<?xml version="1.0" encoding="utf-8"?>
<xbrl
  xmlns="http://www.xbrl.org/2003/instance"
  xmlns:dei="http://xbrl.sec.gov/dei/2026"
  xmlns:ecd="http://xbrl.sec.gov/ecd/2026"
  xmlns:link="http://www.xbrl.org/2003/linkbase"
  xmlns:xlink="http://www.w3.org/1999/xlink">
    <link:schemaRef xlink:href="wsbk-20260928.xsd" xlink:type="simple"/>
    <context id="C_c091c013-9a9c-4667-8827-1308ad42da60">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0002047235</identifier>
        </entity>
        <period>
            <startDate>2025-07-01</startDate>
            <endDate>2026-06-30</endDate>
        </period>
    </context>
    <dei:AmendmentFlag
      contextRef="C_c091c013-9a9c-4667-8827-1308ad42da60"
      id="F_52e74985-c9cd-49c1-a15e-544a7620e3d6">false</dei:AmendmentFlag>
    <dei:EntityCentralIndexKey
      contextRef="C_c091c013-9a9c-4667-8827-1308ad42da60"
      id="F_3d8e7fe5-1fe1-4e92-8824-214d6e9a9c5c">0002047235</dei:EntityCentralIndexKey>
    <dei:DocumentType
      contextRef="C_c091c013-9a9c-4667-8827-1308ad42da60"
      id="F_e0254468-b39b-4de9-b2af-3d1651d1deaa">DEF 14A</dei:DocumentType>
    <dei:EntityRegistrantName
      contextRef="C_c091c013-9a9c-4667-8827-1308ad42da60"
      id="F_04a2d562-1f1e-4f84-9ca2-4416b475ab65">Winchester Bancorp, Inc.</dei:EntityRegistrantName>
    <ecd:InsiderTrdPoliciesProcAdoptedFlag
      contextRef="C_c091c013-9a9c-4667-8827-1308ad42da60"
      id="F_10b4f43d-fac0-437d-be9a-da7ac9fc450f">true</ecd:InsiderTrdPoliciesProcAdoptedFlag>
</xbrl>
