Benefit Plans Fair Value Methodology (Details)	12 Months Ended
Dec. 31, 2019
First Financial Northwest Inc 2008 Equity Incentive Plan [Member] \| Share-based Payment Arrangement, Option [Member]
Fair Value, Option, Changes in Instrument Specific Credit Risk, Gains (Losses) on Liabilities, Methodology	Black-Scholes model

For liabilities with fair values that have been significantly affected during the reporting period by changes in the instrument-specific credit risk, this item represents disclosure of how the gains or losses attributable to changes in instrument-specific credit risk were determined.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (d)(3)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612

Name:	us-gaap_FairValueOptionChangesInInstrumentSpecificCreditRiskGainsLossesOnLiabilitiesMethodology
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration