Basis of Presentation and Critical Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	Basis of presentation - The accompanying unaudited interim consolidated financial statements have been prepared pursuant to the rules and regulations of the U.S. Securities and Exchange Commission ("SEC"). Accordingly, they do not include all of the information and footnotes required by U.S. Generally Accepted Accounting Principles ("GAAP") for complete financial statements. These unaudited interim consolidated financial statements should be read in conjunction with our audited consolidated financial statements and accompanying notes included in the Company's Annual Report on Form 10-KT for the six months ended December 31, 2017. In our opinion, all adjustments (consisting only of normal recurring adjustments) considered necessary for a fair presentation of the consolidated financial statements in accordance with GAAP have been included. The Company changed its fiscal year from June 30 to December 31 effective December 31, 2017. Operating results for the three and six months ended June 30, 2018, are not necessarily indicative of the results that may be expected for future periods. In preparing the unaudited interim consolidated financial statements, we are required to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues, and expenses. Actual results could differ from those estimates. Material estimates that are particularly susceptible to significant change in the near term relate to a determination of the allowance for loan losses ("ALLL"), mortgage servicing rights, fair value of financial instruments, deferred tax assets and liabilities, and the valuation of impaired loans.
Principles of consolidation	Principles of consolidation - The accompanying consolidated financial statements include the accounts of First Northwest Bancorp and its wholly owned subsidiary, First Federal. All material intercompany accounts and transactions have been eliminated in consolidation.
Subsequent Events	Subsequent Events - The Company has evaluated subsequent events for potential recognition and disclosure and determined there are no such events or transactions requiring recognition or disclosure.
Recently adopted and issued accounting pronouncements	Recently adopted accounting pronouncements - In May 2014, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2014-09, Revenue from Contracts with Customers (Topic 606). In August 2015, FASB issued ASU No. 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which postponed the effective date of 2014-09. Subsequently, in March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations. This amendment clarifies that an entity should determine if it is the principal or the agent for each specified good or service promised in a contract with a customer. In April 2016, the FASB issued ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing. The core principle of Topic 606 is that an entity must recognize revenue when it has satisfied a performance obligation of transferring promised goods or services to a customer. In general, the new guidance requires companies to use more judgment and make more estimates than under current guidance, including identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. These standards were effective for interim and annual periods beginning after December 15, 2017. The Company has analyzed its revenue sources of noninterest income to determine when the satisfaction of the performance obligation occurs and the appropriate recognition of revenue. The adoption of these ASUs did not have a material impact on the Company’s consolidated financial statements as the Company did not identify any significant changes in the timing of revenue recognition when considering the amended accounting guidance since it is consistent with the Company’s current accounting policy for contracts. In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Financial Liabilities. ASU 2016-01 requires equity investments (except those accounted for under the equity method of accounting) to be measured at fair value with changes in fair value recognized in net income. In addition, the amendments in this ASU require an entity to disclose the fair value of financial instruments using the exit price notion. Exit price is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The amendments in this ASU were effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The Company has used the exit price notion in the fair value disclosure of financial instruments in Note 9 of this report. The adoption of ASU 2016-01 did not have a material impact on the Company’s consolidated financial statements or disclosures in the Notes to the Consolidated Financial Statements. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments. The ASU provides specific guidance on eight classification issues in order to achieve more consistent reporting. The amendments in this ASU were effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The Company does not currently have items on its cash flow statement that were impacted by adoption of this ASU and therefore adoption of ASU 2016-15 did not have a material impact on the Company’s consolidated financial statements. In March 2017, the FASB issued ASU No. 2017-08, Receivables-Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities. The ASU shortens the amortization period for certain callable debt securities held at a premium using the earliest call date. The amendments do not require an accounting change for securities held at a discount; the discount continues to be amortized to maturity. The amendments in this ASU are effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2019. Early adoption is permitted. The adoption of ASU 2017-08 did not have a material impact on the Company's consolidated financial statements. In May 2017, the FASB issued ASU No. 