XML 19 R38.htm IDEA: XBRL DOCUMENT
v3.20.1

Basis of Presentation And Summary Of Significant Accounting Policies (Narrative) (Details) $ in Thousands |  |  | 3 Months Ended |
--- | --- | --- | --- | ---
Mar. 27, 2020 USD ($) | Mar. 23, 2020 USD ($) | Mar. 31, 2020 USD ($) Segment | Dec. 31, 2019 USD ($)
Significant Accounting Policies [Line Items] |  |  |  |
Retained earnings |  |  | $ 301,243 | $ 313,364
Number of identified portfolio segments | Segment |  |  | 6 |
Offering unsecured personal loans, percentage |  | 2.95% |  |
Maximum [Member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Certificate of deposit penalty fees for withdrawals |  | $ 20,000 |  |
Offering unsecured personal loans, amount |  | $ 5,000 |  |
Offering unsecured personal loans, period |  | 60 months |  |
CARES Act [Member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Estimated amount of relief fund | $ 2,200 |  |  |
ASU 2016-13 [Member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Retained earnings |  |  | $ (8,700) |
Increase in the allowance for credit losses on loans |  |  | 9,600 |
Increase (decrease) in deferred tax assets |  |  | 3,000 |
ASU 2016-13 [Member] | Held To Maturity Investment Securities [Member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Increase in the allowance for credit losses on loans |  |  | 14 |
ASU 2016-13 [Member] | Unfunded Commitments [Member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Increase in the allowance for credit losses on loans |  |  | $ 2,100 |

X
- Definition Certificate of deposit penalty fees for withdrawals. + References No definition available. + Details
Name: | fisi_CertificateOfDepositPenaltyFeesForWithdrawals
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Estimated amount of relief fund. + References No definition available. + Details
Name: | fisi_EstimatedAmountOfReliefFund
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Increase decrease in deferred tax assets. + References No definition available. + Details
Name: | fisi_IncreaseDecreaseInDeferredTaxAssets
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Number of identified portfolio segments. + References No definition available. + Details
Name: | fisi_NumberOfIdentifiedPortfolioSegments
Namespace Prefix: | fisi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Offering unsecured personal loans, amount. + References No definition available. + Details
Name: | fisi_OfferingUnsecuredPersonalLoansAmount
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Offering unsecured personal loans, percentage. + References No definition available. + Details
Name: | fisi_OfferingUnsecuredPersonalLoansPercentage
Namespace Prefix: | fisi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Offering unsecured personal loans, period. + References No definition available. + Details
Name: | fisi_OfferingUnsecuredPersonalLoansPeriod
Namespace Prefix: | fisi_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Significant Accounting Policies [Line Items] + References No definition available. + Details
Name: | fisi_SignificantAccountingPoliciesLineItems
Namespace Prefix: | fisi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of increase (decrease) of allowance reserve for credit loss. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition The cumulative amount of the reporting entity's undistributed earnings or deficit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(30)(a)(3)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(23)(a)(4)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
Name: | us-gaap_RetainedEarningsAccumulatedDeficit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_UnusualOrInfrequentItemAxis=fisi_CoronavirusAidReliefAndEconomicSecurityActMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InformationByCategoryOfDebtSecurityAxis=us-gaap_HeldtomaturitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_UnfundedLoanCommitmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |