<?xml version="1.0" encoding="utf-8"?>
<xbrl
  xmlns="http://www.xbrl.org/2003/instance"
  xmlns:dei="http://xbrl.sec.gov/dei/2026"
  xmlns:fnd="http://xbrl.sec.gov/fnd/2026"
  xmlns:link="http://www.xbrl.org/2003/linkbase"
  xmlns:xlink="http://www.w3.org/1999/xlink">
    <link:schemaRef xlink:href="ft13126-20260615.xsd" xlink:type="simple"/>
    <context id="AsOf2026-06-15">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0002132916</identifier>
        </entity>
        <period>
            <startDate>2026-06-15</startDate>
            <endDate>2026-06-15</endDate>
        </period>
    </context>
    <dei:AmendmentFlag contextRef="AsOf2026-06-15" id="Fact000003">false</dei:AmendmentFlag>
    <dei:EntityCentralIndexKey contextRef="AsOf2026-06-15" id="Fact000004">0002132916</dei:EntityCentralIndexKey>
    <dei:DocumentType contextRef="AsOf2026-06-15" id="Fact000010">S-6</dei:DocumentType>
    <dei:EntityRegistrantName contextRef="AsOf2026-06-15" id="Fact000011">FT 13126</dei:EntityRegistrantName>
    <dei:DocumentPeriodEndDate contextRef="AsOf2026-06-15" id="Fact000012">2026-06-15</dei:DocumentPeriodEndDate>
    <fnd:NmRule35d1TermDfnTextBlock contextRef="AsOf2026-06-15" id="Fact000013">

&lt;p style="font: bold 11pt/12pt Times New Roman, Times, Serif; margin: 4pt 0 1pt"&gt;Objectives.&lt;/p&gt;

&lt;p style="font: 10pt/11pt Times New Roman, Times, Serif; margin: 0 0 2pt; text-indent: 0.25in"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;The
Trust seeks current income, with total return as a secondary objective.&lt;/span&gt;&lt;/p&gt;

</fnd:NmRule35d1TermDfnTextBlock>
    <fnd:NmRule35d1TermSlctnCritTextBlock contextRef="AsOf2026-06-15" id="Fact000014">

&lt;p style="font: bold 11pt/12pt Times New Roman, Times, Serif; margin: 4pt 0 1pt"&gt;Portfolio Selection Process.&lt;/p&gt;

&lt;p style="font: 10pt/11pt Times New Roman, Times, Serif; margin: 0 0 2pt; text-indent: 0.25in"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;The
Trust is a unit investment trust that invests in a diversified portfolio of closed-end funds (&#x201c;CEFs&#x201d;), exchange-traded funds
(&#x201c;ETFs&#x201d;), dividend-paying common stocks and REITs. The portfolio is approximately weighted based on the following allocation:&lt;/span&gt;&lt;/p&gt;

&lt;table cellpadding="0" cellspacing="0" style="font: 10pt Times New Roman, Times, Serif; border-collapse: collapse; width: 100%"&gt;
  &lt;tr style="vertical-align: bottom; background-color: rgb(204,238,255)"&gt;
    &lt;td style="width: 70%; text-align: left; text-indent: 0.25in"&gt;Multi-Strategy CEFs&lt;/td&gt;&lt;td style="width: 10%"&gt;&#160;&lt;/td&gt;
    &lt;td style="width: 1%; text-align: left"&gt;&#160;&lt;/td&gt;&lt;td style="width: 18%; text-align: right"&gt;25%&lt;/td&gt;&lt;td style="width: 1%; text-align: left"&gt;&#160;&lt;/td&gt;&lt;/tr&gt;
  &lt;tr style="vertical-align: bottom; background-color: White"&gt;
    &lt;td style="text-align: left; text-indent: 0.25in"&gt;Senior Loan ETFs&lt;/td&gt;&lt;td&gt;&#160;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&#160;&lt;/td&gt;&lt;td style="text-align: right"&gt;25%&lt;/td&gt;&lt;td style="text-align: left"&gt;&#160;&lt;/td&gt;&lt;/tr&gt;
  &lt;tr style="vertical-align: bottom; background-color: rgb(204,238,255)"&gt;
    &lt;td style="text-align: left; text-indent: 0.25in"&gt;Dividend Paying Stocks and REITs&lt;/td&gt;&lt;td&gt;&#160;&lt;/td&gt;
    &lt;td style="text-align: left"&gt;&#160;&lt;/td&gt;&lt;td style="text-align: right"&gt;50%&lt;/td&gt;&lt;td style="text-align: left"&gt;&#160;&lt;/td&gt;&lt;/tr&gt;
  &lt;/table&gt;


