Note 10 - Benefit Plans (Details Textual)	1 Months Ended	12 Months Ended
	Mar. 22, 2021 USD ($)	Dec. 31, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) shares	Jun. 30, 2017 shares	Dec. 15, 2015 shares
Multiemployer Plans, Requisite Service Period (Year)		1 year
Multiemployer Plans, Benefit Vesting Period (Year)		5 years
Multiemployer Plans, Percentage Of Employer's Contributions		5.00%
Defined Benefit Plan, Plan Assets, Amount, Ending Balance		$ 15,821,000
Multiemployer Plan, Employer Contribution, Cost, Total			$ 364,000
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Period (Year)		15 years
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent		100.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Match		50.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay		6.00%
Defined Contribution Plan, Cost		$ 569,000	$ 380,000
Employee Stock Ownership Plan (ESOP), Minimum Service Period, Hours		1,000
Employee Stock Ownership Plan (ESOP), Requisite Service Period (Month)		12 months
Employee Stock Ownership Plan (ESOP), Shares to be Purchased, Percentage		8.00%
Employee Stock Ownership Plan (ESOP), Shares in ESOP, Total (in shares) | shares		1,048,029	1,048,029		1,048,029
Employee Stock Ownership Plan (ESOP), Number of Shares Purchased, Percentage		100.00%			100.00%
Employee Stock Ownership Plan (ESOP), Weighted Average Purchase Price of Shares Purchased (in dollars per share) | $ / shares		$ 12.45
Employee Stock Ownership Plan (ESOP), Debt Structure, Amortization Period (Year)		20 years
Employee Stock Ownership Plan (ESOP), Debt Structure, Estimated Interest Rate		2.46%
Employee Stock Ownership Plan (ESOP), Principal and Interest Payments from ESOP		$ 835,000	$ 835,000
Employee Stock Ownership Plan (ESOP), Compensation Expense, Net of Dividends Received		$ 675,000	$ 475,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number, Ending Balance (in shares) | shares		236,432	292,892
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | shares		102,033
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total		$ 3,000,000.0
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)		2 years 3 months 3 days
First Northwest Bancorp 2015 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares) | shares		0	0
First Northwest Bancorp 2015 Equity Incentive Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number, Ending Balance (in shares) | shares		103,520
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | shares		102,033	161,224
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)		5 years
First Northwest Bancorp 2020 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares) | shares		520,000
First Northwest Bancorp 2020 Equity Incentive Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares) | shares		344,537
First Northwest Bancorp 2015 and 2020 Equity Incentive Plan [Member]
Share-based Payment Arrangement, Expense		$ 1,800,000	$ 1,300,000
Board of Directors and Officers [Member]
Deferred Compensation Arrangement with Individual, Aggregate Balance Held in Trust		1,300,000
Director [Member] | First Northwest Bancorp 2015 and 2020 Equity Incentive Plan [Member]
Share-based Payment Arrangement, Expense		368,000	$ 358,000
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax		$ 1,600,000
Common Stock [Member]
Stock Repurchased and Retired During Period, Shares (in shares) | shares		349,497	575,859
Common Stock [Member] | First Northwest Bancorp 2015 Equity Incentive Plan [Member]
Stock Repurchased and Retired During Period, Shares (in shares) | shares				523,014
Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Amount, Ending Balance		$ 15,821,000	$ 14,705,000
Defined Benefit Plan, Benefit Obligation, Ending Balance		$ 15,328,000	$ 14,197,000
Pension Plan [Member] | First Federal Defined Benefit Plan [Member]
Defined Benefit Plan, Plan Assets, Amount, Ending Balance	$ 14,705,000
Defined Benefit Plan, Benefit Obligation, Ending Balance	14,197,000
Pentegra Defined Benefit Plan for Financial Institutions [Member]
Multiemployer Plan, Employer Contribution, Cost, Total	$ 2,717,599