Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 06, 2020
Cover [Abstract]
Entity Registrant Name	First Financial Northwest, Inc.
Document Type	10-Q
Document Period End Date	Mar. 31, 2020
Amendment Flag	false
Entity Central Index Key	0001401564
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		10,177,511
Entity Filer Category	Accelerated Filer
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Entity Emerging Growth Company	false
Entity Small Business	true
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes