Document And Entity Information
us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions
Share-Based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount
Statistical Measurement [Axis]
us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions
Share-Based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount
Servicing Asset at Amortized Cost [Table Text Block]
Note To Financial Statement Details Textual
us-gaap_DividendsPayableDateDeclaredDayMonthAndYear
Dividends Payable, Date Declared
fnrn_StockDividendDeclaredPercentage
Stock Dividend Declared, Percentage
Represents information pertaining to the percentage stock dividend declared.
Significant Accounting Policies
Note 3 - Investment Securities
Provision for income taxes
ecd_NonRule10b51ArrTrmntdFlag
Non-Rule 10b5-1 Arrangement Terminated [Flag]
Note 4 - Loans and Allowance for Credit Losses
ecd_NonRule10b51ArrAdoptedFlag
Non-Rule 10b5-1 Arrangement Adopted [Flag]
Note 5 - Mortgage Operations
Trading Arrangements, by Individual [Table]
Note 6 - Fair Value Measurements
Lessee, Operating Lease, Supplemental Cash Flow Information [Table Text Block]
Tabular disclosure of supplemental information related to operating leases.
Debit Card [Member]
Material Terms of Trading Arrangement [Text Block]
Note 7 - Commitments and Contingencies
2031 and thereafter
fnrn_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFour
Amount of lessee's undiscounted obligation for lease payments for operating lease, due after fourth fiscal year following latest fiscal year.
Deposit Account [Member]
Note 8 - Stock Plans
Financial Instrument [Axis]
Note 9 - Accumulated Other Comprehensive Loss
Investment in Debt and Equity Securities and Other Trading Assets [Text Block]
Note 10 - Outstanding Shares and Earnings Per Share
Note 11 - Leases
Minimum [Member]
Note 3 - Investment Securities - Schedule of Marketable Securities (Details)
us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Unrealized Losses, Available-for-sale
Unrealized Gains, Available-for-sale
Note 3 - Investments - Schedule of Contractual Maturities (Details)
Statistical Measurement [Domain]
Note 3 - Investment Securities - Schedule of Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value (Details)
Maximum [Member]
Note 4 - Loans and Allowance for Credit Losses - Schedule of Total Loans Outstanding by Portfolio Segment (Details)
Note 4 - Loans and Allowance for Credit Losses - Allowance for Loan Losses by Portfolio Segment (Details)
Note 4 - Loans and Allowance for Credit Losses - Schedule of Collateral Dependent Loans (Details)
Note 4 - Loans and Allowance for Credit Losses - Schedule of Loans Past Due (Details)
Performance Guarantee [Member]
Note 4 - Loans and Allowance for Credit Losses - Schedule of Modified Financing Receivables (Details)
Note 4 - Loans and Allowance for Credit Losses - Schedule of Financing Receivable Credit Quality Indicators (Details)
Note 5 - Mortgage Operations - Schedule of Assumptions for Fair Value as of Balance Sheet Date of Assets or Liabilities that relate to Transferor's Continuing Involvement (Details)
Note 5 - Mortgage Operations - Schedule of Servicing Assets, At Amortized Value (Details)
Certificates of deposit
Note 6 - Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details)
Note 6 - Fair Value Measurements - Schedule of Fair Value Assets Measured on Nonrecurring Basis (Details)
Note 6 - Fair Value Measurements - Schedule of Fair Value by Balance Sheet Grouping (Details)
Employee Stock Purchase Plan [Member]
Represents information pertaining to the employee stock purchase plan.
Note 7 - Commitments and Contingencies - Schedule of Fair Value, off-Balance-Sheet Risks (Details)
Note 8 - Stock Plans - Schedule of Stock Option Activity (Details)
Note 8 - Stock Plans - Schedule of Restricted Stock Activity (Details)
Note 8 - Stock Plans - Schedule of Share-Based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions (Details)
Note 9 - Accumulated Other Comprehensive Loss - Schedule of Accumulated Other Comprehensive Income (Loss) (Details)
Note 10 - Outstanding Shares and Earnings Per Share - Schedule of Earnings Per Share, Basic and Diluted (Details)
Note 11 - Leases - Schedule of Operating Lease Liability Maturities (Details)
Note 11 - Leases - Schedule of Supplemental Information (Details)
Notes To Financial Statements
Notes To Financial Statements [Abstract]
fnrn_StockPlanParticipationPeriod
Stock Plan, Participation Period (Month)
The participation period for stock plans.
fnrn_StockPurchasePricePercentageOfPrice
Stock Purchase Price, Percentage of Price
Purchase price of stock as percentage of lower of the fair market value on the last trading day before the date of participation or the fair market value on the last trading day during the participation period.
fnrn_PercentageOfEmployeeCompensationThatCanBeWithheldToPurchaseCommonStock
Percentage Of Employee Compensation That Can Be Withheld to Purchase Common Stock
Refers to the percentage of employee compensation that can be withheld to purchase common stock.
