Stock-Based Compensation Disclosure of Share-based Compensation Arrangements by Share-based Payment Award (Details)	3 Months Ended	6 Months Ended
Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding Beginning Balance, Shares \| shares	365,000	315,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures \| shares		50,000
Outstanding Ending Balance, Shares \| shares	365,000	365,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Outstanding Beginning Balance, Weighted Average Exercise Price	$ 11.09	$ 10.34
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price		15.80
Outstanding Ending Balance, Weighted Average Exercise Price	$ 11.09	$ 11.09
Share-based Compensations Arrangement by Share-based Payment Award, Options Outstanding, Weighted Average Remaining Contractual Term [Roll Forward]
Outstanding Beginning Balance, Weighted Average Remaining Contractual Term	5 years 2 months 5 days	5 years 2 months 5 days
Outstanding Ending Balance, Weighted Average Remaining Contractual Term	5 years 2 months 5 days	5 years 2 months 5 days
Share-based Compensation Arrangement by Share-based Payment Award, Options Outstanding, Aggregate Intrinsic [Roll Forward]
Outstanding Beginning Balance, Aggregate Intrinsic Value \| $	$ 1,796,200	$ 1,615,600
Outstanding Ending Balance, Aggregate Intrinsic Value \| $	$ 1,199,800	$ 1,199,800
Share Based Compensation, Stock Option Plan, Additional Disclosures [Abstract]
Expected to Vest, Weighted Average Exercise Price	$ 3.61	$ 3.61
Exercisable at end of period, Shares \| shares	280,000	280,000
Exercisable at end of period, Weighted Average Exercise Price	10.16	10.16
Exercisable at end of period, Weighted Average Remaining Contractual Term in Years	4 years 4 months 4 days	4 years 4 months 4 days
Exercisable at end of period, Aggregate Intrinsic Value \| $	$ 1,118,070	$ 1,118,070
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	$ 3.88	$ 3.69
Granted, Weighted Average Grant Date Fair Value	5.03
Nonvested Ending Balance, Weighted-Average Grant Date Fair Value	$ 3.88	$ 3.88
First Financial Northwest, Inc. 2008 Equity Incentive Plan \| Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested Beginning Balance \| shares	16,698	20,987
Granted, Shares \| shares		16,698
Vested, Shares \| shares		(20,987)
Nonvested Ending Balance \| shares	16,698	16,698
Expected to vest assuming a 3% forfeiture rate over the vesting term, Shares \| shares	16,197	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	$ 16.53	$ 15.90
Granted, Weighted Average Grant Date Fair Value		16.53
Vested, Weighted Average Grant Date Fair Value		15.90
Nonvested Ending Balance, Weighted-Average Grant Date Fair Value	$ 16.53	$ 16.53
Expected to vest assuming a 3% forfeiture rate over the vesting term
Share Based Compensation, Stock Option Plan, Additional Disclosures [Abstract]
Expected to Vest, Shares \| shares	362,450	362,450
Expected to Vest, Weighted Average Exercise Price	$ 11.07	$ 11.07
Expected to Vest, Weighted Average Remaining Contractual Term in Years	5 years 2 months	5 years 2 months
Expected to Vest, Aggregate Intrinsic Value \| $	$ 1,197,348	$ 1,197,348
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Vested, Weighted Average Grant Date Fair Value	$ 3.87
Expected to vest assuming a 3% forfeiture rate over the vesting term \| First Financial Northwest, Inc. 2008 Equity Incentive Plan \| Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested Ending Balance \| shares	0	0
Expected to vest assuming a 3% forfeiture rate over the vesting term, Shares \| shares	16,197	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 Ending Balance, Weighted-Average Grant Date Fair Value	$ 16.53	$ 16.53

Number of options shares expected to vest over the vesting term.

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Share Based Compensation, Stock Option Plan, Additional Disclosures [Abstract]

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Share-based Compensations Arrangement by Share-based Payment Award, Options Outstanding, Weighted Average Remaining Contractual Term [Roll Forward]

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Share-based Compensation Arrangement by Share-based Payment Award, Options Outstanding, Aggregate Intrinsic [Roll Forward] [Roll Forward]

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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Net number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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