XML 12 R1.htm IDEA: XBRL DOCUMENT v3.25.3
Document And Entity Information - shares
 9 Months Ended
Sep. 30, 2025
Oct. 31, 2025
Document Information [Line Items]    
Entity Central Index Key 0001569994  
Entity Registrant Name Waterstone Financial, Inc.  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Document Fiscal Period Focus Q3  
Document Fiscal Year Focus 2025  
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2025  
Document Transition Report false  
Entity File Number 001-36271  
Entity Incorporation, State or Country Code MD  
Entity Tax Identification Number 90-1026709  
Entity Address, Address Line One 11200 W. Plank Court  
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  
Title of 12(b) Security Common Stock, $0.01 Par Value  
Trading Symbol WSBF  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   18,456,918