<?xml version="1.0" encoding="utf-8"?>
<xbrl
  xmlns="http://www.xbrl.org/2003/instance"
  xmlns:dei="http://xbrl.sec.gov/dei/2026"
  xmlns:link="http://www.xbrl.org/2003/linkbase"
  xmlns:xlink="http://www.w3.org/1999/xlink">
    <link:schemaRef xlink:href="sndx-20260804.xsd" xlink:type="simple"/>
    <context id="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001395937</identifier>
        </entity>
        <period>
            <startDate>2026-08-04</startDate>
            <endDate>2026-08-04</endDate>
        </period>
    </context>
    <dei:EntityCentralIndexKey
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_f2248a9c-a53f-4335-8e8e-8fb3b3f8569d">0001395937</dei:EntityCentralIndexKey>
    <dei:AmendmentFlag
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_8d0788f1-3f09-4872-910f-34e6b7789a84">false</dei:AmendmentFlag>
    <dei:DocumentType
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_0798893f-19d1-4a60-a34e-a16ac3387811">8-K</dei:DocumentType>
    <dei:DocumentPeriodEndDate
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_ce381c68-e45f-416f-835b-4c2e6e7ee3a8">2026-08-04</dei:DocumentPeriodEndDate>
    <dei:EntityRegistrantName
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_a7f6f422-5e11-475d-8e33-9843fe1341df">SYNDAX PHARMACEUTICALS, INC.</dei:EntityRegistrantName>
    <dei:EntityIncorporationStateCountryCode
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_12414a5b-5f3f-4f20-a61e-7d819224f1eb">DE</dei:EntityIncorporationStateCountryCode>
    <dei:EntityFileNumber
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_998bbffe-f7d2-4100-8ee6-41507f778f2c">001-37708</dei:EntityFileNumber>
    <dei:EntityTaxIdentificationNumber
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_c57dba43-a93a-4f8f-8508-f82ee8cbf4a2">32-0162505</dei:EntityTaxIdentificationNumber>
    <dei:EntityAddressAddressLine1
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_8913d2cf-fedd-46db-84b9-c6a6da245018">730 THIRD AVENUE</dei:EntityAddressAddressLine1>
    <dei:EntityAddressAddressLine2
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_00e0671b-5f50-47e8-b247-acd4d0c70da0">FLOOR 9</dei:EntityAddressAddressLine2>
    <dei:EntityAddressCityOrTown
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_6011f529-f8a7-4077-8f30-d6016971f7f7">NEW YORK</dei:EntityAddressCityOrTown>
    <dei:EntityAddressStateOrProvince
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_87e82ee3-54c1-4ee4-92ca-4c6e3362050b">NY</dei:EntityAddressStateOrProvince>
    <dei:EntityAddressPostalZipCode
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_b385f6ba-952e-4414-8748-584d4a02077c">10017</dei:EntityAddressPostalZipCode>
    <dei:CityAreaCode
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_6aac312d-11e0-4d53-8220-3926509f7c6f">(781)</dei:CityAreaCode>
    <dei:LocalPhoneNumber
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_1a749e69-143f-4650-8b39-1a980636dc43">419-1400</dei:LocalPhoneNumber>
    <dei:WrittenCommunications
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_ec737cff-59dc-4461-a666-99d24cd68f9f">false</dei:WrittenCommunications>
    <dei:SolicitingMaterial
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_b334db10-77a8-4aa1-b0c7-28783b77c7f6">false</dei:SolicitingMaterial>
    <dei:PreCommencementTenderOffer
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_76f7f0ba-06e8-4e37-b666-2d974625c1be">false</dei:PreCommencementTenderOffer>
    <dei:PreCommencementIssuerTenderOffer
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_1a2a84b9-e741-4129-93d1-5e4540ce9055">false</dei:PreCommencementIssuerTenderOffer>
    <dei:Security12bTitle
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_48492a67-7bd6-464a-b5e5-1174beb814d6">Common Stock</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_873cc14f-ac15-418e-b316-68690ff10d74">SNDX</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_a00eb027-d94c-4b7b-bbd8-83580a8e5476">NASDAQ</dei:SecurityExchangeName>
    <dei:EntityEmergingGrowthCompany
      contextRef="C_d06e02ff-e771-4d8f-9d4e-7fa0f09f597f"
      id="F_dca64047-81a2-46c4-acad-af443c091bdb">false</dei:EntityEmergingGrowthCompany>
</xbrl>
