Employee Benefits - Narrative (Details) - USD ($)			12 Months Ended
	Jul. 09, 2012	Jan. 02, 2012	Dec. 31, 2021	Dec. 31, 2020
Employee Benefits
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		0	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
Employee stock ownership plan (ESOP), interest payments from esop			7,000
ESOP compensation expense for allocated shares			$ 1,773,000	$ 1,307,000