XML 74 R64.htm IDEA: XBRL DOCUMENT
v3.24.0.1

Investment Securities (Amortized Cost and Fair Value of Investment Securities) (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item] |  |
Securities available for sale, Amortized Cost | $ 1,037,990 | $ 1,127,057
Securities available for sale, Unrealized Gains | 1,286 | 338
Securities available for sale, Unrealized Losses | 151,546 | 173,024
Securities available for sale | 887,730 | 954,371
Securities held to maturity, Amortized Cost | 148,160 | 188,980
Securities held to maturity, Unrealized Gains | 34 | 24
Securities held to maturity, Unrealized Losses | 11,164 | 14,816
Securities held to maturity, fair value | 137,030 | 174,188
Allowance for credit losses, Amortized Cost | (4) | (5)
Held To Maturity Securities Net | 148,156 | 188,975
U.S. Government Agencies And Government Sponsored Enterprises [Member] |  |
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item] |  |
Securities available for sale, Amortized Cost | 24,535 | 24,535
Securities available for sale, Unrealized Gains | 0 | 0
Securities available for sale, Unrealized Losses | 2,724 | 3,420
Securities available for sale | 21,811 | 21,115
Securities held to maturity, Amortized Cost | 16,513 | 16,363
Securities held to maturity, Unrealized Gains | 0 | 0
Securities held to maturity, Unrealized Losses | 530 | 848
Securities held to maturity, fair value | 15,983 | 15,515
State And Political Subdivisions [Member] |  |
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item] |  |
Securities held to maturity, Amortized Cost | 68,854 | 97,583
Securities held to maturity, Unrealized Gains | 34 | 24
Securities held to maturity, Unrealized Losses | 5,106 | 7,172
Securities held to maturity, fair value | 63,782 | 90,435
Mortgage-Backed Securities, Federal National Mortgage Association [Member] |  |
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item] |  |
Securities available for sale, Amortized Cost | 449,418 | 545,797
Securities available for sale, Unrealized Gains | 0 | 0
Securities available for sale, Unrealized Losses | 61,219 | 76,193
Securities available for sale | 388,199 | 469,604
Securities held to maturity, Amortized Cost | 5,729 | 8,332
Securities held to maturity, Unrealized Gains | 0 | 0
Securities held to maturity, Unrealized Losses | 467 | 582
Securities held to maturity, fair value | 5,262 | 7,750
Mortgage-Backed Securities, Federal Home Loan Mortgage Corporation [Member] |  |
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item] |  |
Securities available for sale, Amortized Cost | 402,399 | 410,829
Securities available for sale, Unrealized Gains | 488 | 0
Securities available for sale, Unrealized Losses | 59,665 | 68,608
Securities available for sale | 343,222 | 342,221
Securities held to maturity, Amortized Cost | 7,648 | 7,959
Securities held to maturity, Unrealized Gains | 0 | 0
Securities held to maturity, Unrealized Losses | 1,269 | 1,396
Securities held to maturity, fair value | 6,379 | 6,563
Mortgage-Backed Securities, Government National Mortgage Association [Member] |  |
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item] |  |
Securities available for sale, Amortized Cost | 126,417 | 112,202
Securities available for sale, Unrealized Gains | 252 | 1
Securities available for sale, Unrealized Losses | 21,409 | 18,037
Securities available for sale | 105,260 | 94,166
Securities held to maturity, Amortized Cost | 20,223 | 22,541
Securities held to maturity, Unrealized Gains | 0 | 0
Securities held to maturity, Unrealized Losses | 1,703 | 2,116
Securities held to maturity, fair value | 18,520 | 20,425
Collateralized Mortgage Obligations, Federal National Mortgage Association [Member] |  |
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item] |  |
Securities available for sale, Amortized Cost | 10,954 | 12,175
Securities available for sale, Unrealized Gains | 0 | 0
Securities available for sale, Unrealized Losses | 2,343 | 2,603
Securities available for sale | 8,611 | 9,572
Securities held to maturity, Amortized Cost | 11,432 | 14,268
Securities held to maturity, Unrealized Gains | 0 | 0
Securities held to maturity, Unrealized Losses | 851 | 1,119
Securities held to maturity, fair value | 10,581 | 13,149
Collateralized Mortgage Obligations, Federal Home Loan Mortgage Corporation [Member] |  |
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item] |  |
Securities available for sale, Amortized Cost | 19,766 | 21,519
Securities available for sale, Unrealized Gains | 0 | 0
Securities available for sale, Unrealized Losses | 4,186 | 4,163
Securities available for sale | 15,580 | 17,356
Securities held to maturity, Amortized Cost | 14,196 | 17,712
Securities held to maturity, Unrealized Gains | 0 | 0
Securities held to maturity, Unrealized Losses | 968 | 1,253
