<?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="ft13128-20260806.xsd" xlink:type="simple"/>
    <context id="AsOf2026-08-06">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0002133401</identifier>
        </entity>
        <period>
            <startDate>2026-08-06</startDate>
            <endDate>2026-08-06</endDate>
        </period>
    </context>
    <dei:AmendmentFlag contextRef="AsOf2026-08-06" id="Fact000003">false</dei:AmendmentFlag>
    <dei:EntityCentralIndexKey contextRef="AsOf2026-08-06" id="Fact000004">0002133401</dei:EntityCentralIndexKey>
    <dei:DocumentType contextRef="AsOf2026-08-06" id="Fact000010">S-6</dei:DocumentType>
    <dei:EntityRegistrantName contextRef="AsOf2026-08-06" id="Fact000011">FT 13128</dei:EntityRegistrantName>
    <dei:DocumentPeriodEndDate contextRef="AsOf2026-08-06" id="Fact000012">2026-08-06</dei:DocumentPeriodEndDate>
    <fnd:NmRule35d1TermDfnTextBlock contextRef="AsOf2026-08-06" id="Fact000013">

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

&lt;p style="font: 10pt/11pt Times New Roman, Times, Serif; margin: 1pt 0; text-indent: 0.25in"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;The
Trust seeks above-average total return through a combination of capital appreciation and dividend income. Under normal circumstances,
the Trust will invest at least 80% of its assets in dividend-paying securities.&lt;/span&gt;&lt;/p&gt;

</fnd:NmRule35d1TermDfnTextBlock>
    <fnd:NmRule35d1TermSlctnCritTextBlock contextRef="AsOf2026-08-06" 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;Through
our selection process we seek to find the stocks that we believe have the best prospects for above-average total return.&lt;/span&gt;&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;&lt;i&gt;Identify
the Universe.&lt;/i&gt; We begin with the companies listed in the S&amp;amp;P 1500&lt;/span&gt;&lt;span style="font-size: 8pt"&gt;&lt;sup&gt;&#xae;&lt;/sup&gt;&lt;/span&gt; &lt;span style="letter-spacing: -0.05pt"&gt;Index
and eliminate those companies that do not meet our investment criteria as of the date the portfolio was selected. These criteria are designed
to identify companies with the following qualities:&lt;/span&gt;&lt;/p&gt;

&lt;table cellpadding="0" cellspacing="0" style="font: 10pt/11pt Times New Roman, Times, Serif; margin-top: 0; margin-bottom: 2pt; width: 100%"&gt;
  &lt;tr style="vertical-align: top"&gt;
    &lt;td style="width: 0"&gt;&#160;&lt;/td&gt;
    &lt;td style="width: 10pt"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;&#x2022;&lt;/span&gt;&lt;/td&gt;
    &lt;td&gt;&lt;span style="letter-spacing: -0.05pt"&gt;Well-capitalized with strong balance sheets.&lt;/span&gt;&lt;/td&gt;&lt;/tr&gt;
  &lt;/table&gt;
&lt;table cellpadding="0" cellspacing="0" style="font: 10pt/11pt Times New Roman, Times, Serif; margin-top: 0; margin-bottom: 2pt; width: 100%"&gt;
  &lt;tr style="vertical-align: top"&gt;
    &lt;td style="width: 0"&gt;&#160;&lt;/td&gt;
    &lt;td style="width: 10pt"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;&#x2022;&lt;/span&gt;&lt;/td&gt;
    &lt;td&gt;&lt;span style="letter-spacing: -0.05pt"&gt;Record of financial strength and profit growth, as indicated by strong return on equity and long-term debt to market value of equity measures.&lt;/span&gt;&lt;/td&gt;&lt;/tr&gt;
  &lt;/table&gt;
&lt;table cellpadding="0" cellspacing="0" style="font: 10pt/11pt Times New Roman, Times, Serif; margin-top: 0; margin-bottom: 2pt; width: 100%"&gt;
  &lt;tr style="vertical-align: top"&gt;
    &lt;td style="width: 0"&gt;&#160;&lt;/td&gt;
    &lt;td style="width: 10pt"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;&#x2022;&lt;/span&gt;&lt;/td&gt;
    &lt;td&gt;&lt;span style="letter-spacing: -0.05pt"&gt;A history of dividend payments with the ability to generate dividend growth, as indicated by high dividend yields, five-year dividend growth rates and dividend payout ratios.&lt;/span&gt;&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
investment criteria include:&lt;/span&gt;&lt;/p&gt;

&lt;table cellpadding="0" cellspacing="0" style="font: 10pt/11pt Times New Roman, Times, Serif; margin-top: 0; margin-bottom: 2pt; width: 100%"&gt;
  &lt;tr style="vertical-align: top"&gt;
    &lt;td style="width: 0"&gt;&#160;&lt;/td&gt;
    &lt;td style="width: 10pt"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;&#x2022;&lt;/span&gt;&lt;/td&gt;
    &lt;td&gt;&lt;span style="letter-spacing: -0.05pt"&gt;Market Capitalization greater than $1 billion;&lt;/span&gt;&lt;/td&gt;&lt;/tr&gt;
  &lt;/table&gt;
&lt;table cellpadding="0" cellspacing="0" style="font: 10pt/11pt Times New Roman, Times, Serif; margin-top: 0; margin-bottom: 2pt; width: 100%"&gt;
  &lt;tr style="vertical-align: top"&gt;
    &lt;td style="width: 0"&gt;&#160;&lt;/td&gt;
    &lt;td style="width: 10pt"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;&#x2022;&lt;/span&gt;&lt;/td&gt;
    &lt;td&gt;&lt;span style="letter-spacing: -0.05pt"&gt;Long-Term Debt to Market Value of Equity less than 40%;&lt;/span&gt;&lt;/td&gt;&lt;/tr&gt;
  &lt;/table&gt;
&lt;table cellpadding="0" cellspacing="0" style="font: 10pt/11pt Times New Roman, Times, Serif; margin-top: 0; margin-bottom: 2pt; width: 100%"&gt;
  &lt;tr style="vertical-align: top"&gt;
    &lt;td style="width: 0"&gt;&#160;&lt;/td&gt;
    &lt;td style="width: 10pt"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;&#x2022;&lt;/span&gt;&lt;/td&gt;
    &lt;td&gt;&lt;span style="letter-spacing: -0.05pt"&gt;Return on Equity greater than 10%;&lt;/span&gt;&lt;/td&gt;&lt;/tr&gt;
  &lt;/table&gt;
&lt;table cellpadding="0" cellspacing="0" style="font: 10pt/11pt Times New Roman, Times, Serif; margin-top: 0; margin-bottom: 2pt; width: 100%"&gt;
  &lt;tr style="vertical-align: top"&gt;
    &lt;td style="width: 0"&gt;&#160;&lt;/td&gt;
    &lt;td style="width: 10pt"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;&#x2022;&lt;/span&gt;&lt;/td&gt;
    &lt;td&gt;&lt;span style="letter-spacing: -0.05pt"&gt;Dividend Yield greater than 1.5%;&lt;/span&gt;&lt;/td&gt;&lt;/tr&gt;
  &lt;/table&gt;
&lt;table cellpadding="0" cellspacing="0" style="font: 10pt/11pt Times New Roman, Times, Serif; margin-top: 0; margin-bottom: 2pt; width: 100%"&gt;
  &lt;tr style="vertical-align: top"&gt;
    &lt;td style="width: 0"&gt;&#160;&lt;/td&gt;
    &lt;td style="width: 10pt"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;&#x2022;&lt;/span&gt;&lt;/td&gt;
    &lt;td&gt;&lt;span style="letter-spacing: -0.05pt"&gt;5-Year Dividend Growth Rate greater than 5%; and&lt;/span&gt;&lt;/td&gt;&lt;/tr&gt;
  &lt;/table&gt;
&lt;table cellpadding="0" cellspacing="0" style="font: 10pt/11pt Times New Roman, Times, Serif; margin-top: 0; margin-bottom: 2pt; width: 100%"&gt;
  &lt;tr style="vertical-align: top"&gt;
    &lt;td style="width: 0"&gt;&#160;&lt;/td&gt;
    &lt;td style="width: 10pt"&gt;&lt;span style="letter-spacing: -0.05pt"&gt;&#x2022;&lt;/span&gt;&lt;/td&gt;
    &lt;td&gt;&lt;span style="letter-spacing: -0.05pt"&gt;Dividend Payout Ratio less than 50%.&lt;/span&gt;&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;&lt;i&gt;Examine
Historical Financial Results.&lt;/i&gt; The next step in our process is to look for those companies that have earned a net cash flow return
on investment that is above the average of their peers. &lt;/span&gt;&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;&lt;i&gt;Select
Companies with Attractive Valuations.&lt;/i&gt; The final step in our process is to select companies based on the fundamental analysis of our
team of research analysts. The stocks selected for the portfolio are those that meet our investment objectives, trade at attractive valuations
and, in our opinion, are likely to exceed market expectations of future cash flows.&lt;/span&gt;&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;While
not a part of the Trust&#x2019;s portfolio selection process, the Trust also invests in foreign securities and companies with various market
capitalizations.&lt;/span&gt;&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;As
with any similar investments, there can be no assurance that the objective of the Trust will be achieved. See &#x201c;Risk Factors&#x201d;
for a discussion of the risks of investing in the Trust.&lt;/span&gt;&lt;/p&gt;

</fnd:NmRule35d1TermSlctnCritTextBlock>
</xbrl>
