<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.21.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>165</ContextCount>
  <ElementCount>304</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>38</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001001 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001002 - Statement - Condensed Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/CondensedConsolidatedBalanceSheets</Role>
      <ShortName>Condensed Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1002003 - Statement - Condensed Consolidated Balance Sheets (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/CondensedConsolidatedBalanceSheetsParentheticals</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1003004 - Statement - Condensed Consolidated Statements of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/CondensedConsolidatedStatementsofOperations</Role>
      <ShortName>Condensed Consolidated Statements of Operations</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1004005 - Statement - Condensed Consolidated Statements of Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/CondensedConsolidatedStatementsofStockholdersEquity</Role>
      <ShortName>Condensed Consolidated Statements of Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1005006 - Statement - Condensed Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/CondensedConsolidatedStatementsofCashFlows</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>2101101 - Disclosure - Description of Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/DescriptionofBusiness</Role>
      <ShortName>Description of Business</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2102102 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/SummaryofSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2108103 - Disclosure - Inventory</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/Inventory</Role>
      <ShortName>Inventory</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2112104 - Disclosure - Federal Income Tax Receivable</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/FederalIncomeTaxReceivable</Role>
      <ShortName>Federal Income Tax Receivable</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2114105 - Disclosure - Rental Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/RentalEquipment</Role>
      <ShortName>Rental Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2117106 - Disclosure - Credit Facility</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/CreditFacility</Role>
      <ShortName>Credit Facility</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2119107 - Disclosure - Stock-Based and Other Long-Term Incentive Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensation</Role>
      <ShortName>Stock-Based and Other Long-Term Incentive Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2127108 - Disclosure - (Loss) Earnings per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/LossEarningsperShare</Role>
      <ShortName>(Loss) Earnings per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2130109 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/CommitmentsandContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2132110 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/SubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2203201 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/SummaryofSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.ngsgi.com/role/SummaryofSignificantAccountingPolicies</ParentRole>
      <Position>17</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2304301 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/SummaryofSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngsgi.com/role/SummaryofSignificantAccountingPolicies</ParentRole>
      <Position>18</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2309302 - Disclosure - Inventory (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/InventoryTables</Role>
      <ShortName>Inventory (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngsgi.com/role/Inventory</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2315303 - Disclosure - Rental Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/RentalEquipmentTables</Role>
      <ShortName>Rental Equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngsgi.com/role/RentalEquipment</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2320304 - Disclosure - Stock-Based and Other Long-Term Incentive Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationTables</Role>
      <ShortName>Stock-Based and Other Long-Term Incentive Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensation</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2328305 - Disclosure - (Loss) Earnings per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/LossEarningsperShareTables</Role>
      <ShortName>(Loss) Earnings per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngsgi.com/role/LossEarningsperShare</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2405401 - Disclosure - Summary of Significant Accounting Policies - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/SummaryofSignificantAccountingPoliciesNarrativeDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - Summary of Significant Accounting Policies - Disaggregation of Revenue (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/SummaryofSignificantAccountingPoliciesDisaggregationofRevenueDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Disaggregation of Revenue (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2407403 - Disclosure - Summary of Significant Accounting Policies - Contract Balances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/SummaryofSignificantAccountingPoliciesContractBalancesDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Contract Balances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2410404 - Disclosure - Inventory - Schedule of Inventory (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/InventoryScheduleofInventoryDetails</Role>
      <ShortName>Inventory - Schedule of Inventory (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2411405 - Disclosure - Inventory - Schedule of Inventory Allowance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/InventoryScheduleofInventoryAllowanceDetails</Role>
      <ShortName>Inventory - Schedule of Inventory Allowance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2413406 - Disclosure - Federal Income Tax Receivable - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/FederalIncomeTaxReceivableNarrativeDetails</Role>
      <ShortName>Federal Income Tax Receivable - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2416407 - Disclosure - Rental Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/RentalEquipmentDetails</Role>
      <ShortName>Rental Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.ngsgi.com/role/RentalEquipmentTables</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2418408 - Disclosure - Credit Facility (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/CreditFacilityDetails</Role>
      <ShortName>Credit Facility (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.ngsgi.com/role/CreditFacility</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2421409 - Disclosure - Stock-Based and Other Long-Term Incentive Compensation - Stock Option Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationStockOptionActivityDetails</Role>
      <ShortName>Stock-Based and Other Long-Term Incentive Compensation - Stock Option Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2422410 - Disclosure - Stock-Based and Other Long-Term Incentive Compensation - Stock Options Outstanding (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationStockOptionsOutstandingDetails</Role>
      <ShortName>Stock-Based and Other Long-Term Incentive Compensation - Stock Options Outstanding (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2423411 - Disclosure - Stock-Based and Other Long-Term Incentive Compensation - Stock Option, Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationStockOptionNarrativeDetails</Role>
      <ShortName>Stock-Based and Other Long-Term Incentive Compensation - Stock Option, Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2424412 - Disclosure - Stock-Based and Other Long-Term Incentive Compensation - Restricted Stock, Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationRestrictedStockNarrativeDetails</Role>
      <ShortName>Stock-Based and Other Long-Term Incentive Compensation - Restricted Stock, Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2425413 - Disclosure - Stock-Based and Other Long-Term Incentive Compensation - Restricted Stock (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationRestrictedStockDetails</Role>
      <ShortName>Stock-Based and Other Long-Term Incentive Compensation - Restricted Stock (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2426414 - Disclosure - Stock-Based and Other Long-Term Incentive Compensation - Other Long-Term Incentive Compensation, Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationOtherLongTermIncentiveCompensationNarrativeDetails</Role>
      <ShortName>Stock-Based and Other Long-Term Incentive Compensation - Other Long-Term Incentive Compensation, Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2429415 - Disclosure - (Loss) Earnings per Share - Schedule of Earnings Per Share, Basic and Diluted (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/LossEarningsperShareScheduleofEarningsPerShareBasicandDilutedDetails</Role>
      <ShortName>(Loss) Earnings per Share - Schedule of Earnings Per Share, Basic and Diluted (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="ngs-20210930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2431416 - Disclosure - Commitments and Contingencies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngsgi.com/role/CommitmentsandContingenciesDetails</Role>
      <ShortName>Commitments and Contingencies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.ngsgi.com/role/CommitmentsandContingencies</ParentRole>
      <Position>38</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="ngs-20210930.htm">ngs-20210930.htm</File>
    <File>ngs-20210930.xsd</File>
    <File>ngs-20210930_cal.xml</File>
    <File>ngs-20210930_def.xml</File>
    <File>ngs-20210930_lab.xml</File>
    <File>ngs-20210930_pre.xml</File>
    <File>ngs093021-ex311.htm</File>
    <File>ngs093021-ex312.htm</File>
    <File>ngs093021-ex321.htm</File>
    <File>ngs093021-ex322.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/us-gaap/2021-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2021-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2021</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
