Basis of Presentation and Critical Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2020
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-K for the year ended December 31, 2019. 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 end from June 30 to December 31 effective December 31, 2017. Operating results for the three months ended March 31, 2020, 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"), fair value of financial instruments, and deferred tax assets and liabilities.
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 has included additional information where appropriate.
Recently Adopted Accounting Policies, Regulatory Rule and Issued Accounting Pronouncements	Recently issued accounting pronouncements not yet adopted Credit Losses 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. Additional updates were issued in ASU No. 2019-04, Codification Improvements to Topic 326, Financial Instruments - Credit Losses, Topic 815, Derivatives and Hedging (Topic 825), Financial Instruments. This ASU clarifies and improves guidance related to the previously issued standards on credit losses, hedging and recognition and measurement of financial instruments. The amendments provide entities with various measurement alternatives and policy elections related to accounting for credit losses and accrued interest receivable balances. Entities are also able to elect a practical expedient to separately disclose the total amount of accrued interest included in the amortized cost basis as a single balance to meet certain disclosure requirements. The amendments clarify that the estimated allowance for credit losses should include all expected recoveries of financial assets and trade receivables that were previously written off and expected to be written off. The amendments also allow entities to use projections of future interest rate environments when using a discounted cash flow method to measure expected credit losses on variable-rate financial instruments. In addition, new updates were issued through ASU No. 2019-05, Financial Instruments - Credit Losses (Topic 326): Targeted Transition Relief. This amendment allows entities to elect the fair value option on certain financial instruments. On adoption, an entity is allowed to irrevocably elect the fair value option on an instrument-by-instrument basis. This alternative is available for all instruments in the scope of Subtopic 326-20 except for existing held-to-maturity debt securities. If an entity elects the fair value option, the difference between the instrument’s fair value and carrying amount is recognized as a cumulative-effect adjustment. In November 2019, the FASB issued ASU 2019-10 which defers the effective date for this guidance for smaller reporting companies from the interim and annual periods beginning after December 15, 2020 to the interim and annual periods beginning after December 15, 2022. Early adoption is permitted for interim and annual periods beginning after December 15, 2018. The Company plans to defer adoption of CECL until January 1, 2023. The Company is evaluating the provisions of ASU No. 2016-13, ASU No. 2019-04 and ASU No. 2019-05, and will closely monitor developments and additional guidance to determine the potential impact on the Company’s consolidated financial statements. At this time, we cannot reasonably estimate the impact the implementation of these ASUs will have on the Company's consolidated financial statements. The Company's internal project management team continues to review models, work with our third-party vendor, and discuss changes to processes and procedures to ensure the Company is fully compliant with the amendments at the adoption date. Other Pronouncements In December 2019, FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. ASU 2019-12 simplifies various aspects related to accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The standard also clarifies and amends existing guidance to improve consistent application. This ASU, which is effective for fiscal years beginning after December 15, 2020, is not expected to have a material impact on the Company's financial statements. Early adoption is permitted. In January 2020, the FASB issued ASU No. 2020-01, Investments-Equity Securities (Topic 321), Investments-Equity Method and Joint Ventures (Topic 323), and Derivatives and Hedging (Topic 815)-Clarifying the Interactions between Topic 321, Topic 323, and Topic 815. ASU 2020-01 clarifies the interaction between accounting standards related to equity securities, equity method investments, and certain derivatives including accounting for the transition into and out of the equity method and measuring certain purchased options and forward contracts to acquire investments. The ASU, which is effective for fiscal years beginning after December 15, 2020, is not expected to have a material effect on the Company's financial statements. In March 2020, the FASB issued ASU No. 2020-04 Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting. ASU 2020-04 provides optional guidance for a limited period of time to ease the potential burden in accounting for (or recognizing the effects of) reference rate reform on financial reporting. The amendments are effective for the Company as of March 12, 2020 through December 31, 2022. The Company does not believe this standard will have a material impact on its 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	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 an 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 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: 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 receivable, net - At March 31, 2020, the fair value of loans 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 were valued at a price that represents the Company’s exit price or the price at which these instruments would be sold or transferred. Mortgage servicing rights, net - The estimated fair value of mortgage servicing rights is based on market prices for comparable mortgage servicing contracts when available. If no comparable contract is available, the estimated fair value is based on a valuation model that calculates the present value of estimated future net servicing income.