Cancelled / Forfeited, weighted average exercise price (in dollars per share)
Expired, weighted average exercise price (in dollars per share)
Granted, weighted average exercise price (in dollars per share)
Exercised, weighted average exercise price (in dollars per share)
Salaries and employee benefits
Financial Instruments [Domain]
Asset Pledged as Collateral [Member]
us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet
Increase in interest receivable and other assets
us-gaap_SharesOutstanding
Balance (in shares)
Balance (in shares)
us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions
us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions
Non-vested restricted stock, Outstanding, Weighted Average Remaining Contractual Term (Year)
Dividends [Axis]
Dividends [Domain]
Segment Reporting [Text Block]
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months
Financial Instruments Owned and Pledged as Collateral [Table]
Award Type [Axis]
Adjustments to reconcile net income to net cash provided by operating activities:
Plan Name [Axis]
us-gaap_Liabilities
Total Liabilities
Plan Name [Domain]
us-gaap_AllocatedShareBasedCompensationExpense
Share-Based Payment Arrangement, Expense
us-gaap_LiabilitiesAndStockholdersEquity
Total Liabilities and Stockholders’ Equity
Pledged Status [Domain]
fnrn_PolicyTable
Title and Position [Axis]
Public Deposits or Other Purposes [Member]
Represents information pertaining to public deposits or other purposes.
Segment Reporting [Table]
Pledged Status [Axis]
Mortgage servicing rights, net of valuation allowance, additions
The value of new servicing assets, subsequently measured at amortized value, acquired or created during the current period through purchases or from transfers of financial assets, net of valuation allowance.
fnrn_ServicingAssetAtAmortizedCostNetOfValuationAllowance
Mortgage servicing rights, net of valuation allowance, balance
Mortgage servicing rights, net of valuation allowance, balance
Amortized carrying amount (balance) as of the balance sheet date of an asset representing net future revenues from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements, net of valuation allowance.
fnrn_ImpairedLoansMeasurementInput
Impaired Loans, Measurement Input
Value of input used to measure impaired loans.
Stock-based compensation
us-gaap_ShareBasedCompensation
fnrn_ServicingAssetAtAmortizedCostNetOfValuationAllowanceReductions
Mortgage servicing rights, net of valuation allowance, reductions
Amount of expense charged against earnings for the periodic recognition of servicing assets relative to estimated net servicing income, net of valuation allowance.
fnrn_MortgageLoansOnRealEstateServicedForOhers
Mortgage Loans on Real Estate, Serviced for Ohers
The balance represents the amount of loans serviced for others that are secured by real estate mortgages.
Collateral, Market, Income, Enterprise, Liquidation Methods [Member]
Represents information pertaining to collateral, market, income, enterprise, and liquidation valuation methods.
Share-Based Payment Arrangement [Text Block]
Changes in assets and liabilities:
Portion at Fair Value Measurement [Member]
Reported Value Measurement [Member]
Fair Value, by Balance Sheet Grouping [Table]
us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect
Net (decrease) increase in Cash and Cash Equivalents
us-gaap_CashAndCashEquivalentsFairValueDisclosure
Cash and cash equivalents
Estimate of Fair Value Measurement [Member]
us-gaap_ServicingAssetAtFairValueAmount
Servicing Asset at Fair Value, Amount
New Accounting Pronouncements, Policy [Policy Text Block]
Lessor, Lease, Description [Table]
Schedule of Fair Value, off-Balance-Sheet Risks [Table Text Block]
us-gaap_ServicingAssetAtAmortizedValueDisposals
Mortgage servicing rights, reductions
us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations
Cash and Cash Equivalents, beginning of period
Cash and Cash Equivalents, end of period
Mortgage servicing rights, additions
us-gaap_ServicingAssetAtAmortizedValue
Mortgage servicing rights, balance
Mortgage servicing rights, balance
Antidilutive Securities, Name [Domain]
Servicing Asset at Fair Value, Activity [Table]
Statement [Table]
Discount rate
Operating cash flows from operating leases
Constant prepayment rate
Fair Value, off-Balance-Sheet Risk [Table]
Interest payable and other liabilities
us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss
Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table Text Block]
Collateral dependent loans
Net loans
Recognition of right-of-use assets obtained in exchange for operating lease liabilities
Debt Securities, Available-for-Sale, Unrealized Loss Position
Financing Receivable, Allowance for Credit Loss [Table Text Block]
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Non-vested Restricted stock, Outstanding (in shares)
Non-vested Restricted stock, Outstanding (in shares)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
Non-vested Restricted stock Outstanding, Weighted Average Grant Date Fair Value (in dollars per share)
Non-vested Restricted stock Outstanding, Weighted Average Grant Date Fair Value (in dollars per share)
Financing Receivable, Past Due [Table Text Block]
Non-vested Restricted stock, Granted (in shares)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Non-vested Restricted stock, Granted, Weighted Average Grant Date Fair Value (in dollars per share)
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
Non-vested Restricted stock, Cancelled / Forfeited (in shares)
Financing Receivable Credit Quality Indicators [Table Text Block]
Financing Receivable, Modified [Table Text Block]
Product and Service [Domain]
Product and Service [Axis]
Non-vested Restricted stock, Exercised/Released/Vested, Weighted Average Grant Date Fair Value (in dollars per share)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
Non-vested Restricted stock, Exercised/Released/Vested (in shares)
Share-Based Payment Arrangement, Option, Exercise Price Range [Table]
Collateralized Mortgage Obligations [Member]
us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue
Total lease payments
Weighted-average discount rate – operating leases
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share)
Commercial Real Estate [Member]
2027
us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod
Cancelled / Forfeited, shares (in shares)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercises in Period, Intrinsic Value
Loans
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod
Expired, shares (in shares)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized (in shares)
Commitments to Extend Credit [Member]
2026 (remaining 6 months)
us-gaap_LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear
Exercisable (vested) at End of Period, shares (in shares)
us-gaap_LoansAndLeasesReceivableImpairedCommitmentToLend
Financing Receivable, Modified, Commitment to Lend
Lessee, Operating Lease, Liability, to be Paid, Maturity [Table Text Block]
Exercisable (vested) at End of Period, weighted average exercise price (in dollars per share)
Measurement Frequency [Axis]
2029
us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree
Assets
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Options outstanding, shares (in shares)
Options outstanding, shares (in shares)
Commitments and Contingencies Disclosure [Text Block]
2028
us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Options outstanding, weighted average exercise price (in dollars per share)
Options outstanding, weighted average exercise price (in dollars per share)
Statement [Line Items]
2030
us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour
Fair Value Hierarchy and NAV [Axis]
Options outstanding at End of Period, Intrinsic value
Common Stock, Shares, Issued (in shares)
Common Stock, Shares, Outstanding (in shares)
Award Type [Domain]
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
Risk Free Interest Rate
Common Stock, Shares Authorized (in shares)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate
Expected Price Volatility
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
Expected Dividend Yield
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]
Schedule of Share-Based Compensation Arrangements by Share-Based Payment Award [Table]
Common stock, no par value; 32,000,000 shares authorized; 16,395,303 shares issued and outstanding at June 30, 2026 and 16,406,281 shares issued and outstanding at December 31, 2025
us-gaap_MortgageLoansInProcessOfForeclosureAmount
Mortgage Loans in Process of Foreclosure, Amount
us-gaap_PolicyTextBlockAbstract
Accounting Policies
us-gaap_DisclosureTextBlockAbstract
Notes to Financial Statements
us-gaap_TableTextBlock
Notes Tables
Comprehensive Income (Loss) Note [Text Block]
us-gaap_ComprehensiveIncomeNetOfTax
Comprehensive income
Significant Accounting Policies [Text Block]
Proceeds from calls or maturities of available-for-sale securities
us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount
Less: interest
Amortization of operating lease right-of-use asset
Options outstanding at End of Period, Remaining Contractual Term (Year)
Present value of lease liabilities
Operating Lease, Liability
Loans held-for-sale
us-gaap_DepositsFairValueDisclosure
Time deposits
us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1
Expected Life in Years (Year)
Exercisable (vested) at End of Period, Remaining Contractual Term (Year)
Standby Letters of Credit [Member]
Agricultural Portfolio Segment [Member]
Represents information pertaining to the agricultural portfolio segment.
Financial Standby Letter of Credit [Member]
us-gaap_ContractuallySpecifiedServicingFeesAmount
Contractually Specified Servicing Fees, Amount
Payment Deferral [Member]
Extended Maturity [Member]
Statement of Stockholders' Equity [Abstract]
Statement of Comprehensive Income [Abstract]
Loan Restructuring Modification [Axis]
Loan Restructuring Modification [Domain]
Proceeds from sales of loans held-for-sale
Statement of Cash Flows [Abstract]
Financing Receivable Portfolio Segment [Axis]
Statutory Accounting Practices [Table]
Other comprehensive income, net of taxes
Current period other comprehensive loss
Stockholders' Equity:
Equity Components [Axis]
fnrn_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossAndReserveForUnfundedCommitments
Total
Total
Amount of allowance for credit loss on financing receivable and reserve for unfunded commitments.
Unfunded Loan Commitment [Member]
fnrn_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossAndReserveForUnfundedCommitmentsChargeoffs
Total, Charge-offs
Amount of write-off of financing receivable, charged against allowance for credit loss and reserve for unfunded commitments.
fnrn_OffBalanceSheetCreditLossLiabilityChargeoffs
Reserve for unfunded commitments, Charge-offs
Amount of charge-offs for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.
Financing Receivable Portfolio Segment [Domain]
Equity Component [Domain]
Fair Value Measurements, Nonrecurring [Table Text Block]
Fair Value, Inputs, Level 2 [Member]
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Fair Value, Inputs, Level 3 [Member]
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member]
Reserve for unfunded commitments, Recovery
Amount of recoveries for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.
AOCI Attributable to Parent [Member]
Fair Value, Inputs, Level 1 [Member]
Additional Paid-in Capital [Member]
Total, Recovery
Amount of increase in allowance for credit loss on financing receivable from recovery and reserve for unfunded commitments.
Retained Earnings [Member]
Agriculture Production Loans [Member]
Represents information pertaining to agriculture production loans.
Farmland [Member]
Represents information pertaining to farmland.
Other Non-farm, Non-Residential Properties [Member]
Represents information pertaining to other non-farm, non-residential properties.
Owner-occupied, Non-farm, Non-residential Properties [Member]
Represents information pertaining to owner-occupied, non-farm, and non-residential properties.
Real Estate, First Lien [Member]
Represents information pertaining to the first lien on real estate.
Financing Receivable, Collateral Dependent Loans [Table Text Block]
The entire tabular disclosure for collateral dependent financing receivables.
Real Estate, Revolving Liens [Member]
Represents information pertaining to revolving liens on real estate.
Residential Real Estate [Member]
Real Estate, Junior Lien [Member]
Represents information pertaining to junior liens on real estate.
Schedule of Gain (Loss) on Securities [Table]
Financing Receivable, Excluding Accrued Interest, Year One, Originated, Current Fiscal Year, Recoveries
Year-to-date Recoveries
Amount, excluding accrued interest, of recoveries of financing receivable originated in current fiscal year. Excludes net investment in lease.
Premises and equipment, net
Financial Asset, Aging [Axis]
Financing Receivable, Excluding Accrued Interest, Year Two, Originated, Fiscal Year before Current Fiscal Year, Recoveries
Year-to-date Recoveries
Amount, excluding accrued interest, of recoveries of financing receivable originated in fiscal year before current fiscal year. Excludes net investment in lease.
Financial Asset, Past Due And Nonaccrual [Member]
Represents information pertaining to past due and nonaccrual financial assets.
Extended Maturity and Payment Deferral [Member]
Represents information pertaining to extended maturity and payment.
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss
Pass
Financing Receivable, Excluding Accrued Interest, Year One, Originated, Current Fiscal Year, Net Charge-offs
Year-to-date Net Charge-offs
Amount, excluding accrued interest, of write-offs (net of recoveries) for financing receivable originated in current fiscal year. Excludes net investment in lease.
Financing Receivable, Excluding Accrued Interest, Revolving, Recoveries
Year-to-date Recoveries
Amount, excluding accrued interest, of recoveries of financing receivable that can be withdrawn, repaid, and redrawn.
Financing Receivable, Excluding Accrued Interest, Year Three, Originated, Two Years before Current Fiscal Year, Net Charge-offs
Year-to-date Net Charge-offs
Amount, excluding accrued interest, of write-offs (net of recoveries) for financing receivable originated two years before current fiscal year. Excludes net investment in lease.
Financing Receivable, Excluding Accrued Interest, Year Two, Originated, Fiscal Year before Current Fiscal Year, Net Charge-offs
Year-to-date Net Charge-offs
Amount, excluding accrued interest, of write-offs (net of recoveries) for financing receivable originated in fiscal year before current fiscal year. Excludes net investment in lease.
Financing Receivable, Excluding Accrued Interest, Year Four, Originated, Three Years before Current Fiscal Year, Recoveries
Year-to-date Recoveries
Amount, excluding accrued interest, of recoveries of financing receivable originated three years before current fiscal year. Excludes net investment in lease.
Financing Receivable, Excluding Accrued Interest, Year Three, Originated, Two Years before Current Fiscal Year, Recoveries
Year-to-date Recoveries
Amount, excluding accrued interest, of recoveries of financing receivable originated two years before current fiscal year. Excludes net investment in lease.
Financial Asset, Aging [Domain]
Financing Receivable, Excluding Accrued Interest, Originated, More than Five Years before Current Fiscal Year, Recoveries
Year-to-date Recoveries
Amount, excluding accrued interest, of recoveries of financing receivable originated more than five years before current fiscal year. Excludes net investment in lease.
