PARENT COMPANY ONLY FINANCIAL INFORMATION - Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Assets
Cash and due from banks	$ 10,525	$ 12,043
Other assets	17,238	12,195
TOTAL ASSETS	2,632,898	2,286,391
Liabilities and Stockholders' Equity
Subordinated note, net	49,461	49,394
Other liabilities	30,997	25,243
Total liabilities	2,401,201	2,038,884
Total stockholders' equity	231,697	247,507	$ 230,007	$ 200,242
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	2,632,898	2,286,391
FS Bancorp, Inc.
Assets
Cash and due from banks	7,195	19,883
Investment in subsidiary	274,092	277,390
Other assets	704	407
TOTAL ASSETS	281,991	297,680
Liabilities and Stockholders' Equity
Subordinated note, net	49,461	49,394
Other liabilities	833	779
Total liabilities	50,294	50,173
Total stockholders' equity	231,697	247,507
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 281,991	$ 297,680