Stock-Based Compensation - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 270,000	$ 327,000	$ 497,000	$ 641,000
Tax benefit from compensation expense	$ 57,000	$ 69,000	$ 104,000	$ 135,000
Grants in period	40,000		40,000
Cost not yet recognized, period of recognition			3 years 9 months 18 days
First Financial Northwest Inc 2016 Equity Incentive Plan | Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized					1,400,000
Available for grant (shares)	848,598		848,598
Expiration period			10 years
Shares issued (in shares)			40,000
First Financial Northwest Inc 2016 Equity Incentive Plan | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized					400,000
Available for grant (shares)	144,299		144,299
First Financial Northwest, Inc. 2008 Equity Incentive Plan | Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			5 years
Percentage of options vesting per year			20.00%
Expiration period			10 years
Vested and available for exercise (in shares)	122,500		122,500
Compensation cost not yet recognized	$ 111,140		$ 111,140
First Financial Northwest, Inc. 2008 Equity Incentive Plan | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost not yet recognized	$ 284,000		$ 284,000
Compensation cost not yet recognized, weighted average vesting period			5 months