Financing Receivable, Excluding Accrued Interest, Year Five, Originated, Four Years before Current Fiscal Year, Recoveries
Year-to-date Recoveries
Amount, excluding accrued interest, of recoveries of financing receivable originated four years before current fiscal year. Excludes net investment in lease.
fnrn_InvestmentInFederalHomeLoanBankStockAndOtherEquitySecuritiesFairValueDisclosure
Stock in Federal Home Loan Bank and other equity securities
Fair value portion of investments in Federal Home Loan Bank (FHLBank) stock and other equity securities.
fnrn_CertificateOfDepositsFairValueDisclosure
Certificates of deposit
Represents information pertaining to the fair value of certificate of deposit assets.
fnrn_InterestPayableFairValueDisclosure
Interest payable
This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure.
fnrn_InterestReceivableFairValueDisclosure
Interest receivable
This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure.
Financing Receivable, Excluding Accrued Interest, Year Five, Originated, Four Years before Current Fiscal Year, Net Charge-offs
Year-to-date Net Charge-offs
Amount, excluding accrued interest, of write-offs (net of recoveries) for financing receivable originated four years before current fiscal year. Excludes net investment in lease.
Financing Receivable, Excluding Accrued Interest, Year Four, Originated, Three Years before Current Fiscal Year, Net Charge-offs
Year-to-date Net Charge-offs
Amount, excluding accrued interest, of write-offs (net of recoveries) for financing receivable originated three years before current fiscal year. Excludes net investment in lease.
Provision for credit losses
Provision for Loan, Lease, and Other Losses
Financing Receivable, Excluding Accrued Interest, Revolving, Net Charge-offs
Year-to-date Net Charge-offs
Amount, excluding accrued interest, of write-offs (net of recoveries) for financing receivable that can be withdrawn, repaid, and redrawn.
Financing Receivable, Excluding Accrued Interest, Originated, More than Five Years before Current Fiscal Year, Net Charge-offs
Year-to-date Net Charge-offs
Amount, excluding accrued interest, of write-offs (net of recoveries) for financing receivable originated more than five years before current fiscal year. Excludes net investment in lease.
Financial Asset, 30 to 59 Days Past Due [Member]
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss
Total, amortized cost
Schedule of Share-Based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions [Table Text Block]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest
Allowance for credit losses
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest
Financial Asset, 60 to 89 Days Past Due [Member]
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest
Share-Based Payment Arrangement, Option, Activity [Table Text Block]
Cash in lieu of fractional shares (in shares)
Cash in lieu of fractional shares (in shares)
Number of shares issued equal to the amount of reduction to retained earnings resulting from cash paid in lieu of fractional shares.
fnrn_CashPaidInLieuOfFractionalSharesValue
Cash in lieu of fractional shares
Reduction to retained earnings resulting from cash dividends paid in lieu of fractional shares during the period.
Financing Receivable, Excluding Accrued Interest, Year Five, Originated, Four Years before Current Fiscal Year
Pass
Financing Receivable, Excluding Accrued Interest, Originated, More than Five Years before Current Fiscal Year
Pass
Financing Receivable, Excluding Accrued Interest, Year Three, Originated, Two Years before Current Fiscal Year
Pass
Financing Receivable, Excluding Accrued Interest, Year Four, Originated, Three Years before Current Fiscal Year
Pass
Fair Value Disclosures [Text Block]
Financing Receivable, Excluding Accrued Interest, Year One, Originated, Current Fiscal Year
Pass
Fair Value, by Balance Sheet Grouping [Table Text Block]
Financing Receivable, Excluding Accrued Interest, Year Two, Originated, Fiscal Year before Current Fiscal Year
Pass
Share-Based Payment Arrangement, Option [Member]
Supplemental Disclosures of Cash Flow Information:
fnrn_FinancingReceivableExcludingAccruedInterestRecoveries
Year-to-date Recoveries
The amount of recoveries from financing receivables, excluding accrued interest.
Financing Receivable, Excluding Accrued Interest, Revolving
Pass
us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingConvertedToTermLoan
Financing Receivable, Excluding Accrued Interest, Revolving, Converted to Term Loan
fnrn_FinancingReceivableExcludingAccruedInterestNetChargeoffs
Year-to-date Net Charge-offs
The amount of net charge-offs from financing receivables excluding accrued interest.
us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments
Accretion and amortization of investment securities premiums and discounts, net
Income taxes
Collateral Held [Domain]
Collateral Held [Axis]
Other Commitments [Table]
Common Stock, No Par Value (in dollars per share)
Accumulated other comprehensive loss, net
Basis of Accounting [Text Block]
us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest
Income before provision for income taxes
Data processing
Additional paid-in capital
us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]
us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
Purchases of premises and equipment
Lessee, Operating Leases [Text Block]
Stock-based compensation
Collateral Pledged [Member]
Weighted average life (years) (Year)
us-gaap_StockRepurchasedAndRetiredDuringPeriodValue
Stock repurchase and retirement
us-gaap_NoninterestExpense
Total non-interest expenses
Non-interest expenses:
Stock dividend distributed
Financial Asset, Not Past Due [Member]
us-gaap_PaymentsToAcquireRestrictedCertificatesOfDeposit
Purchases of certificates of deposit
Stock in Federal Home Loan Bank and other equity securities, at cost
The amount of federal home loan bank stock and equity securities.
Fair Value, Recurring and Nonrecurring [Table]
Savings and MMDAs
The amount of savings deposits and money market deposits.
us-gaap_OffBalanceSheetCreditLossLiability
Off-Balance-Sheet, Credit Loss, Liability
Reserve for unfunded commitments
Reserve for unfunded commitments
Stock options exercised, net of swapped shares (in shares)
Exercised, shares (in shares)
Common shares issued related to restricted stock grants (in shares)
Depreciation
Stock dividend adjustment
us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
Stock repurchase and retirement (in shares)
Cash Flows From Operating Activities
Cash Flows From Investing Activities
us-gaap_NetCashProvidedByUsedInOperatingActivities
Net cash provided by operating activities
Cash Flows From Financing Activities
us-gaap_NetCashProvidedByUsedInInvestingActivities
Net cash (used in) provided by investing activities
us-gaap_NetCashProvidedByUsedInFinancingActivities
Net cash provided by (used in) financing activities
us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt
Purchases of available-for-sale securities
Proceeds from sales of available-for-sale securities
Proceeds from Sale of Debt Securities, Available-for-Sale
us-gaap_LiabilitiesFairValueDisclosure
Liabilities, Fair Value Disclosure
Total assets at fair value
Advertising
Net increase (decrease) in deposits
US Government Corporations and Agencies Securities [Member]
Accounting Policies [Abstract]
Interest receivable and other assets
Mortgage-Backed Security, Issued by US Government-Sponsored Enterprise [Member]
US Treasury Securities [Member]
US States and Political Subdivisions Debt Securities [Member]
Loans, net of allowance for credit losses of $14,795 at June 30, 2026 and $14,519 at December 31, 2025
Loans, net
Measurement Input Type [Axis]
us-gaap_PaymentsForProceedsFromLoansAndLeases
Net increase in loans
Income Statement [Abstract]
Reserve for unfunded commitments, Credit Loss Expense (Reversal)
Measurement Input, Cost to Sell [Member]
Investment securities – available-for-sale, at estimated fair value, net of allowance for credit losses of $0; amortized cost of $647,387 at June 30, 2026 and $639,532 at December 31, 2025
Total, fair value
Debt Securities, Available-for-Sale, Excluding Accrued Interest
us-gaap_NumberOfReportableSegments
Number of Reportable Segments
us-gaap_AmortizationOfDeferredLoanOriginationFeesNet
Increase in deferred loan origination costs, net
Net income
Net income
Net income
Net income
us-gaap_DividendsPayableDateOfRecordDayMonthAndYear
Dividends Payable, Date of Record
Amortization of intangible assets
Measurement Input Type [Domain]
us-gaap_InterestIncomeExpenseNet
Net interest income
Accumulated Other Comprehensive Income (Loss) [Table]
O 2026 Q1 Dividends [Member]
Represents O 2026 Q1 Dividends.
Restricted Stock [Member]
us-gaap_FinancingReceivableNonaccrualInterestIncome
Financing Receivable, Nonaccrual, Interest Income
Accounting Standards Update and Change in Accounting Principle [Table]
Occupancy and equipment
Other expense
us-gaap_NoninterestIncome
Total non-interest income
Non-interest income:
Stationery and supplies
Transfers and Servicing of Financial Assets [Text Block]
us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Vested in Period, Fair Value
Investment and brokerage services income
Measurement Basis [Axis]
Fair Value Hierarchy and NAV [Domain]
Other income
Gains on sales of loans held-for-sale
Gains on sales of loans held-for-sale
Measurement Frequency [Domain]
us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance
Valuation allowance, balance
Valuation allowance, balance
Statement of Financial Position [Abstract]
us-gaap_DividendPayableDateToBePaidDayMonthAndYear
Dividends Payable, Date to be Paid
Financing Receivable, Excluding Accrued Interest, Credit Loss Expense (Reversal)
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Recovery
Year-to-date Recoveries
us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Charge-offs
Year-to-date Charge-offs
Directors’ fees
Fair Value, Nonrecurring [Member]
Schedule of Assumptions for Fair Value as of Balance Sheet Date of Assets or Liabilities that relate to Transferor's Continuing Involvement [Table]
Fair Value, Recurring [Member]
us-gaap_PaymentsForProceedsFromFederalHomeLoanBankStock
Purchases of Federal Home Loan Bank stock and other equity securities, at cost
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff (Recovery)
Year-to-date Net Charge-offs
us-gaap_PaymentsForRepurchaseOfCommonStock
Repurchases of common stock
Document Type
Document Information [Line Items]
Document Quarterly Report
us-gaap_LesseeOperatingLeaseRenewalTerm
Lessee, Operating Lease, Renewal Term (Year)
Valuation allowance, additions
Weighted-average remaining lease term – operating leases, in years (Year)
Document Information [Table]
us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsRecoveries
Valuation allowance, reductions
us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares)
Antidilutive Securities [Axis]
us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
Effect of dilutive shares (in shares)
Entity Interactive Data Current
Document Transition Report
us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Adjusted weighted average common shares outstanding (in shares)
us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis
Subtotal, amortized cost
us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDate
Subtotal, fair value
us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Weighted average common shares outstanding (in shares)
Basic earnings per common share (in dollars per share)
Basic EPS (in dollars per share)
Financing Receivable, Excluding Accrued Interest, Nonaccrual, No Allowance
Earnings Per Share, Diluted, by Common Class, Including Two-Class Method [Table]
Financing Receivable, Excluding Accrued Interest, Nonaccrual
Demand deposits
Title and Position [Domain]
Interest-bearing transaction deposits
us-gaap_Deposits
Total deposits
Deposits:
Residential Mortgage [Member]
Intangible assets, net
Document Fiscal Period Focus
Pledging Purpose [Axis]
Document Fiscal Year Focus
Pledging Purpose [Domain]
Amendment Flag
Financing Receivables [Text Block]
Common Stock [Member]
Financing Receivable, Past Due [Table]
Deferred origination fees and costs, net
us-gaap_DebtSecuritiesAvailableForSaleRealizedGain
Debt Securities, Available-for-Sale, Realized Gain
Document Period End Date
Interest and dividend income:
Non-vested restricted stock, Outstanding, Aggregate Intrinsic Value
Residential Portfolio Segment [Member]
Granted, shares (in shares)
Legal Entity [Axis]
us-gaap_InterestAndDividendIncomeOperating
Total interest and dividend income
us-gaap_OperatingLeaseCost
Operating Lease, Cost
Consumer Portfolio Segment [Member]
Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Table]
Commercial Portfolio Segment [Member]
Entity File Number
Entity Tax Identification Number
Supplemental disclosures of non-cash investing and financing activities:
us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss
us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss
Entity [Domain]
Entity Central Index Key
us-gaap_DebtSecurities
Debt Securities
Entity Registrant Name
Doubtful [Member]
Entity Current Reporting Status
us-gaap_DebtSecuritiesAvailableForSaleRealizedLoss
Debt Securities, Available-for-Sale, Realized Loss
Special Mention [Member]
Losses on sales/calls of available-for-sale securities
Losses on sales/calls of available-for-sale securities
Substandard [Member]
Entity Incorporation, State or Country Code
Pass [Member]
Current Fiscal Year End Date
Unrealized holding (losses) gains on available-for-sale securities, net of taxes
Entity Filer Category
Entity Shell Company
Entity Emerging Growth Company
Entity Small Business
Stock dividend adjustment (in shares)
Stock dividend adjustment (in shares)
Valuation Approach and Technique [Axis]
Derivative [Member]
Financing Receivable, Modified [Table]
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]
Valuation Approach and Technique [Domain]
Deposits
Entity Address, State or Province
us-gaap_PaymentsForOriginationAndPurchasesOfLoansHeldForSale
Originations of loans held-for-sale
Entity Address, City or Town
Marketable Securities [Table]
Retained earnings
us-gaap_InterestExpenseOperating
Total interest expense
Entity Address, Address Line One
City Area Code
Due after one year through five years, amortized cost
Due after five years through ten years, amortized cost
Interest expense:
Local Phone Number
Due in one year or less, amortized cost
Entity Address, Postal Zip Code
Decrease in interest payable and other liabilities
Mortgage-backed securities & collateralized mortgage obligations, fair value
Title of 12(b) Security
Security Exchange Name
Due after five years through ten years, fair value
Trading Symbol
Due after ten years, fair value
Due in one year or less, fair value
Due after one year through five years, fair value
Due after ten years, amortized cost
Mortgage-backed securities & collateralized mortgage obligations, amortized cost
Class of Stock [Axis]
FHLB advances
Class of Financing Receivable [Axis]
Class of Financing Receivable [Domain]
Entity Common Stock, Shares Outstanding
Other comprehensive (loss) income, net of tax:
Earnings Per Share [Text Block]
us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax
Financing Receivable, Credit Quality Indicator [Table]
Director [Member]
Unrealized holding (losses) gains arising during the period, net of tax effect of $(565) and $1,111 for the three months ended June 30, 2026 and June 30, 2025, respectively, and $(1,837) and $4,824 for the six months ended June 30, 2026 and June 30, 2025, respectively
Internal Credit Assessment [Axis]
Officer [Member]
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability
us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss
Net interest income after provision for credit losses
Share-Based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity [Table Text Block]
Investments Classified by Contractual Maturity Date [Table Text Block]
Due from banks interest bearing accounts
Exercisable (vested) at End of Period, Intrinsic Value
Internal Credit Assessment [Domain]
Taxable
Non-taxable
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax
us-gaap_Assets
Total Assets
us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
Less: reclassification adjustment due to losses realized on sales of securities, net of tax effect of $33 and $10 for the three months ended June 30, 2026 and June 30, 2025, respectively, and $33 and $26 for the six months ended June 30, 2026 and June 30, 2025, respectively
Cash and cash equivalents
Other earning assets
Financing Receivable, Excluding Accrued Interest, Modified in Period, Amount
Investment securities:
Financing Receivable, Excluding Accrued Interest, Modified in Period, to Total Financing Receivables, Percentage
us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansNetOfTax
Other comprehensive (loss) income, net of tax
Diluted earnings per common share (in dollars per share)
Diluted EPS (in dollars per share)
us-gaap_FinancingReceivableExcludingAccruedInterestModifiedSubsequentDefault
Financing Receivable, Excluding Accrued Interest, Modified, Subsequent Default
Financing Receivable, Modified, Weighted Average Interest Rate Decrease from Modification
Other real estate owned
Schedule of Assumptions for Fair Value as of Balance Sheet Date of Assets or Liabilities that relate to Transferor's Continuing Involvement [Table Text Block]
Construction [Member]
Financing Receivable, Modified, Weighted Average Term Increase from Modification (Month)
Accounts and Financing Receivables [Table]
Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block]
Interest
Participating Mortgage Loans [Table]
Time, over $250,000
The aggregate amount of domestic and foreign time deposit liabilities (including certificates of deposits, individual retirement accounts and open accounts) in denominations over $250,000.
Time, $250,000 or less
The aggregate amount of domestic or foreign time deposits (including certificates of deposits, individual retirement accounts and open accounts) that total to $250,000 or less.
us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff
Year-to-date Charge-offs
Year-to-date Charge-offs
us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff
Year-to-date Charge-offs
Year-to-date Charge-offs
us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff
Year-to-date Charge-offs
Year-to-date Charge-offs
us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff
Year-to-date Charge-offs
Year-to-date Charge-offs
Loan Servicing [Member]
Represents information pertaining to loan servicing.
us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff
Year-to-date Charge-offs
Year-to-date Charge-offs
us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff
Year-to-date Charge-offs
Year-to-date Charge-offs
us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff
Year-to-date Charge-offs
Year-to-date Charge-offs
us-gaap_StockholdersEquity
Total Stockholders’ Equity
Balance
Balance
Non-vested Restricted stock, Cancelled/Forfeited , Weighted Average Grant Date Fair Value (in dollars per share)
Collateral Dependent Loans [Member]
Represents collateral dependent loans.
Principal repayments on available-for-sale securities
The cash inflow associated with principal repayments on securities not classified as either held-to-maturity securities or trading securities which are classified as available-for-sale securities.
Market value of shares tendered in-lieu of cash to pay for exercise of options
The market value of shares tendered in-lieu of cash to pay for exercise of options.
fnrn_CashDividendsPaidInLieuOfFractionalShares
Cash dividends paid in lieu of fractional shares
Represents the amount of cash dividends paid in lieu of fractional shares held.
us-gaap_OperatingLeaseRightOfUseAsset
Operating Lease, Right-of-Use Asset
Proceeds from maturities of certificates of deposit
Amount of cash inflow from maturities of certificates of deposit.
ecd_Rule10b51ArrTrmntdFlag
Rule 10b5-1 Arrangement Terminated [Flag]
fnrn_LesseeOperatingLeaseNumberOfBranchAndAdministrativeLocations
Lessee, Operating Lease, Number of Branch and Administrative Locations
Number of branch and administration locations leased by the entity.
ecd_Rule10b51ArrAdoptedFlag
Rule 10b5-1 Arrangement Adopted [Flag]
Revenue from Contract with Customer, Including Assessed Tax
Transfer of premises and equipment to other real estate owned
Value of premises and equipment transferred to other real estate owned (OREO) in noncash transactions.
Commitments and contingencies (Note 7)
Marketable Securities [Table Text Block]