Benefit Plans Disclosure of Share-based Compensation Arrangements by Share-based Payment Award (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Exercised	(2,000)	(137,940)
First Financial Northwest, Inc. 2008 Equity Incentive Plan | Stock Options
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount | $	$ 32,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning balance	315,000
Granted	50,000	0
Exercised	(2,000)
Outstanding at ending balance	313,000	315,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Outstanding beginning balance, Weighted Average Exercise Price | $ / shares	$ 10.34
Granted | $ / shares	15.80
Exercised | $ / shares	10.77
Outstanding ending balance, Weighted Average Exercise Price | $ / shares	$ 10.34	$ 10.34
Share-based Compensations Arrangement by Share-based Payment Award, Options Outstanding, Weighted Average Remaining Contractual Term [Roll Forward]
Outstanding ending balance, Weighted Average Remaining Contractual Term	3 years 11 months 24 days
Share Based Compensation, Stock Option Plan, Additional Disclosures [Abstract]
Outstanding beginning balance, Agregate Intrinsic Value | $	$ 1,615,600
Exercised | $	7,240
Outstanding ending balance, Agregate Intrinsic Value | $	$ 1,440,310	$ 1,615,600
Expected to Vest, Shares	312,760
Expected to Vest, Weighted Average Exercise Price | $ / shares	$ 10.34
Expected to Vest, Weighted Average Remaining Contracutal Term in Years	3 years 11 months 24 days
Expected to Vest, Aggregate Intrinsic Value | $	$ 1,439,854
Exercisable ending balance, Shares	305,000
Exercisable ending balance, Weighted Average Exercise Price	10.27
Exercisable ending balance, Weighted Average Remaining Contracutal Term in Years	3 years 11 months 5 days
Exercisable ending balance, Aggregate Intrinsic Value | $	$ 1,425,110
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period	50,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price | $ / shares	$ 15.80
First Financial Northwest, Inc. 2008 Equity Incentive Plan | Restricted Stock
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount | $	$ 46,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested beginning balance, Shares	20,987
Granted, Shares	25,278
Vested, Shares	(29,567)
Nonvested ending balance, Shares	16,698	20,987
Expected to vest assuming a 3% forfeiture rate over the vesting term, Shares	16,197
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Nonvested beginning balance, Weighted-Average Grant Date Fair Value | $ / shares	$ 15.90
Granted, Weighted-Average Grant Date Fair Value | $ / shares	15.88
Vested, Weighted-Average Grant Date Fair Value | $ / shares	15.53
Nonvested ending balance, Weighted-Average Grant Date Fair Value | $ / shares	$ 16.53	$ 15.90