&lt;p style="font: 10pt/11pt Times New Roman, Times, Serif; margin: 0 0 2pt; text-indent: 0.25in"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;The
multi-strategy segment of the portfolio consists of CEFs which invest in income and preferred stock funds. The ETFs invest in senior loans.
Loans in a company&#x2019;s top two debt tranches (i.e., first or second lien) are considered to be senior loans. The dividend-paying stocks
and REITs are selected by applying a disciplined investment strategy which adheres to pre-determined screens and factors, which require
having a market capitalization greater than $1 billion, a three-month average daily trading volume greater than $1 million and a current
indicated dividend yield greater than twice that of the S&amp;amp;P 500&lt;sup&gt;&#xae;&lt;/sup&gt; Index at the time of selection. The stocks are then
ranked by three equally-weighted factors: price to cash flow; return on assets; and 3-, 6- and 12-month price appreciation. These screens
and factors are designed to identify companies that, in our opinion, have above-average dividend yields and trade at attractive valuations.&lt;/span&gt;&lt;/p&gt;

&lt;p style="font: 10pt/10pt Times New Roman, Times, Serif; margin: 0 0 2pt; text-indent: 0.25in"&gt;The Funds were selected by our research
department based on the following factors: the size and liquidity of the Funds (requiring a minimum market capitalization of $50,000,000),
the current dividend yield of the Funds (prioritizing Funds with the highest dividend yields) and the quality and character of the securities
held by the Funds (mostly focusing on credit quality, maturity and duration, which are balanced to varying degrees based on current economic
conditions). The Sponsor does not require specific credit quality, maturity or duration investment policies when selecting the ETFs for
the portfolio. The Trust&#x2019;s portfolio may include both actively managed ETFs and ETFs that track an index.&lt;/p&gt;

&lt;p style="font: 10pt/10pt Times New Roman, Times, Serif; margin: 0 0 2pt; text-indent: 0.25in"&gt;In connection with the Trust&#x2019;s investments
in ETFs advised by First Trust Advisors L.P., an affiliate of the Trust&#x2019;s Sponsor, First Trust Advisors L.P. will receive advisory
fees from the underlying ETFs which it would not otherwise receive if the Trust invested solely in ETFs advised by unaffiliated third-parties.
This may provide an incentive for the Sponsor to select ETFs advised by First Trust Advisors L.P. over ETFs advised by unaffiliated third-parties.
The Sponsor may invest in an affiliated ETF even in circumstances where an unaffiliated ETF may have lower fees or better performance
over certain time periods. However, the Sponsor selected what it considered to be the best suited ETFs to achieve the Trust&#x2019;s investment
objectives even though there may be other ETFs, including those advised by unaffiliated third-parties, that provide similar results.&lt;/p&gt;

&lt;p style="font: 10pt/10pt Times New Roman, Times, Serif; margin: 0 0 2pt; text-indent: 0.25in"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;While
not a part of the Trust&#x2019;s portfolio selection process, the Trust also has exposure to foreign securities, depositary receipts, emerging
and/or developing market companies and companies with various market capitalizations through the Trust&#x2019;s investment in the Common
Stocks, and to floating-rate securities, high-yield securities, covenant-lite loans, foreign securities, depositary receipts, emerging
and/or developing market companies and companies with various market capitalizations through the Trust&#x2019;s investment in the Funds.
&lt;/span&gt;&lt;/p&gt;

&lt;p style="font: 10pt/10pt Times New Roman, Times, Serif; margin: 0 0 2pt; text-indent: 0.25in"&gt;As with any similar investments, there
can be no assurance that the objectives of the Trust will be achieved. See &#x201c;Risk Factors&#x201d; for a discussion of the risks of
investing in the Trust.&lt;/p&gt;

</fnd:NmRule35d1TermSlctnCritTextBlock>
</xbrl>
