| Document and Entity Information | Dec. 17, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Dec. 17, 2020 |
| Entity Registrant Name | Waterstone Financial, Inc. |
| Entity Incorporation, State or Country Code | MD |
| Entity File Number | 001-36271 |
| Entity Tax Identification Number | 90-1026709 |
| Entity Address, Address Line One | 11200 West Plank Ct |
| Entity Address, City or Town | Wauwatosa |
| Entity Address, State or Province | WI |
| Entity Address, Postal Zip Code | 53226 |
| City Area Code | 414 |
| Local Phone Number | 761-1000 |
| 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 | 0001569994 |
| Title of 12(b) Security | Common Stock, $0.01 Par Value |
| Trading Symbol | WSBF |
| Security Exchange Name | NASDAQ |