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting. This ASU provides clarity on the guidance related to stock compensation when there have been changes to the terms or conditions of a share-based payment award to which an entity would be required to apply modification accounting under ASC 718. The ASU provides the three following criteria must be met in order to not account for the effect of the modification of terms or conditions: the fair value, the vesting conditions and the classification as an equity or liability instrument of the modified award is the same as the original award immediately before the original award is modified. The amendments in this ASU were effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017. The Company has not had any modifications on share-based payment awards and therefore the adoption of ASU 2017-09 did not have a material impact on the Company’s consolidated financial statements. In March 2018, FASB issued ASU No. 2018-05, Income Taxes (Topic 740). This ASU was issued to provide guidance on the income tax accounting implications of the Tax Cuts and Jobs Act, and allows for entities to report provisional amounts for specific income tax effects of the Act for which the accounting under Topic 740 was not yet complete but a reasonable estimate could be determined. A measurement period of one-year is allowed to complete the accounting effects under Topic 740 and revise any previous estimates reported. Any provisional amounts or subsequent adjustments included in an entity’s financial statements during the measurement period should be included in income from continuing operations as an adjustment to tax expense in the reporting period the amounts are determined. The Company adopted this ASU with the provisional adjustments as reported in the Consolidated Financial Statements on Form 10-KT as of December 31, 2017. As of June 30, 2018, the Company did not incur any adjustments to the provisional recognition. Recently issued accounting pronouncements - In February 2016, the FASB issued ASU No. 2016-02, Leases. ASU 2016-02 is intended to increase transparency and comparability among organizations by requiring the recognition of lease assets and lease liabilities on the balance sheet and disclosure of key information about leasing arrangements. The principal change required by this ASU relates to lessee accounting, and is that for operating leases, a lessee is required to (1) recognize a right-of-use asset and a lease liability, initially measured at the present value of the lease payments, in the statement of financial position, (2) recognize a single lease cost, calculated so that the cost of the lease is allocated over the lease term on a generally straight-line basis, and (3) classify all cash payments within operating activities in the statement of cash flows. For leases with a term of 12 months or less, a lessee is permitted to make an accounting policy election by class of underlying asset not to recognize lease assets and lease liabilities. If a lessee makes this election, it should recognize lease expense for such leases generally on a straight-line basis over the lease term. ASU 2016-02 also changes disclosure requirements related to leasing activities, and requires certain qualitative disclosures along with specific quantitative disclosures. The amendments in ASU 2016-02 are effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2018. Early application of the amendments in ASU 2016-02 is permitted. The Company is compiling an inventory of all leased assets to determine the impact of ASU 2016-02 on its financial condition and results of operations. Once adopted, we expect to report higher assets and liabilities on our Consolidated Balance Sheets as a result of including right-of-use assets and lease liabilities related to certain banking offices and certain equipment under noncancelable operating lease agreements, which currently are not reflected in our Consolidated Balance Sheets. We do not expect the guidance to have a material impact on the Consolidated Statements of Income or Consolidated Statements of Changes in Shareholders' Equity. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Loss, which updates the guidance on recognition and measurement of credit losses for financial assets. The new requirements, known as the current expected credit loss model (CECL) will require entities to adopt an impairment model based on expected losses rather than incurred losses. ASU No. 2016-13 is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Upon adoption, the Company will change processes and procedures to calculate the allowance for loan losses, including changes in assumptions and estimates to consider expected credit losses over the life of the loan versus the current accounting practice that utilizes the incurred loss model. In addition, the current accounting policy and procedures for other-than-temporary impairment on investment securities available for sale will be replaced with an allowance approach. At this time, we do not anticipate an increase to the ALLL as a result of the implementation of this ASU based on the preliminary review and testing of different models being evaluated. The Company has formed an internal project management team which will coordinate and monitor implementation progress, work with our third-party vendor, and implement changes to processes and procedures to ensure the Company is fully compliant with the amendments at the adoption date. In August 2017, FASB issued ASU No. 2017-12, Derivatives and Hedging (Topic 815). This ASU was issued to provide investors better insight to an entity’s risk management hedging strategies by permitting companies to recognize the economic results of its hedging strategies in its financial statements. The amendments in this ASU permit hedge accounting for hedging relationships involving nonfinancial risk and interest rate risk by removing certain limitations in cash flow and fair value hedging relationships. In addition, the ASU requires an entity to present the earnings effect of the hedging instrument in the same income statement line item in which the earnings effect of the hedged item is reported. This ASU is effective for fiscal years beginning after December 15, 2018, and early adoption is permitted. Adoption of ASU 2017-12 is not expected to have a material impact on the Company’s consolidated financial statements.
Reclassifications	Reclassifications - Certain amounts in the unaudited interim consolidated financial statements for prior periods have been reclassified to conform to the current unaudited financial statement presentation with no effect on net income or shareholders' equity.
Earnings per Share	Earnings per Share Basic earnings per share is computed by dividing income available to common shareholders by the weighted average number of common shares outstanding for the period. Diluted earnings per share reflect the potential dilution that could occur if securities or other contracts to issue common stock were exercised or converted into common stock or resulted in the issuance of common stock that then shared in the earnings of the entity. In addition, nonvested share-based payment awards that contain nonforfeitable rights to dividends or dividend equivalents are considered participating securities and are included in the computation of earnings per share.
Fair Value Accounting and Measurement	Fair value is the price to sell an asset or transfer a liability in an orderly transaction between market participants in the Company’s principal market. The Company has established and documented its process for determining the fair values of its assets and liabilities, where applicable. Fair value is based on quoted market prices, when available, for identical or similar assets or liabilities. In the absence of quoted market prices, management determines the fair value of the Company’s assets and liabilities using valuation models or third-party pricing services, both of which rely on market-based parameters when available, such as interest rate yield curves, option volatilities and credit spreads, or unobservable inputs. Unobservable inputs may be based on management’s judgment, assumptions, and estimates related to credit quality, liquidity, interest rates, and other relevant inputs. Any changes to valuation methodologies are reviewed by management to ensure they are relevant and justified. Valuation methodologies are refined as more market-based data becomes available. A three-level valuation hierarchy is used in determining fair value that is based on the transparency of the inputs used in the valuation process. The inputs used in determining fair value in each of the three levels of the hierarchy are as follows: Level 1 - Quoted prices (unadjusted) in active markets for identical assets or liabilities. Level 2 - Either: (i) quoted prices for similar assets or liabilities; (ii) observable inputs, such as interest rates or yield curves; or (iii) inputs derived principally from or corroborated by observable market data. Level 3 - Unobservable inputs. The hierarchy gives the highest ranking to Level 1 inputs and the lowest ranking to Level 3 inputs. The level in the fair value hierarchy within which the fair value measurement in its entirety falls is determined based on the lowest level input that is significant to the overall fair value measurement. Qualitative disclosures of valuation techniques - Securities available for sale: where quoted prices are available in an active market, securities are classified as Level 1. Level 1 instruments include highly liquid government bonds, securities issued by the U.S. Treasury, and exchange-traded equity securities. If quoted prices are not available, management determines fair value using pricing models, quoted prices of similar securities, which are considered Level 2, or discounted cash flows. In certain cases, where there is limited activity in the market for a particular instrument, assumptions must be made to determine their fair value. Such instruments are classified as Level 3. Assets and liabilities measured at fair value on a recurring basis - Assets and liabilities are considered to be fair valued on a recurring basis if fair value is measured regularly (i.e., daily, weekly, monthly, or quarterly).
Fair Value of Financial Instruments	Financial assets and liabilities other than investment securities are not traded in active markets. Estimated fair values require subjective judgments and are approximate. The estimates of fair value in the previous table are not necessarily representative of amounts that could be realized in actual market transactions, or of the underlying value of the Company. The methods and assumptions used by the Company in estimating fair values of financial instruments as set forth below in accordance with ASC Topic 825, Financial Instruments, as amended by ASU 2016-01 requiring public entities to use the exit price notion effective January 1, 2018, are as follows: Financial instruments with a carrying amount equal to fair value - The fair value of financial instruments that are short-term or reprice frequently and that have little or no risk are considered to have a fair value equal to the carrying amount. These instruments include cash and due from banks, interest bearing deposits with banks, FHLB stock, accrued interest receivable, and accrued interest payable. FHLB stock is not publicly traded, however, it may be redeemed on a dollar-for-dollar basis, for any amount the Bank is not required to hold, subject to the FHLB's discretion. The fair value is therefore equal to the carrying amount. Securities - Fair values for investment securities are primarily measured using information from a third-party pricing service. The pricing service uses pricing models based on market data. In the event that limited or less transparent information is provided by the third-party pricing service, fair value is estimated using secondary pricing services or non-binding third-party broker quotes. Loans held for sale - The fair value of loans held for sale is based on quoted market prices from Federal Home Loan Mortgage Corporation ("Freddie Mac"), which are updated daily and represent prices at which loans are exchanged in high volumes and in a liquid market. Loans receivable, net - At June 30, 2018, the fair value of loans and leases is estimated by discounting the future cash flows using the current rate at which similar loans and leases would be made to borrowers with similar credit and for the same remaining maturities. Additionally, to be consistent with the requirements under FASB ASC Topic 820 for Fair Value Measurements and Disclosures, the loans and leases were valued at a price that represents the Company’s exit price or the price at which these instruments would be sold or transferred. At December 31, 2017, fair values were estimated for portfolios of loans with similar financial characteristics. Loans were segregated by type, including fixed and variable one- to four-family residential real estate, commercial, and consumer loans. There is an accurate and reliable secondary market for one- to four-family residential mortgage production, and available market benchmarks are used to establish discount factors for estimating fair value for these types of loans. Commercial and consumer loans use market benchmarks when available; however, due to the varied term structures and credit issues involved, they mainly rely on cash flow projections and repricing characteristics within the loan portfolio. These amounts are discounted further by embedded probable losses expected to be realized in the portfolio. Mortgage servicing rights, net - The estimated fair value of mortgage servicing rights is based on market prices for comparable mortgage servicing contracts, when available, or alternatively, is based on a valuation model that calculates the present value of estimated future net servicing income. Deposits - The fair value of deposits with no stated maturity, such as non-interest bearing deposits, savings and interest checking accounts, and money market accounts, is equal to the amount payable on demand as of June 30, 2018 and December 31, 2017. The fair value of certificates of deposit is based on the discounted value of contractual cash flows. The discount rate is estimated using the rates currently offered for deposits of similar remaining maturities. Borrowings - The fair value of FHLB advances and other borrowings are calculated using a discounted cash flow method, adjusted for market interest rates and terms to maturity. Off-balance-sheet financial instruments - Commitments to extend credit represent all off-balance-sheet financial instruments. The fair value of these commitments is not significant.