FIRST FINANCIAL NORTHWEST, INC. AND SUBSIDIARIES CONSOLIDATED STATEMETNS OF STOCKHOLDERS' EQUITY (PARENTHETICALS) - $ / shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Statement of Stockholders' Equity [Abstract]
Cash dividend declared and paid per share (usd per share)	$ 0.0035	$ 0.0031
Allocated shares	112,853	112,853

Number of shares of capital stock issued (purchased by employees) in connection with an employee stock ownership plan.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration