<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>164</ContextCount>
  <ElementCount>284</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>35</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>7</UnitCount>
  <MyReports>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000 - Document - Document And Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-document-and-entity-information</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>001 - Statement - Consolidated Balance Sheets (Current Period Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-consolidated-balance-sheets-current-period-unaudited</Role>
      <ShortName>Consolidated Balance Sheets (Current Period Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>002 - Statement - Consolidated Balance Sheets (Current Period Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-consolidated-balance-sheets-current-period-unaudited-parentheticals</Role>
      <ShortName>Consolidated Balance Sheets (Current Period Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>003 - Statement - Consolidated Statements of Operations (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-consolidated-statements-of-operations-unaudited</Role>
      <ShortName>Consolidated Statements of Operations (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>004 - Statement - Consolidated Statement of Stockholders' Equity, Series A and Series B Convertible Preferred Stock (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-consolidated-statement-of-stockholders-equity-series-a-and-series-b-convertible-preferred-stock-unaudited</Role>
      <ShortName>Consolidated Statement of Stockholders' Equity, Series A and Series B Convertible Preferred Stock (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>005 - Statement - Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-consolidated-statements-of-cash-flows-unaudited</Role>
      <ShortName>Consolidated Statements of Cash Flows (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>006 - Disclosure - Note 1 - Background and Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-1-background-and-basis-of-presentation</Role>
      <ShortName>Note 1 - Background and Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>007 - Disclosure - Note 2 - Prepaid Expenses and Other Current Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-2-prepaid-expenses-and-other-current-assets</Role>
      <ShortName>Note 2 - Prepaid Expenses and Other Current Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>008 - Disclosure - Note 3 - Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-3-investment-securities</Role>
      <ShortName>Note 3 - Investment Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>009 - Disclosure - Note 4 - Sale of Equity Interests in Rio Bravo</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-4-sale-of-equity-interests-in-rio-bravo-</Role>
      <ShortName>Note 4 - Sale of Equity Interests in Rio Bravo</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>010 - Disclosure - Note 5 - Property, Plant and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-5-property-plant-and-equipment</Role>
      <ShortName>Note 5 - Property, Plant and Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>011 - Disclosure - Note 6 - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-6-leases</Role>
      <ShortName>Note 6 - Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>012 - Disclosure - Note 7 - Other Non-current Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-7-other-noncurrent-assets</Role>
      <ShortName>Note 7 - Other Non-current Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>013 - Disclosure - Note 8 - Accrued Liabilities and Other Current Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-8-accrued-liabilities-and-other-current-liabilities</Role>
      <ShortName>Note 8 - Accrued Liabilities and Other Current Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>014 - Disclosure - Note 9 - Preferred Stock and Common Stock Warrants</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-9-preferred-stock-and-common-stock-warrants</Role>
      <ShortName>Note 9 - Preferred Stock and Common Stock Warrants</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>015 - Disclosure - Note 10 - Net Loss Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-10-net-loss-per-share-</Role>
      <ShortName>Note 10 - Net Loss Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>016 - Disclosure - Note 11 - Share-based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-11-sharebased-compensation</Role>
      <ShortName>Note 11 - Share-based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>017 - Disclosure - Note 12 - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-12-income-taxes</Role>
      <ShortName>Note 12 - Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>018 - Disclosure - Note 13 - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-13-commitments-and-contingencies</Role>
      <ShortName>Note 13 - Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>019 - Disclosure - Note 14 - Recent Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-14-recent-accounting-pronouncements</Role>
      <ShortName>Note 14 - Recent Accounting Pronouncements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>020 - Disclosure - Note 15 - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-15-subsequent-events</Role>
      <ShortName>Note 15 - Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>021 - Disclosure - Note 2 - Prepaid Expenses and Other Current Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-2-prepaid-expenses-and-other-current-assets-tables</Role>
      <ShortName>Note 2 - Prepaid Expenses and Other Current Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-2-prepaid-expenses-and-other-current-assets</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>022 - Disclosure - Note 3 - Investment Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-3-investment-securities-tables</Role>
      <ShortName>Note 3 - Investment Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-3-investment-securities</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>023 - Disclosure - Note 5 - Property, Plant and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-5-property-plant-and-equipment-tables</Role>
      <ShortName>Note 5 - Property, Plant and Equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-5-property-plant-and-equipment</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>024 - Disclosure - Note 6 - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-6-leases-tables</Role>
      <ShortName>Note 6 - Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-6-leases</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>025 - Disclosure - Note 7 - Other Non-current Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-7-other-noncurrent-assets-tables</Role>
      <ShortName>Note 7 - Other Non-current Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-7-other-noncurrent-assets</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>026 - Disclosure - Note 8 - Accrued Liabilities and Other Current Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-8-accrued-liabilities-and-other-current-liabilities-tables</Role>
      <ShortName>Note 8 - Accrued Liabilities and Other Current Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-8-accrued-liabilities-and-other-current-liabilities</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>027 - Disclosure - Note 9 - Preferred Stock and Common Stock Warrants (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-9-preferred-stock-and-common-stock-warrants-tables</Role>
      <ShortName>Note 9 - Preferred Stock and Common Stock Warrants (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-9-preferred-stock-and-common-stock-warrants</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>028 - Disclosure - Note 10 - Net Loss Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-10-net-loss-per-share-tables</Role>
      <ShortName>Note 10 - Net Loss Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-10-net-loss-per-share-</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>029 - Disclosure - Note 11 - Share-based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-11-sharebased-compensation-tables</Role>
      <ShortName>Note 11 - Share-based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-11-sharebased-compensation</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>030 - Disclosure - Note 1 - Background and Basis of Presentation (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-1-background-and-basis-of-presentation-details-textual</Role>
      <ShortName>Note 1 - Background and Basis of Presentation (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-1-background-and-basis-of-presentation</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>031 - Disclosure - Note 2 - Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-2-prepaid-expenses-and-other-current-assets-summary-of-prepaid-expenses-and-other-current-assets-details</Role>
      <ShortName>Note 2 - Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>032 - Disclosure - Note 3 - Investment Securities (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-3-investment-securities-details-textual</Role>
      <ShortName>Note 3 - Investment Securities (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-3-investment-securities-tables</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>033 - Disclosure - Note 3 - Investment Securities - Summary of Investment Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-3-investment-securities-summary-of-investment-securities-details</Role>
      <ShortName>Note 3 - Investment Securities - Summary of Investment Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>034 - Disclosure - Note 4 - Sale of Equity Interests in Rio Bravo (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-4-sale-of-equity-interests-in-rio-bravo-details-textual</Role>
      <ShortName>Note 4 - Sale of Equity Interests in Rio Bravo (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-4-sale-of-equity-interests-in-rio-bravo-</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>035 - Disclosure - Note 5 - Property, Plant and Equipment (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-5-property-plant-and-equipment-details-textual</Role>
      <ShortName>Note 5 - Property, Plant and Equipment (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-5-property-plant-and-equipment-tables</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>036 - Disclosure - Note 5 - Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-5-property-plant-and-equipment-summary-of-property-plant-and-equipment-details</Role>
      <ShortName>Note 5 - Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>037 - Disclosure - Note 6 - Leases - Supplemental Balance Sheet Information Related to Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-6-leases-supplemental-balance-sheet-information-related-to-leases-details</Role>
      <ShortName>Note 6 - Leases - Supplemental Balance Sheet Information Related to Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>038 - Disclosure - Note 6 - Leases - Lease Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-6-leases-lease-expense-details</Role>
      <ShortName>Note 6 - Leases - Lease Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>039 - Disclosure - Note 6 - Leases - Maturity of Operating Lease (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-6-leases-maturity-of-operating-lease-details</Role>
      <ShortName>Note 6 - Leases - Maturity of Operating Lease (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>040 - Disclosure - Note 7 - Other Non-current Assets - Schedule of Other Non-current Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-7-other-noncurrent-assets-schedule-of-other-noncurrent-assets-details</Role>
      <ShortName>Note 7 - Other Non-current Assets - Schedule of Other Non-current Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>041 - Disclosure - Note 8 - Accrued Liabilities and Other Current Liabilities - Summary of Accrued Liabilities and Other Current Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-8-accrued-liabilities-and-other-current-liabilities-summary-of-accrued-liabilities-and-other-current-liabilities-details</Role>
      <ShortName>Note 8 - Accrued Liabilities and Other Current Liabilities - Summary of Accrued Liabilities and Other Current Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>042 - Disclosure - Note 9 - Preferred Stock and Common Stock Warrants (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-9-preferred-stock-and-common-stock-warrants-details-textual</Role>
      <ShortName>Note 9 - Preferred Stock and Common Stock Warrants (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-9-preferred-stock-and-common-stock-warrants-tables</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>043 - Disclosure - Note 9 - Preferred Stock and Common Stock Warrants - Fair Value Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-9-preferred-stock-and-common-stock-warrants-fair-value-assumptions-details</Role>
      <ShortName>Note 9 - Preferred Stock and Common Stock Warrants - Fair Value Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>044 - Disclosure - Note 10 - Net Loss Per Share (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-10-net-loss-per-share-details-textual</Role>
      <ShortName>Note 10 - Net Loss Per Share (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-10-net-loss-per-share-tables</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>045 - Disclosure - Note 10 - Net Loss Per Share - Basic and Diluted Common Shares (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-10-net-loss-per-share-basic-and-diluted-common-shares-details</Role>
      <ShortName>Note 10 - Net Loss Per Share - Basic and Diluted Common Shares (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>046 - Disclosure - Note 10 - Net Loss Per Share - Anti-dilutive Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-10-net-loss-per-share-antidilutive-securities-details</Role>
      <ShortName>Note 10 - Net Loss Per Share - Anti-dilutive Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>047 - Disclosure - Note 11 - Share-based Compensation - Summary of Share-based Compensation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-11-sharebased-compensation-summary-of-sharebased-compensation-details</Role>
      <ShortName>Note 11 - Share-based Compensation - Summary of Share-based Compensation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="next20200930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>048 - Disclosure - Note 13 - Commitments and Contingencies (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.next-decade.com/20200930/role/statement-note-13-commitments-and-contingencies-details-textual</Role>
      <ShortName>Note 13 - Commitments and Contingencies (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.next-decade.com/20200930/role/statement-note-13-commitments-and-contingencies</ParentRole>
      <Position>49</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="10-Q" original="next20200930_10q.htm">next20200930_10q.htm</File>
    <File>ex_202674.htm</File>
    <File>ex_202675.htm</File>
    <File>ex_202676.htm</File>
    <File>ex_202677.htm</File>
    <File>ex_209499.htm</File>
    <File>ex_209504.htm</File>
    <File>next-20200930.xsd</File>
    <File>next-20200930_cal.xml</File>
    <File>next-20200930_def.xml</File>
    <File>next-20200930_lab.xml</File>
    <File>next-20200930_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>zrp4pccgc10qimg001.gif</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
