XML 52 R40.htm IDEA: XBRL DOCUMENT
v3.26.1

Note 4 - Loans and Allowance for Credit Losses - Schedule of Loans Past Due (Details) - USD ($) $ in Thousands | Jun. 30, 2026 | Dec. 31, 2025
--- | --- | ---
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | $ 1,106,100 | $ 1,064,259
Financing Receivable, Excluding Accrued Interest, Nonaccrual | 4,567 | 6,030
Financing Receivable, Excluding Accrued Interest, Nonaccrual, No Allowance | 4,567 | 6,030
Commercial Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 214,185 | 146,178
Financing Receivable, Excluding Accrued Interest, Nonaccrual | 289 | 139
Financing Receivable, Excluding Accrued Interest, Nonaccrual, No Allowance | 289 | 139
Commercial Real Estate Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 688,569 | 702,455
Financing Receivable, Excluding Accrued Interest, Nonaccrual | 894 | 657
Financing Receivable, Excluding Accrued Interest, Nonaccrual, No Allowance | 894 | 657
Agricultural Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 86,474 | 93,627
Financing Receivable, Excluding Accrued Interest, Nonaccrual | 2,758 | 4,423
Financing Receivable, Excluding Accrued Interest, Nonaccrual, No Allowance | 2,758 | 4,423
Residential Mortgage [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 98,448 | 100,684
Financing Receivable, Excluding Accrued Interest, Nonaccrual | 156 | 174
Financing Receivable, Excluding Accrued Interest, Nonaccrual, No Allowance | 156 | 174
Residential Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 4,292 | 5,837
Financing Receivable, Excluding Accrued Interest, Nonaccrual | 0 | 0
Financing Receivable, Excluding Accrued Interest, Nonaccrual, No Allowance | 0 | 0
Consumer Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 14,132 | 15,478
Financing Receivable, Excluding Accrued Interest, Nonaccrual | 470 | 637
Financing Receivable, Excluding Accrued Interest, Nonaccrual, No Allowance | 470 | 637
Financial Asset, 30 to 59 Days Past Due [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 1,715 | 1,392
Financial Asset, 30 to 59 Days Past Due [Member] | Commercial Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 210 | 470
Financial Asset, 30 to 59 Days Past Due [Member] | Commercial Real Estate Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 354 | 231
Financial Asset, 30 to 59 Days Past Due [Member] | Agricultural Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 2 | 0
Financial Asset, 30 to 59 Days Past Due [Member] | Residential Mortgage [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 1,149 | 691
Financial Asset, 30 to 59 Days Past Due [Member] | Residential Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, 30 to 59 Days Past Due [Member] | Consumer Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, 60 to 89 Days Past Due [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 1,236 | 598
Financial Asset, 60 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 996 | 597
Financial Asset, 60 to 89 Days Past Due [Member] | Commercial Real Estate Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, 60 to 89 Days Past Due [Member] | Agricultural Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 137 | 1
Financial Asset, 60 to 89 Days Past Due [Member] | Residential Mortgage [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 103 | 0
Financial Asset, 60 to 89 Days Past Due [Member] | Residential Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, 60 to 89 Days Past Due [Member] | Consumer Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Real Estate Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Agricultural Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Residential Mortgage [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Residential Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, Past Due And Nonaccrual [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 7,518 | 8,020
Financial Asset, Past Due And Nonaccrual [Member] | Commercial Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 1,495 | 1,206
Financial Asset, Past Due And Nonaccrual [Member] | Commercial Real Estate Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 1,248 | 888
Financial Asset, Past Due And Nonaccrual [Member] | Agricultural Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 2,897 | 4,424
Financial Asset, Past Due And Nonaccrual [Member] | Residential Mortgage [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 1,408 | 865
Financial Asset, Past Due And Nonaccrual [Member] | Residential Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 0 | 0
Financial Asset, Past Due And Nonaccrual [Member] | Consumer Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 470 | 637
Financial Asset, Not Past Due [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 1,098,582 | 1,056,239
Financial Asset, Not Past Due [Member] | Commercial Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 212,690 | 144,972
Financial Asset, Not Past Due [Member] | Commercial Real Estate Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 687,321 | 701,567
Financial Asset, Not Past Due [Member] | Agricultural Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 83,577 | 89,203
Financial Asset, Not Past Due [Member] | Residential Mortgage [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 97,040 | 99,819
Financial Asset, Not Past Due [Member] | Residential Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 4,292 | 5,837
Financial Asset, Not Past Due [Member] | Consumer Portfolio Segment [Member] |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | $ 13,662 | $ 14,841

X
- Definition Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-5 Reference 2: 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 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 3B -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B + Details
Name: | us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost excluding accrued interest, of financing receivable on nonaccrual status. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 16 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-16 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477657/942-210-S45-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147478478/942-210-S50-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(7)(e)(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 + Details
Name: | us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrual
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost excluding accrued interest, of financing receivable on nonaccrual status with no allowance for credit loss. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 16 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-16 + Details
Name: | us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrualNoAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=fnrn_AgriculturalPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables30To59DaysPastDueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables60To89DaysPastDueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablesPeriodPastDueAxis=fnrn_FinancialAssetPastDueAndNonaccrualMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancialAssetNotPastDueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |