EMPLOYEE BENEFITS - Narrative (Details) - USD ($)		12 Months Ended
	Jan. 01, 2012	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2012
Schedule of Defined Contribution Plans Disclosures [Line Items]
Defined contribution plan, minimum age requirement		18 years
Percentage vested in deferral contributions account		100.00%
Employer contributions		$ 1,900,000	$ 1,700,000	$ 1,500,000
Employee Stock Ownership Plan (ESOP), Interest Payments from ESOP			7,000
Number of hours of service required for participation in ESOP, per first 12 month period (in hours)	1000 hours
Vesting percentage after requisite service period is met (as a percent)					100.00%
Requisite service period (in years)	2 years
Number of hours per service period year (in hours)	1000 hours
ESOP borrowings from FS Bancorp	$ 2,600,000
Shares acquired (in shares)	518,420			479,330
Employee stock ownership plan (ESOP), weighted average purchase price of shares purchased (in dollars per share)	$ 5.09
Amortization period of ESOP loan	10 years
Employee Stock Ownership Plan (ESOP), Debt Structure, Employer Loan, Interest Rate	2.30%
Employee stock ownership plan (ESOP), periodic installment payments from esop, amount paid			288,000
ESOP compensation expense for allocated shares			$ 1,773,000	$ 1,307,000
Contributions between 1% to 3%
Schedule of Defined Contribution Plans Disclosures [Line Items]
Employer matching percentage		100.00%
Contributions between 1% to 3% | Minimum
Schedule of Defined Contribution Plans Disclosures [Line Items]
Defined contribution plan, employee contribution (percent)		1.00%
Contributions between 1% to 3% | Maximum
Schedule of Defined Contribution Plans Disclosures [Line Items]
Defined contribution plan, employee contribution (percent)		3.00%
Contributions between 4% and 5%
Schedule of Defined Contribution Plans Disclosures [Line Items]
Employer matching percentage		50.00%
Contributions between 4% and 5% | Minimum
Schedule of Defined Contribution Plans Disclosures [Line Items]
Defined contribution plan, employee contribution (percent)		4.00%
Contributions between 4% and 5% | Maximum
Schedule of Defined Contribution Plans Disclosures [Line Items]
Defined contribution plan, employee contribution (percent)		5.00%