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Benefit Plans - Stock Based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2017	Nov. 16, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instruments other than options, granted (in shares)		65,000
Stock options granted in the period (in shares)	0	0
First Northwest Bancorp 2015 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized				1,834,050
Share based compensation expense	$ 1,100	$ 1,100
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock repurchased and retired (in shares)	477,837	623,813
Common Stock \| First Northwest Bancorp 2015 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized				1,310,036
Stock repurchased and retired (in shares)			523,014
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instruments other than options, granted (in shares)	64,900
Award vesting period	5 years
Compensation cost not yet recognized	$ 3,400
Compensation cost not yet recognized, recognition period	3 years 2 months 4 days
Restricted Stock \| First Northwest Bancorp 2015 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized				524,014
Director \| First Northwest Bancorp 2015 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense	$ 342	$ 343