<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.24.4</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>416</ContextCount>
  <ElementCount>26</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>47</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report instance="8dd3b4347d1b15b.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>030000 - Disclosure - Form N-CSR Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/oef/role/NcsrCover</Role>
      <ShortName>Form N-CSR Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="8dd3b4347d1b15b.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>030001 - Disclosure - Shareholder Report</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/oef/role/ShareholderReport</Role>
      <ShortName>Shareholder Report</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="8dd3b4347d1b15b.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>040020 - Disclosure - Shareholder Report, Holdings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/oef/role/ShareholderReportHoldingsDetails</Role>
      <ShortName>Shareholder Report, Holdings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="N-CSRS" isDefinitelyNotFs="true" isNcsr="true" isOEF="true" isRRorOEF="true" isUsgaap="true" original="8dd3b4347d1b15b.htm">8dd3b4347d1b15b.htm</File>
    <File>jhbt-20250110.xsd</File>
  </InputFiles>
  <SupplementalFiles>
    <File>chartimages_5593678.jpg</File>
    <File>chartimages_5593729.jpg</File>
    <File>chartimages_5593756.jpg</File>
    <File>chartimages_5593771.jpg</File>
    <File>chartimages_5593798.jpg</File>
    <File>chartimages_5593849.jpg</File>
    <File>chartimages_5593876.jpg</File>
    <File>chartimages_5593891.jpg</File>
    <File>chartimages_5593954.jpg</File>
    <File>chartimages_5593969.jpg</File>
    <File>chartimages_5593996.jpg</File>
    <File>chartimages_5594011.jpg</File>
    <File>chartimages_5594038.jpg</File>
    <File>chartimages_5594053.jpg</File>
    <File>chartimages_5594116.jpg</File>
    <File>chartimages_5594131.jpg</File>
    <File>chartimages_5594158.jpg</File>
    <File>chartimages_5594173.jpg</File>
    <File>chartimages_5594200.jpg</File>
    <File>chartimages_5594251.jpg</File>
    <File>chartimages_5594278.jpg</File>
    <File>chartimages_5594293.jpg</File>
    <File>chartimages_5594320.jpg</File>
    <File>images_4800.jpg</File>
    <File>images_4801.jpg</File>
    <File>images_4802.jpg</File>
    <File>images_4854.jpg</File>
    <File>img00fe12c11.gif</File>
    <File>img073eb6801.gif</File>
    <File>img152a6d1d3.gif</File>
    <File>img6fc69db42.gif</File>
    <File>img74bab9d32.gif</File>
    <File>img7634eb463.gif</File>
    <File>img8f10335e3.gif</File>
    <File>img92bf92bf3.gif</File>
    <File>img9b83a05b2.gif</File>
    <File>imga0ca8a433.gif</File>
    <File>imgbed949821.gif</File>
    <File>imgcdc710f31.gif</File>
    <File>imgcf8fb36f1.gif</File>
    <File>imge6244aca2.gif</File>
    <File>imgf62828a42.gif</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="46">http://fasb.org/us-gaap/2023</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2023</BaseTaxonomy>
    <BaseTaxonomy items="714">http://xbrl.sec.gov/oef/2023</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>false</HasPresentationLinkbase>
  <HasCalculationLinkbase>false</HasCalculationLinkbase>
</FilingSummary>