Securities held to maturity, fair value | 13,228 | 16,459
Collateralized Mortgage Obligations, Privately Issued [Member] |  |
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item] |  |
Securities available for sale, Amortized Cost | 0 | 0
Securities available for sale, Unrealized Gains | 325 | 337
Securities available for sale, Unrealized Losses | 0 | 0
Securities available for sale | 325 | 337
Collateralized Mortgage Obligations Government National Mortgage Association [Member] |  |
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item] |  |
Securities available for sale, Amortized Cost | 4,501 |
Securities available for sale, Unrealized Gains | 221 |
Securities available for sale, Unrealized Losses | 0 |
Securities available for sale | 4,722 |
Securities held to maturity, Amortized Cost | 3,565 | 4,222
Securities held to maturity, Unrealized Gains | 0 | 0
Securities held to maturity, Unrealized Losses | 270 | 330
Securities held to maturity, fair value | 3,295 | 3,892
Mortgage-Backed Securities [Member] |  |
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item] |  |
Securities available for sale, Amortized Cost | 1,013,455 | 1,102,522
Securities available for sale, Unrealized Gains | 1,286 | 338
Securities available for sale, Unrealized Losses | 148,822 | 169,604
Securities available for sale | 865,919 | 933,256
Securities held to maturity, Amortized Cost | 62,793 | 75,034
Securities held to maturity, Unrealized Gains | 0 | 0
Securities held to maturity, Unrealized Losses | 5,528 | 6,796
Securities held to maturity, fair value | $ 57,265 | $ 68,238

X
- Definition Allowance for credit losses, amortized cost. + References No definition available. + Details
Name: | fisi_AllowanceForCreditLossesAmortizedCost
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Debt securities available for sale and held to maturity securities. + References No definition available. + Details
Name: | fisi_DebtSecuritiesAvailableForSaleAndHeldToMaturitySecuritiesLineItems
Namespace Prefix: | fisi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Held-to-maturity securities, net. + References No definition available. + Details
Name: | fisi_HeldToMaturitySecuritiesNet
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 + Details
Name: | us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 + Details
Name: | us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479130/326-30-45-1 + Details
Name: | us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (aa) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481830/320-10-45-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479130/326-30-45-1 + Details
Name: | us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479344/326-20-45-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 14 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-14 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section 50 -Paragraph 7A -SubTopic 10 -Topic 310 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-7A + Details
Name: | us-gaap_HeldToMaturitySecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5A Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 + Details
Name: | us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5A -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5A Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 + Details
Name: | us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5A Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1 Reference 4: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (aa) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-5 + Details
Name: | us-gaap_HeldToMaturitySecuritiesFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fisi_CollateralizedMortgageBackedSecuritiesFederalNationalMortgageAssociationCertificatesAndObligationsFnmaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fisi_CollateralizedMortgageBackedSecuritiesFederalHomeLoanMortgageCorporationCertificatesAndObligationsFhlmcMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fisi_CollateralizedMortgageBackedSecuritiesGovernmentNationalMortgageAssociationCertificatesAndObligationsGnmaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fisi_CollateralizedMortgageObligationsFederalNationalMortgageAssociationCertificatesAndObligationsFnmaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fisi_CollateralizedMortgageObligationsFederalHomeLoanMortgageCorporationCertificatesAndObligationsFhlmcMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fisi_CollateralizedMortgageObligationsMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=fisi_CollateralizedMortgageObligationsGovernmentNationalMortgageAssociationCertificatesAndObligationsGnmaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |