Business Combination - Amount Recognized for Assets Acquired and Liabilities Assumed (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Nov. 15, 2018	Dec. 31, 2018	Dec. 31, 2019
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Bargain purchase gain		$ (7,414)
Anchor Bancorp
Business Acquisition [Line Items]
Fair value of FS Bancorp common stock at $46.54 (1) per share for 725,518 shares	$ 33,766
Cash paid	30,805
Total purchase price	$ 64,571
Share price	$ 46.54
Common stock outstanding	725,518
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract]
Cash and cash equivalents	$ 54,558
Securities available-for-sale	19,555
Loans receivable, net	356,275
Premises and equipment	11,765
OREO	689
Deferred tax asset	739
Mortgage servicing rights	782
Intangible assets - CDI	5,251
Other assets	25,249
Total assets and identifiable intangible assets acquired	474,863
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract]
Deposits	356,811	$ 321,100	$ 299,000
Borrowings	36,718
Other liabilities	9,349
Total liabilities assumed	402,878
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Total fair value of identifiable net assets	71,985
Bargain purchase gain	$ (7,414)
Number of shares received	0.291
Consideration received by anchor shareholder in cash for each share	$ 12.40