<?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:rway="http://runwaygrowth.com/20260713"
  xmlns:us-gaap="http://fasb.org/us-gaap/2026"
  xmlns:xbrldi="http://xbrl.org/2006/xbrldi"
  xmlns:xlink="http://www.w3.org/1999/xlink">
    <link:schemaRef xlink:href="rway-20260713.xsd" xlink:type="simple"/>
    <context id="C_ac6be455-5357-4adc-bdf7-5477ad04e154">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001653384</identifier>
            <segment>
                <xbrldi:explicitMember dimension="us-gaap:StatementClassOfStockAxis">rway:Notes7.25Due2031Member</xbrldi:explicitMember>
            </segment>
        </entity>
        <period>
            <startDate>2026-07-13</startDate>
            <endDate>2026-07-13</endDate>
        </period>
    </context>
    <context id="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001653384</identifier>
        </entity>
        <period>
            <startDate>2026-07-13</startDate>
            <endDate>2026-07-13</endDate>
        </period>
    </context>
    <context id="C_0a4734c0-c405-473d-a773-a4aaee934f09">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001653384</identifier>
            <segment>
                <xbrldi:explicitMember dimension="us-gaap:StatementClassOfStockAxis">rway:CommonStockParValue0.01PerShareMember</xbrldi:explicitMember>
            </segment>
        </entity>
        <period>
            <startDate>2026-07-13</startDate>
            <endDate>2026-07-13</endDate>
        </period>
    </context>
    <context id="C_4969d579-7a1b-40bc-a0fc-1b79c9d34ed5">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001653384</identifier>
            <segment>
                <xbrldi:explicitMember dimension="us-gaap:StatementClassOfStockAxis">rway:Notes9.00Due2027Member</xbrldi:explicitMember>
            </segment>
        </entity>
        <period>
            <startDate>2026-07-13</startDate>
            <endDate>2026-07-13</endDate>
        </period>
    </context>
    <context id="C_7b90f965-3974-4a74-b5eb-7f06dbaaf2cc">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001653384</identifier>
            <segment>
                <xbrldi:explicitMember dimension="us-gaap:StatementClassOfStockAxis">rway:Sec7.50NotesDue2027Member</xbrldi:explicitMember>
            </segment>
        </entity>
        <period>
            <startDate>2026-07-13</startDate>
            <endDate>2026-07-13</endDate>
        </period>
    </context>
    <dei:AmendmentFlag
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_cc5dcf0e-551d-458a-a8e2-3e0e83eca640">false</dei:AmendmentFlag>
    <dei:EntityCentralIndexKey
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_7ffa151f-5cff-4be9-a0c4-3786037ef6a2">0001653384</dei:EntityCentralIndexKey>
    <dei:DocumentType
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_e2593cd3-d1fc-4e51-8414-e69ad3f62d98">8-K</dei:DocumentType>
    <dei:DocumentPeriodEndDate
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_269b15e2-4b5a-466d-8b10-80a1ec14fefb">2026-07-13</dei:DocumentPeriodEndDate>
    <dei:EntityRegistrantName
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_6907c192-9945-4c81-b0c9-1305e92a9ed2">Runway Growth Finance Corp.</dei:EntityRegistrantName>
    <dei:EntityIncorporationStateCountryCode
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_730cbd7a-7177-42d4-a384-703fdf66ba6f">MD</dei:EntityIncorporationStateCountryCode>
    <dei:EntityFileNumber
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_09ff6717-c0c2-4701-bb4d-b2795a39cd4e">814-01180</dei:EntityFileNumber>
    <dei:EntityTaxIdentificationNumber
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_f5c6dc67-7e6e-4508-a4bb-d61c73a6ba4a">47-5049745</dei:EntityTaxIdentificationNumber>
    <dei:EntityAddressAddressLine1
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_98138eba-c3c4-4d7b-8fe8-f507c9108e96">205 N. Michigan Ave.</dei:EntityAddressAddressLine1>
    <dei:EntityAddressAddressLine2
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_f344ef0e-e2d4-4cd6-afa6-fb8d44d193e0">Suite 4200</dei:EntityAddressAddressLine2>
    <dei:EntityAddressCityOrTown
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_e7caed09-8de4-47e2-9ba8-d02d16ea8a88">Chicago</dei:EntityAddressCityOrTown>
    <dei:EntityAddressStateOrProvince
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_71f124df-586d-4194-80f4-732ec6ad523c">IL</dei:EntityAddressStateOrProvince>
    <dei:EntityAddressPostalZipCode
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_c24f39e1-73e4-463d-acef-ee4ae6943ae6">60601</dei:EntityAddressPostalZipCode>
    <dei:CityAreaCode
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_706bf311-c4bd-482d-8d01-aeb2ef9b9822">(312)</dei:CityAreaCode>
    <dei:LocalPhoneNumber
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_543a5245-312c-4ff1-a7d7-3bfd6255063c">698-6902</dei:LocalPhoneNumber>
    <dei:WrittenCommunications
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_b2394bd4-8b51-4ace-9dd3-646d2a4558ae">false</dei:WrittenCommunications>
    <dei:SolicitingMaterial
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_909a5bb8-f04e-4075-9f67-8d5658333c70">false</dei:SolicitingMaterial>
    <dei:PreCommencementTenderOffer
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_5a7d6551-3a80-4bdf-8282-1fe551677488">false</dei:PreCommencementTenderOffer>
    <dei:PreCommencementIssuerTenderOffer
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_3193f9c4-0c03-4030-b76f-2e6c5d69ac5a">false</dei:PreCommencementIssuerTenderOffer>
    <dei:Security12bTitle
      contextRef="C_0a4734c0-c405-473d-a773-a4aaee934f09"
      id="F_6c848fa3-4cdb-4a2f-9f82-299462c1d68f">Common Stock, par value $0.01 per share</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="C_0a4734c0-c405-473d-a773-a4aaee934f09"
      id="F_a067d0e5-33e1-4c72-a7b3-6f8197ef962a">RWAY</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="C_0a4734c0-c405-473d-a773-a4aaee934f09"
      id="F_4fc992f1-4e81-4749-a93e-72a4a45b9e0d">NASDAQ</dei:SecurityExchangeName>
    <dei:Security12bTitle
      contextRef="C_7b90f965-3974-4a74-b5eb-7f06dbaaf2cc"
      id="F_495c67c1-1eec-4e85-a4dd-9339d35f901f">7.50% Notes due 2027</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="C_7b90f965-3974-4a74-b5eb-7f06dbaaf2cc"
      id="F_3b118e83-e152-4c37-a6d6-c81d2aa95807">RWAYL</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="C_7b90f965-3974-4a74-b5eb-7f06dbaaf2cc"
      id="F_d667662b-b21a-4095-9732-723bccd00d9c">NASDAQ</dei:SecurityExchangeName>
    <dei:Security12bTitle
      contextRef="C_ac6be455-5357-4adc-bdf7-5477ad04e154"
      id="F_50bbb51d-8207-4bee-9a8c-392a0a78ff40">7.25% Notes due 2031</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="C_ac6be455-5357-4adc-bdf7-5477ad04e154"
      id="F_6977207b-1510-48b7-958f-fe78335e18e3">RWAYI</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="C_ac6be455-5357-4adc-bdf7-5477ad04e154"
      id="F_9d207a66-ef17-4d06-ab00-d6ed050a5934">NASDAQ</dei:SecurityExchangeName>
    <dei:Security12bTitle
      contextRef="C_4969d579-7a1b-40bc-a0fc-1b79c9d34ed5"
      id="F_ad817b93-b732-43d4-a447-08cb3dd89bf0">9.00% Notes due 2027</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="C_4969d579-7a1b-40bc-a0fc-1b79c9d34ed5"
      id="F_5b127366-7baa-428e-8ed7-b5442bdb31d0">SWKHL</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="C_4969d579-7a1b-40bc-a0fc-1b79c9d34ed5"
      id="F_95c291b8-7770-4425-a55c-e85cf5c3f36c">NASDAQ</dei:SecurityExchangeName>
    <dei:EntityEmergingGrowthCompany
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_3ebe86ea-0bc6-4c80-9087-bf2222727462">true</dei:EntityEmergingGrowthCompany>
    <dei:EntityExTransitionPeriod
      contextRef="C_d1c6f49b-d162-4eda-94dc-78fca1455d9b"
      id="F_c93ffcf9-1307-44fd-a2cc-99c1fcfb7440">false</dei:EntityExTransitionPeriod>
</xbrl>
