Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Mar. 09, 2018	Jun. 30, 2017
Document And Entity Information
Entity Registrant Name	FS Bancorp, Inc.
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Entity Central Index Key	0001530249
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		3,695,552
Entity Filer Category	Accelerated Filer
Entity Voluntary Filers	No
Entity Public Float			$ 121,778,776
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY