v3.25.4
Document and Entity Information - shares
 6 Months Ended
Dec. 31, 2025
Feb. 02, 2026
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Amendment Flag false  
Document Period End Date Dec. 31, 2025  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q2  
Current Fiscal Year End Date --06-30  
Document Transition Report false  
Entity File Number 001-42627  
Entity Registrant Name WINCHESTER BANCORP, INC.  
Entity Central Index Key 0002047235  
Entity Tax Identification Number 33-3361275  
Entity Incorporation, State or Country Code MD  
Entity Address, Address Line One 661 Main Street  
Entity Address, City or Town Winchester  
Entity Address, State or Province MA  
Entity Address, Postal Zip Code 01890  
City Area Code 781  
Local Phone Number 729-2130  
Title of 12(b) Security Common Stock, par value $0.01 per share  
Trading Symbol WSBK  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   9,295,376