| Document and Entity Information | Oct. 13, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 13, 2020 |
| Entity Registrant Name | FS BANCORP, INC. |
| Entity Incorporation, State or Country Code | WA |
| Entity File Number | 001-35589 |
| Entity Tax Identification Number | 45-4585178 |
| Entity Address, Address Line One | 6920 220th Street SW |
| Entity Address, Address Line Two | |
| Entity Address, City or Town | Mountlake Terrace |
| Entity Address, State or Province | WA |
| Entity Address, Postal Zip Code | 98043 |
| City Area Code | 425 |
| Local Phone Number | 771-5299 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001530249 |
| Title of 12(b) Security | Common Stock, par value $.01 per share |
| Trading Symbol | FSBW |
| Security Exchange Name | NASDAQ |
