Parent Company Only Financial Information - Condensed Statements of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Condensed Income Statements, Captions [Line Items]
Interest from subsidiary	$ 84,706	$ 58,616
Interest expense on subordinated note	(679)	(679)
Dividends received from subsidiary	4,919	3,710
Income tax benefit	5,413	4,223
NET INCOME	22,717	24,347
FS Bancorp, Inc.
Condensed Income Statements, Captions [Line Items]
Interest from subsidiary	2	2
Interest expense on subordinated note	(679)	(679)
Dividends received from subsidiary	3,935	1,436
Other expenses	(142)	(157)
Income before income tax benefit and equity in undistributed net income of subsidiary	3,116	602
Income tax benefit	172	175
Equity in undistributed earnings of subsidiary	19,429	23,570
NET INCOME	$ 22,717	$ 24,347