Stock-Based Compensation - Nonvested Restricted Stock Shares Activity (Details) - Restricted Stock - First Financial Northwest, Inc. 2008 Equity Incentive Plan	3 Months Ended	9 Months Ended
	Sep. 30, 2023 $ / shares shares	Sep. 30, 2023 $ / shares shares
Shares
Nonvested Beginning Balance (in shares) | shares	27,618	31,272
Granted (in shares) | shares	13,000	40,618
Vested (in shares) | shares	(13,000)	(44,272)
Nonvested Ending Balance (in shares) | shares	27,618	27,618
Weighted-Average Grant Date Fair Value
Nonvested Beginning Balance, Weighted-Average Grant Date Fair Value (in dollars per share) | $ / shares	$ 14.92	$ 16.93
Granted (in dollars per share) | $ / shares	12.69	14.21
Vested (in dollars per share) | $ / shares	12.69	15.68
Nonvested Ending Balance, Weighted-Average Grant Date Fair Value (in dollars per share) | $ / shares	$ 14.92	$ 14.92
Vested and expected to vest assuming a 3% forfeiture rate over the vesting term
Shares
Expected to vest assuming a 3% forfeiture rate over the vesting term (in shares) | shares	26,789	26,789
Weighted-Average Grant Date Fair Value
Nonvested Ending Balance, Weighted-Average Grant Date Fair Value (in dollars per share) | $ / shares	$ 14.92	$ 14.92
Forfeiture rate, percent	3.00%	3.00%