: | xbrli:gYearMonthItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of months hedging exposure to variability in future cash flows for forecasted transactions. + References No definition available. + Details
Name: | fisi_NumberOfMonthsHedgingExposureToVariabilityInFutureCashFlowsForForecastedTransactions
Namespace Prefix: | fisi_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of outstanding forward starting interest rate derivative. + References No definition available. + Details
Name: | fisi_NumberOfOutstandingForwardStartingInterestRateDerivative
Namespace Prefix: | fisi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Nominal or face amount used to calculate payment on derivative. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 + Details
Name: | us-gaap_DerivativeNotionalAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

X
- Definition The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121549185&loc=d3e80748-113994 + Details
Name: | us-gaap_InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |