Document And Entity Information Statistical Measurement [Axis] Note To Financial Statement Details Textual Significant Accounting Policies Note 2 - Securities Benefit from income taxes ecd_NonRule10b51ArrTrmntdFlag Non-Rule 10b5-1 Arrangement Terminated [Flag] Note 3 - Loans Receivable Change in unrealized gain (loss) on fair value hedge The amount of change in unrealized gain (loss) of plan. ecd_NonRule10b51ArrAdoptedFlag Non-Rule 10b5-1 Arrangement Adopted [Flag] Note 4 - Allowance for Credit Losses on Loans Trading Arrangements, by Individual [Table] Note 5 - Deposits Material Terms of Trading Arrangement [Text Block] Note 6 - Borrowings Note 8 - (Loss) Earnings Per Common Share Financial Instrument [Axis] Note 9 - Employee Benefits Investment in Debt and Equity Securities and Other Trading Assets [Text Block] Note 10 - Stock-based Compensation fnwb_EmployeeStockOwnershipPlanEsopDebtStructureEstimatedInterestRate Employee Stock Ownership Plan (ESOP), Debt Structure, Estimated Interest Rate The percentage of estimated interest paid by the ESOP to a lender. Note 11 - Fair Value Measurements Minimum [Member] Note 12 - Change in Accumulated Other Comprehensive Income ("AOCI") us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax Securities available for sale, gross unrealized losses fnwb_DepositsPublicFund Deposits, Public Fund Amount of public fund deposit liabilities. Note 13 - Derivatives and Hedging Activities Securities available for sale, gross unrealized gains Statistical Measurement [Domain] Note 2 - Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Securities (Details) Maximum [Member] Schedule of Interest on Deposits Liabilities, Type [Table Text Block] The tabular disclosure of interest on deposits liabilities by type. Note 2 - Securities - Unrealized Gross Losses and Fair Value of Securities in a Continuous Loss Position (Details) Note 2 - Securities - Amortized Cost and Estimated Fair Value of Investment Securities by Contractual Maturity (Details) Note 3 - Loans Receivable - Loans Receivable (Details) First Northwest Bancorp 2020 Equity Incentive Plan [Member] Represents First Northwest Bancorp 2020 equity incentive plan. Litigation Case [Domain] Note 3 - Loans Receivable - Nonaccrual Loans by Class (Details) Note 3 - Loans Receivable - Past Due Loans (Details) fnwb_EmployeeStockOwnershipPlanEsopDebtStructureAmortizationPeriod Employee Stock Ownership Plan (ESOP), Debt Structure, Amortization Period (Year) The amortization period of the loan made by a third-party lender directly to the ESOP. Note 3 - Loans Receivable - Credit Quality Indicators by Class of Loan (Details) Note 3 - Loans Receivable - Collateral Dependent Loans (Details) fnwb_EmployeeStockOwnershipPlanEsopPrincipalAndInterestPaymentsFromEsop Employee Stock Ownership Plan (ESOP), Principal and Interest Payments from ESOP The amount of principal and interest paid by the ESOP to a lender. Note 3 - Loans Receivable -Other Real Estate Activity (Details) us-gaap_ProceedsFromIssuanceOfSubordinatedLongTermDebt Proceeds from Issuance of Subordinated Long-Term Debt Note 4 - Allowance for Credit Losses on Loans - Allowance for Loan Losses (Details) Note 5 - Deposits - Deposits (Details) Note 5 - Deposits - Maturities of Certificates (Details) Business Description and Accounting Policies [Text Block] Office Building [Member] Note 5 - Deposits - Interest on Deposits by Type (Details) Note 6 - Borrowings - Advances from FHLB (Details) Amortization of unrecognized DB plan prior service cost us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit Note 8 - (Loss) Earnings Per Common Share - Components Used to Compute Basic and Diluted Earnings Per Share (Details) Note 9 - Employee Benefits - Shares Issued to the ESOP (Details) Note 10 - Stock-based Compensation - Non-vested Stock Awards (Details) Note 11 - Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) Note 11 - Fair Value Measurement - Quantitative Information (Details) Note 11 - Fair Value Measurement - Changes in Level 3 Assets (Details) Time Deposits [Member] Represents time deposits. Note 11 - Fair Value Measurement - Assets Measured at Fair Value on a Nonrecurring Basis (Details) Accrued interest payable fnwb_InterestPayableFairValueDisclosure The fair value portion of interest payable. Note 11 - Fair Value Measurement - Carrying Values and Estimated Fair Values of Financial Instruments (Details) Note 12 - Change in Accumulated Other Comprehensive Income ("AOCI") - Change in Accumulated Other Comprehensive Income (Details) Counterparty Name [Axis] fnwb_PolicyTable Note 13 - Derivatives and Hedging Activities - Derivatives (Details) us-gaap_EffectiveIncomeTaxRateContinuingOperations Effective Income Tax Rate Reconciliation, Percent Note 13 - Derivatives and Hedging Activities - Derivatives in Statement of Financial Position (Details) Note 13 - Derivatives and Hedging Activities - Derivatives on the Income Statement (Details) us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent fnwb_EmployeeStockOwnershipPlanEsopCompensationExpenseNetOfDividendsReceived Employee Stock Ownership Plan (ESOP), Compensation Expense, Net of Dividends Received The amount of plan compensation cost recognized during the period, net of dividends received. Notes To Financial Statements Change in unrealized (loss) gain on securities available for sale Notes To Financial Statements [Abstract] Additions to servicing rights on sold loans, net The value of new servicing assets, net reductions, subsequently measured at fair value, acquired or created during the current period through purchases or from transfers of financial assets. Closed Portfolio of Loans Receivable [Member] Relating to closed portfolio of loans receivable. Single Family Residence [Member] Represents single family residence. Compensation and benefits Performance Shares [Member] Collateral Dependent Loans [Table Text Block] Tabular disclosure for collateral dependent loans. fnwb_FederalReserveBankAdvancesAvailableAmount Federal Reserve Bank Advances, Available Amount The available amount of advances from the Federal Reserve Bank. us-gaap_DividendsCash Cash dividends declared Prepaid expenses and other assets fnwb_UnearnedESOPSharesCost Unearned employee stock ownership plan ("ESOP") shares Cost of unearned shares that qualify as permanent equity that are held by an employee stock ownership plan (ESOP). Financial Instruments [Domain] Total FHLB long-term advances, weighted-average interest rate Asset Pledged as Collateral [Member] Litigation Case [Axis] Weighted Average [Member] Federal Home Loan Bank, Long-term Advances [Member] Represents Federal Home Loan Bank, Long-term Advances us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet Increase in accrued interest receivable Counterparty Name [Domain] Federal Home Loan Bank, Overnight Variable-rate Advances [Member] Represents Federal Home Loan Bank, Overnight Variable-rate Advances Decrease in accrued interest payable Segment Reporting [Text Block] Securities available for sale, fair value, twelve months or longer Securities available for sale, fair value, less than twelve months Award Type [Axis] Adjustments to reconcile net income to net cash from operating activities: Preferred stock, $0.01 par value; 5,000,000 shares authorized; no shares issued or outstanding Plan Name [Axis] us-gaap_Liabilities Total liabilities Plan Name [Domain] Litigation Status [Axis] Litigation Status [Domain] fnwb_FederalReserveBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged Federal Reserve Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged Amount of the collateral pledged in support of federal reserve bank advances outstanding. us-gaap_AllocatedShareBasedCompensationExpense Share-Based Payment Arrangement, Expense us-gaap_LiabilitiesAndStockholdersEquity Total liabilities and shareholders' equity Pledged Status [Domain] Title and Position [Axis] Segment Reporting [Table] Pledged Status [Axis] us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets Increase in prepaid expenses and other assets us-gaap_ShareBasedCompensation Share-based compensation expense Subsequent Event [Member] Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member] LIABILITIES AND SHAREHOLDERS' EQUITY Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member] Federal Home Loan Bank of San Francisco [Member] Committed to be released shares (in shares) us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationTax Reclassification adjustment for change in fair value of hedged items, tax effect Unallocated shares (in shares) us-gaap_UndistributedEarningsLossAllocatedToParticipatingSecuritiesBasic Dividends and undistributed earnings allocated to participating securities us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationBeforeTax Reclassification adjustment for change in fair value of hedged items, before tax Allocated shares (in shares) us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax Reclassification adjustment for change in fair value of hedged items, after tax Federal Home Loan Bank of Des Moines [Member] Schedule of Available-for-sale and Held-to-Maturity Securities Reconciliation [Table Text Block] Tabular disclosure of available-for-sale and held-to-maturity securities reconciliation Share-Based Payment Arrangement [Text Block] us-gaap_EmployeeStockOwnershipPlanESOPCompensationExpense Allocation of ESOP shares us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP Total ESOP shares issued (in shares) Change in assets and liabilities: Preferred stock, shares issued (in shares) Preferred stock, shares outstanding (in shares) Preferred stock, shares authorized (in shares) Portion at Fair Value Measurement [Member] Reported Value Measurement [Member] Employee Stock Ownership Plan (ESOP) [Table] Preferred stock, par value (in dollars per share) Fair value of unallocated shares us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect Net increase in cash and cash equivalents Cash and cash equivalents us-gaap_CashAndCashEquivalentsFairValueDisclosure Estimate of Fair Value Measurement [Member] Cash dividends declared, per share (in dollars per share) Servicing rights on sold loans Servicing rights on sold loans, at fair value Balance Balance New Accounting Pronouncements, Policy [Policy Text Block] us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period US Government Agencies Debt Securities [Member] Sold Loan Servicing Rights [Member] Represents sold loan servicing rights. Statement [Table] Credit Facility [Axis] us-gaap_TimeDeposits Total certificates of deposit Credit Facility [Domain] Revolving Credit Facility [Member] Accrued expenses and other liabilities us-gaap_DebtInstrumentRepurchasedFaceAmount Debt Instrument, Repurchased Face Amount us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss Securities available for sale, gross unrealized losses, total Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table Text Block] Loans receivable, net us-gaap_LoansReceivableFairValueDisclosure us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 Debt Instrument, Basis Spread on Variable Rate Lease liabilities arising from obtaining right-of-use assets Securities available for sale, fair value, total Changes in fair value due to changes in model inputs or assumptions (1) Amount of change in fair value to contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer. us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity Line of Credit Facility, Maximum Borrowing Capacity Line of Credit Facility, Lender [Domain] Goodwill and other intangible assets, net FHLB stock us-gaap_InvestmentInFederalHomeLoanBankStockFairValueDisclosure Interest rate swaps - securities us-gaap_DerivativeLiabilities Financing Receivable, Allowance for Credit Loss [Table Text Block] us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber Non-vested, shares (in shares) Non-vested, shares (in shares) Lender Name [Axis] us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue Non-vested, weighted-average grant date fair value (in dollars per share) Non-vested, weighted-average grant date fair value (in dollars per share) Multifamily [Member] Gas Station [Member] Represents gas station. us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity Line of Credit Facility, Remaining Borrowing Capacity Granted, shares (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) Financing Receivable, Past Due [Table Text Block] Granted, weighted-average grant date fair value (in dollars per share) Long-Term Debt, Type [Axis] us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod Forfeited, shares (in shares) FHLB Borrowings us-gaap_FederalHomeLoanBankBorrowingsFairValueDisclosure Financing Receivable Credit Quality Indicators [Table Text Block] Capital ABS Master Fund LP Against First Fed [Member] Represents Capital ABS Master Fund LP Against First Fed. Line of Credit us-gaap_LineOfCredit Fed Funds Borrowing Facility [Member] Represents Fed Funds Borrowing Facility. Pacific Coast Bankers Bank [Member] Represents Pacific Coast Bankers Bank. Line of Credit [Member] Vested, weighted-average grant date fair value (in dollars per share) us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount fnwb_DebtInstrumentMinimumAverageDemandDepositAccountBalance Debt Instrument, Minimum Average Demand Deposit Account Balance Minimum average demand deposit account balance required under debt instrument. us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod Vested, shares (in shares) us-gaap_LoansReceivableHeldForSaleAmount Loans held for sale fnwb_LossContingencyAllegedMisappropriationOfFundsAmount Loss Contingency, Alleged Misappropriation of Funds Amount The amount of alleged illegal and unauthorized use of money in the legal matter. The 352 Capital [Member] Represents the 352 Capital. Commercial Real Estate [Member] Asset Class [Axis] Cumulative Effect, Period of Adoption, Adjusted Balance [Member] Cumulative Effect, Period of Adoption [Axis] Cumulative Effect, Period of Adoption [Domain] Interest and fees on loans receivable Total amounts recognized in interest and fees on loans receivable us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized (in shares) us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Available for Grant (in shares) fnwb_FederalHomeLoanBankAdvancesMaximumAvailableCreditToBankAssetsPercentage Federal Home Loan Bank, Advances, Maximum Available Credit to Bank Assets, Percentage The percentage of maximum available credit to bank assets for advances from federal home loan bank. ASSETS Measurement Frequency [Axis] Commitments and Contingencies Disclosure [Text Block] Statement [Line Items] us-gaap_LongTermDebtWeightedAverageInterestRateOverTime Annualized Liability Class [Axis] Fair Value Hierarchy and NAV [Axis] Common stock, shares issued (in shares) Net increase in advances from borrowers for taxes and insurance Common stock, shares outstanding (in shares) Balance (in shares) Balance (in shares) Award Type [Domain] Common stock, shares authorized (in shares) 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] Change in fair value of sold loan servicing rights The amount of fair value adjustment of sold loan servicing rights. NexBank [Member] Related to NexBank. Common stock, $0.01 par value; 75,000,000 shares authorized; 9,508,205 and 9,467,925 shares issued and outstanding, respectively Accrued interest payable Proceeds from BOLI death benefit 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_FederalHomeLoanBankAdvancesActivityForYearAverageBalanceOfAgreementsOutstanding Balance outstanding Early surrender of BOLI policies us-gaap_ComprehensiveIncomeNetOfTax Comprehensive (loss) income us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresAmountOfAvailableUnusedFunds Federal Home Loan Bank, Advances, General Debt Obligations, Amount of Available, Unused Funds Federal Home Loan Bank Advances, Disclosure [Text Block] us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateForYear Annualized us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd Period End After one year through two years, weighted average interest rate Proceeds from maturities, calls, and principal repayments of securities available for sale Net increase in line of credit Lease liability, net Within one year or less, weighted average interest rate Construction Loans [Member] Deposits us-gaap_DepositsFairValueDisclosure Redemption of partnership investment Debt Instrument, Notes Due 2031 [Member] Information on subordinated debt due in 2031. After two years through three years us-gaap_TimeDepositMaturitiesRollingYearThree After three years through four years us-gaap_TimeDepositMaturitiesRollingYearFour us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue Balance Balance Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Within one year or less us-gaap_TimeDepositMaturitiesNextRollingTwelveMonths After one year through two years us-gaap_TimeDepositMaturitiesRollingYearTwo Total investment securities, amortized cost Debt Securities, Available-for-Sale, Amortized Cost Subordinated debt, net us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations Income from BOLI death benefit, net The amount of payout income from bank owned life insurance. Federal Home Loan Bank ("FHLB") stock, at cost After four years through five years us-gaap_TimeDepositMaturitiesRollingYearFive us-gaap_LongTermDebt Balance outstanding Payment Deferral [Member] Due after one through five years, amortized cost Due after five through ten years, amortized cost Other comprehensive (loss) income: Statement of Stockholders' Equity [Abstract] Proceeds from sale of real estate owned Due within one year, amortized cost Loan Restructuring Modification [Axis] Statement of Comprehensive Income [Abstract] Loan Restructuring Modification [Domain] Equity and partnership investments Proceeds from sale of loans held for sale Statement of Cash Flows [Abstract] Financing Receivable Portfolio Segment [Axis] Due after five through ten years, estimated fair value Due after ten years, estimated fair value Due within one year, estimated fair value Due after one through five years, estimated fair value Due after ten years, amortized cost Shareholders' Equity Long-Term Debt, Type [Domain] Equity Components [Axis] Unfunded Loan Commitment [Member] Decrease in lease liabilities Financing Receivable Portfolio Segment [Domain] Home Equity Loan [Member] Equity Component [Domain] Offsetting Liabilities [Table] Fair Value Measurements, Nonrecurring [Table Text Block] Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 3 [Member] AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] AOCI Attributable to Parent [Member] Fair Value, Inputs, Level 1 [Member] Within one year or less Additional Paid-in Capital [Member] After one year through two years Corporate Debt Securities [Member] Retained Earnings [Member] us-gaap_DerivativeLiabilityFairValueOfCollateral Derivative Liability, Fair Value of Collateral Subordinated Debt [Member] us-gaap_SubordinatedDebt Subordinated Debt, Ending Balance Premises and equipment, net Financial Asset, Aging [Axis] Compensation and Employee Benefit Plans [Text Block] Subsequent Event Type [Domain] Financing Receivable, Nonaccrual [Table Text Block] Subsequent Event Type [Axis] Deferred tax asset, net Financial Asset, Aging [Domain] Provision for credit losses on loans Provision for (recapture of) credit losses on loans us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss Debt Securities, Held-to-Maturity, Excluding Accrued Interest, before Allowance for Credit Loss Financial Asset, 30 to 59 Days Past Due [Member] Available-for-Sale, Amortized Cost Share-based compensation expense Automobiles [Member] Derivative basis adjustment Represents the amount of derivative basis adjustment on financing receivables. Nonaccrual loans with allowance Amortized cost of financing receivable on nonaccrual status with allowance for credit loss. Excludes net investment in lease. Financing Receivable, Allowance for Credit Loss Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest Balance Balance Financial Asset, 60 to 89 Days Past Due [Member] Financial Asset, Equal to or Greater than 90 Days Past Due [Member] ESOP shares committed to be released ESOP shares committed to be released Securities available for sale, allowance for credit losses Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest Derivative, fair value Credit Risk Derivatives, at Fair Value, Net Schedule of Cash Flow Hedging Instruments, Statements of Financial Performance and Financial Position, Location [Table Text Block] Originated four years prior Originated years prior Originated two years prior Originated three years prior Fair Value Disclosures [Text Block] us-gaap_GainLossOnDerivativeInstrumentsNetPretax Derivative, gain (loss) Originated current year Fair Value, by Balance Sheet Grouping [Table Text Block] Originated prior year Supplemental disclosures of cash flow information: Revolving us-gaap_DebtInstrumentVariableInterestRateTypeExtensibleEnumeration Debt Instrument, Variable Interest Rate, Type [Extensible Enumeration] us-gaap_LettersOfCreditOutstandingAmount Letters of Credit Outstanding, Amount Secured Overnight Financing Rate (SOFR) [Member] us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments Amortization and accretion of premiums and discounts on investments, net fnwb_IncreaseDecreaseInRightOfUseAsset Amortization of ROU asset Represents the amount of increase (decrease) in right of use assets during the period. Loans held for sale us-gaap_LoansHeldForSaleFairValueDisclosure fnwb_PaymentsForProceedsFromPurchaseSaleOfPropertyPlantAndEquipmentNet Net (purchase) sale of premises and equipment The cash outflow (inflow) from the purchase and sale of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale. Collateral Held [Domain] Collateral Held [Axis] ARC Loans [Member] Relating to ARC Loans. us-gaap_PaymentsToAcquireEquityMethodInvestments Capital contributions to partnership investments us-gaap_GainLossRelatedToLitigationSettlement Legal settlement Other Commitments [Table] us-gaap_PaymentsToAcquireLifeInsurancePolicies Purchase of BOLI policies Accumulated other comprehensive loss, net of tax us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest Loss before benefit from income taxes Data processing us-gaap_GainsLossesOnExtinguishmentOfDebt Gain (Loss) on Extinguishment of Debt Gain on extinguishment of subordinated debt Additional paid-in capital Subsequent Events, Policy [Policy Text Block] us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1 Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1 Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Collateral Pledged [Member] Collateralized Mortgage-Backed Securities [Member] us-gaap_NoninterestExpense Total noninterest expense NONINTEREST EXPENSE us-gaap_LongtermDebtWeightedAverageInterestRate Period End us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures Restricted stock awards canceled Financial Asset, Not Past Due [Member] Restricted stock award grants, net of forfeitures Fair Value, Recurring and Nonrecurring [Table] Credit Loss, Financial Instrument [Text Block] Debt Instrument, Name [Domain] Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] us-gaap_DebtInstrumentInterestRateStatedPercentage Debt Instrument, Interest Rate, Stated Percentage us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited Restricted stock awards canceled (in shares) Restricted stock award grants, net of forfeitures (in shares) us-gaap_DebtInstrumentInterestRateEffectivePercentage Debt Instrument, Interest Rate, Effective Percentage Schedule of Long-Term Debt Instruments [Table] Debt Instrument [Axis] Cash flows from operating activities: Cash flows from investing activities: us-gaap_NetCashProvidedByUsedInOperatingActivities Net cash used by operating activities Cash flows from financing activities: us-gaap_NetCashProvidedByUsedInInvestingActivities Net cash (used) provided by investing activities us-gaap_NetCashProvidedByUsedInFinancingActivities Net cash provided (used) by financing activities Total FHLB long-term advances Advances from borrowers for taxes and insurance us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt Purchase of securities available for sale Total assets measured at fair value us-gaap_AssetsFairValueDisclosure Assets, fair value us-gaap_OtherRealEstateDisposals Sales Market value adjustments us-gaap_OtherRealEstateValuationAdjustments Fair value adjustment on real estate owned Loans, Notes, Trade and Other Receivables Disclosure [Text Block] Redemption of historic tax credit partnerships Represents the cash inflow from historic tax credit partnerships. Loans transferred to other real estate owned Net increase (decrease) in deposits Advertising fnwb_ServicingAssetAtAmortizedCostAdditionsDisposals Servicing rights that result from transfers and sale of financial assets Amount of additions (disposals) to contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer. Accounting Policies [Abstract] Socotra [Member] Represents Socotra. Socotra Complaint [Member] Represents Socotra Complaint. Mortgage-Backed Security, Issued by US Government-Sponsored Enterprise [Member] fnwb_ProceedsReceivedToPayDownLien Proceeds Received to Pay Down Lien Amount of proceeds received to pay down lien. Mortgage-Backed Securities, Issued by Private Enterprises [Member] US States and Political Subdivisions Debt Securities [Member] Loans receivable (net of allowance for credit losses on loans of $16,388 and $16,987, respectively) Total loans receivable, net Measurement Input Type [Axis] us-gaap_PaymentsForProceedsFromLoansAndLeases Decrease in loans receivable, net Income Statement [Abstract] us-gaap_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal Off-Balance-Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal) Investment securities available for sale Investment securities available for sale, at fair value (amortized cost of $348,325 and $295,849, respectively) Measurement Input, Constant Prepayment Rate [Member] us-gaap_NumberOfReportableSegments Number of Reportable Segments us-gaap_AmortizationOfDeferredLoanOriginationFeesNet Accretion of deferred loan fees and purchased premiums, net Measurement Input, Discount Rate [Member] Net (loss) income Net (loss) income Borrowings us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic (Loss) earnings allocated to common shareholders Financial Asset, Past Due [Member] Amortization of debt issuance costs Loss [Member] A category of financing receivables considered to be losses by management. Amortization of core deposit intangible Valuation Technique, Discounted Cash Flow [Member] Real Estate [Table] Valuation Technique, Consensus Pricing Model [Member] Measurement Input Type [Domain] fnwb_PaymentsToAcquireLoans Purchase of loans Represents the amount of payments to acquire loans during the period. us-gaap_InterestIncomeExpenseNet Net interest income Series A equity investment acquired upon conversion of commercial business loan Accumulated Other Comprehensive Income (Loss) [Table] Other Real Estate, Roll Forward [Table Text Block] fnwb_FinancingReceivableIndividuallyEvaluatedForCreditLossStillAccruing Financing Receivable, Individually Evaluated for Credit Loss, Still Accruing The amount of financing receivable individually evaluated for credit loss that is still accruing. Income Tax Disclosure [Text Block] Restricted Stock [Member] us-gaap_OtherRealEstateOwnedMeasurementInput Other real estate owned, measurement input Sold loan servicing rights, measurement input Sold loan servicing rights, measurement input us-gaap_FinancingReceivableNonaccrualInterestIncome Financing Receivable, Nonaccrual, Interest Income Accounting Standards Update and Change in Accounting Principle [Table] Asset-Backed Securities [Member] Capital disbursements received from partnership investments Proceeds from Equity Method Investment, Distribution, Return of Capital Occupancy and equipment Other expense Increase in BOLI cash surrender value Increase in BOLI cash surrender value, net us-gaap_BankOwnedLifeInsuranceIncome Income from BOLI death benefit, net us-gaap_NoninterestIncome Total noninterest income NONINTEREST INCOME Measurement Basis [Axis] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Table] Net gain on sales Gain on sale of real estate owned Other income Fair Value Hierarchy and NAV [Domain] Net gain (loss) on sale of loans Gain on sale of loans, net Measurement Frequency [Domain] Loan principal transferred to real estate owned and repossessed assets, net Statement of Financial Position [Abstract] us-gaap_TimeDepositsAtOrAboveFDICInsuranceLimit Time Deposits, at or Above FDIC Insurance Limit us-gaap_RepaymentsOfSubordinatedDebt Redemption of subordinated debt, net Provision for Credit Losses us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss Unrealized Gains (Losses) Cash paid for income taxes Recoveries Loan receivable, gross, write offs Charge-offs Fair Value, Nonrecurring [Member] Professional fees Fair Value, Recurring [Member] us-gaap_PaymentsForProceedsFromFederalHomeLoanBankStock (Purchase) redemption of FHLB stock Document Type Demand Deposits [Member] Document Information [Line Items] Document Quarterly Report Document Information [Table] us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares) us-gaap_WeightedAverageNumberOfSharesEmployeeStockOwnershipPlanSharesNotCommittedToBeReleased Weighted average unallocated ESOP shares (in shares) Nonvested Restricted Stock Shares Activity [Table Text Block] Entity Interactive Data Current Bank-owned life insurance ("BOLI"), net Document Transition Report us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding Total diluted weighted average common shares outstanding (in shares) Interest-bearing demand deposits, weighted-average interest rate us-gaap_LossContingencyDamagesSoughtValue Loss Contingency, Damages Sought, Value Total deposits, weighted-average interest rate Dilutive restricted stock awards (in shares) Basic weighted average common shares outstanding (in shares) Total basic weighted average common shares outstanding (in shares) us-gaap_WeightedAverageNumberOfSharesRestrictedStock Weighted average unvested restricted stock awards (in shares) Derivative Instrument [Axis] The Equity Incentive Plans [Member] Related to the equity incentive plans. Basic and diluted (loss) earnings per common share (in dollars per share) fnwb_SharebasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCanceledInPeriod Canceled, shares (in shares) The amount of canceled equity instruments other than options during the period in a share based compensation arrangement. Savings accounts, weighted-average interest rate Earnings Per Share, Basic, by Common Class, Including Two-Class Method [Table] Canceled, weighted-average grant date fair value (in dollars per share) The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan canceled during period. Nonaccrual loans without allowance Nonaccrual loans Effective Income Tax Rate Reconciliation [Table] Money market accounts, weighted-average interest rate Title and Position [Domain] Interest-bearing demand deposits Loan principal transferred from held-for-investment to held-for-sale Deposits Deposit Liabilities Disclosures [Text Block] FDIC insurance premium Amount of (income )expense for Federal Deposit Insurance Corporation (FDIC) insurance. Money market accounts Noninterest-bearing demand deposits Provision for (recapture of) credit losses on unfunded commitments The amount of provision for credit losses on unfunded commitments. Document Fiscal Period Focus Pledging Purpose [Axis] Document Fiscal Year Focus Pledging Purpose [Domain] Savings accounts Amendment Flag Federal Home Loan Bank Branch [Domain] Common Stock [Member] Federal Home Loan Bank Branch [Axis] Financing Receivable, Past Due [Table] us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount Financing Receivable, Unamortized Loan Cost (Fee) and Purchase Premium (Discount) Bellevue Washington Branch [Member] Represent the Bellevue Washington Branch. Document Period End Date Certificates of deposit, customer, weighted average interest rate Represents the weighted average domestic deposit percent relating to retail deposits. INTEREST INCOME Legal Entity [Axis] fnwb_InterestExpenseTimeDepositsRetail Certificates of deposit, customer The aggregate interest expense incurred on retail time deposits, including certificates of deposits, in domestic offices. Certificates of deposit, brokered, weighted-average interest rate Represents the weighted average rate of domestic deposits, relating to brokered deposits. us-gaap_InterestAndDividendIncomeOperating Total interest income Consumer Portfolio Segment [Member] Certificates of deposit, customer Represents retail time deposits. Financing Receivable, Allowance for Credit Loss [Table] Commercial Portfolio Segment [Member] Certificates of deposit, brokered Represents brokered time deposits. Loans, individually evaluated us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment Financing Receivable, Individually Evaluated for Credit Loss Entity File Number Entity Tax Identification Number Supplemental disclosures of noncash investing activities: Business Assets [Member] Represents business assets. Condominium [Member] Represents condominium. us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss Securities available for sale, gross unrealized losses, less than twelve months Entity [Domain] fnwb_InterestExpenseTimeDepositsBrokered Certificates of deposit, brokered The aggregate interest expense incurred on brokered time deposits, including certificates of deposits, in domestic offices. us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss Securities available for sale, gross unrealized losses, twelve months or longer Entity Central Index Key Valuation Technique, Underlying Value of Collateral [Member] Valuation technique calculating underlying value of collateral. us-gaap_DebtSecurities Debt Securities Entity Registrant Name Entity Current Reporting Status Special Mention [Member] Substandard [Member] Entity Incorporation, State or Country Code Pass [Member] Current Fiscal Year End Date us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax Unrealized holding (losses) gains on investments available for sale arising during the period, after tax us-gaap_OtherComprehensiveIncomeLossBeforeTax Other comprehensive (loss) income, before tax Entity Filer Category Entity Shell Company Entity Emerging Growth Company Derivative, carrying amount Derivative, Notional Amount Net other comprehensive (loss) income Other comprehensive income (loss), net of tax Entity Small Business Valuation Approach and Technique [Axis] Investments [Domain] Hedging Designation [Domain] Interest rate swaps - securities us-gaap_DerivativeAssets Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] INTEREST EXPENSE us-gaap_InterestExpenseBorrowings Borrowings Canceled dividends payable on forfeited unvested restricted stock awards Canceled dividends payable on forfeited unvested restricted stock awards Interest-earning deposits in banks Valuation Approach and Technique [Domain] Derivative Instruments and Hedging Activities Disclosure [Text Block] Interest on investment securities Total amounts recognized in interest on investment securities us-gaap_InterestExpenseDeposits Total interest expense on deposits Deposits Entity Address, State or Province us-gaap_PaymentsForOriginationAndPurchasesOfLoansHeldForSale Origination of loans held for sale Entity Address, City or Town Retained earnings us-gaap_InterestExpenseOperating Total interest expense Entity Address, Address Line One City Area Code Proceeds from long-term FHLB advances Amount of cash inflow from collection of advance by Federal Home Loan Bank (FHLBank), classified as financing activity. fnwb_PaymentsOfFhlbankBorrowingsFinancingActivitiesLongterm Repayment of long-term FHLB advances Amount of cash outflow for repayment of long-term Federal Home Loan Bank (FHLBank) borrowing, classified as financing activity. fnwb_PaymentsOfFhlbankBorrowingsFinancingActivitiesShortterm Net increase (decrease) in short-term FHLB advances Amount of cash outflow for repayment of short-term Federal Home Loan Bank (FHLBank) borrowing, classified as financing activity. us-gaap_InterestExpenseSavingsDeposits Savings accounts Local Phone Number Entity Address, Postal Zip Code us-gaap_InterestExpenseMoneyMarketDeposits Money market accounts Derivative [Table] Increase (decrease) in accrued expenses and other liabilities us-gaap_InterestExpenseDemandDepositAccounts Demand deposits Sold loan servicing fees and servicing rights mark-to-market Income from mortgage banking activities (fees and commissions) and servicing real estate mortgages, credit cards, and other financial assets held by others net of any related impairment of capitalized service costs and amortization. Also includes any premiums received in lieu of regular servicing fees on such loans only as earned over the life of the loans. Title of 12(b) Security Security Exchange Name Trading Symbol Measurement input Class of Stock [Axis] Total investment securities, estimated fair value Class of Financing Receivable [Axis] us-gaap_OtherComprehensiveIncomeLossTax Other comprehensive (loss) income, tax effect Class of Financing Receivable [Domain] Variable Rate [Domain] fnwb_DepositsFundsHeldByFederallyRecognizedTribes Deposits, Funds Held by Federally Recognized Tribes Represents funds held by federally recognized tribes as deposits. Entity Common Stock, Shares Outstanding Interest Rate Contract [Member] Earnings Per Share [Text Block] Hedging Designation [Axis] fnwb_PaymentsToAcquireLowincomeHousingTaxCreditPartnerships Capital contributions to low-income housing tax credit partnerships The cash outflow from purchase of low-income housing tax credit partnerships. Financing Receivable, Credit Quality Indicator [Table] us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax Unrealized holding (losses) gains on investments available for sale arising during the period, tax effect Director [Member] Loan receivable, gross Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Fee, and Loan in Process us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax Unrealized holding (losses) gains on investments available for sale arising during the period, before tax Internal Credit Assessment [Axis] Weighted average common shares outstanding (in shares) Funds Held by Federally Recognized Tribes [Member] Represents funds held by federally recognized tribes. Net loss us-gaap_ProfitLoss Net (loss) income available to common shareholders us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss Net interest income after provision for (recapture of) credit losses us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements Principal payments and maturities Collateral dependent loans Collateral dependent loans The fair value of collateral dependent loans. Measurement Input, Quoted Price [Member] Purchases us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases Other comprehensive loss before reclassification Investments Classified by Contractual Maturity Date [Table Text Block] Interest on deposits in banks and other Internal Credit Assessment [Domain] Common stock, par value (in dollars per share) Regulatory assessments and state taxes Basis of Accounting, Policy [Policy Text Block] Amounts reclassified from accumulated other comprehensive income Derivative Contract [Domain] Cash and due from banks us-gaap_Assets Total assets Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Designated as Hedging Instrument [Member] Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Amortization of unrecognized defined benefit ("DB") plan prior service cost, before tax us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditTax Amortization of unrecognized defined benefit ("DB") plan prior service cost, tax effect fnwb_OtherComprehensiveIncomeLossFairValueHedgeGainLossAfterReclassificationBeforeTax Recognized on hedged items Amount, before tax and after reclassification, of gain (loss) from derivative instrument designated and qualifying fair value hedge included in assessment of hedge effectiveness. us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPeriod Financing Receivable, Excluding Accrued Interest, Modified in Period, Amount Amortization of unrecognized defined benefit ("DB") plan prior service cost, after tax Diluted (loss) earnings per common share (in dollars per share) Accumulated Gain (Loss), Net, Fair Value Hedge, Parent [Member] Related to accumulated gain (loss), net, fair value hedge, parent. fnwb_FinancingReceivableCreditLossExpenseReversalIncludingUnfundedCommitments Provision for (recapture of) credit losses Amount of credit loss expense (reversal of expense) for financing receivable, including unfunded commitments. Other real estate owned us-gaap_OtherRealEstate Other Real Estate Beginning balance Ending balance Time Deposit Maturities [Table Text Block] Depreciation and amortization of fixed assets Interest rate swap derivative - securities and loans Interest rate swap derivative - securities and loans Schedule of Derivative Instruments [Table Text Block] Asset Class [Domain] Accounts and Financing Receivables [Table] Investment Type [Axis] Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Loan and deposit service fees Consolidation, Policy [Policy Text Block] Fair Value by Liability Class [Domain] Interest rate swap derivative - securities and loans us-gaap_InterestRateDerivativeLiabilitiesAtFairValue Accrued interest receivable Accrued interest receivable Cash paid for interest on deposits and borrowings Interest Rate Swap [Member] Securities Investment [Member] Settled Litigation [Member] Schedule of Long-Term Debt Instruments [Table Text Block] us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest Total shareholders' equity Balance Balance Originated years prior, write offs Revolving, write offs Originated three years prior, write offs Originated four years prior, write offs Originated prior year, write offs Originated two years prior, write offs Extended Maturity and Interest Rate Reduction [Member] Originated current year, write offs FHLB dividends Amount of operating dividend income on federal home loan bank. Employee Stock Ownership Plan (ESOP) Disclosures [Table Text Block] Deposit Liabilities, Type [Table Text Block] us-gaap_StockholdersEquity BALANCE BALANCE Forfeited, weighted-average grant date fair value (in dollars per share) us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLoss Financing Receivable, Accrued Interest, after Allowance for Credit Loss Supplies, postage, and telephone The aggregate amount of expenses directly related to the supplies, postage ans telephone. Unearned ESOP Shares [Member] Represents unearned employee stock ownership plan shares. us-gaap_PaymentsOfDividends Payment of dividends US Small Business Administration Securities [Member] Represents US Small Business Administration (SBA) securities. Corporate Issued Asset-backed Securities [Member] Represents corporate issued asset-backed securities. Real Estate Portfolio Segment [Member] Portfolio segment of the company's total financing receivables related to real estate. us-gaap_PaymentsForLegalSettlements Payments for Legal Settlements One-to-four Family Loan [Member] Represents one-to-four family loan. Multi-family Loan [Member] Represents multi-family loan. Right-of-use ("ROU") asset, net us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLoss Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss Investment Securities, Excluding Mortgage Backed Securities [Member] Represents investment securities other than mortgage backed securities. ecd_Rule10b51ArrTrmntdFlag Rule 10b5-1 Arrangement Terminated [Flag] us-gaap_PaymentsForRepurchaseOfOtherEquity Restricted stock awards canceled ecd_Rule10b51ArrAdoptedFlag Rule 10b5-1 Arrangement Adopted [Flag] Automobile and Other Loan [Member] Loan to finance the purchase of a vehicle and other loans. Watch [Member] A category of financing receivables considered to have potential weaknesses that deserve management's close attention. If left uncorrected, those potential weaknesses may result in a deterioration of the repayment prospects for the asset or of the creditor's position at some future date.