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    <Log type="Info">Process Flow-Through: 1001501 - Statement - Balance Sheets (Parenthetical)</Log>
    <Log type="Info">Process Flow-Through: 1002000 - Statement - Statements of Income</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Dec. 31, 2014'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Sep. 30, 2014'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Jun. 30, 2014'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Mar. 31, 2014'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Dec. 31, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Sep. 30, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Jun. 30, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Mar. 31, 2013'</Log>
    <Log type="Info">Process Flow-Through: 1004000 - Statement - Statements of Cash Flows</Log>
  </Logs>
  <InputFiles>
    <File>ngs-20141231.xml</File>
    <File>ngs-20141231.xsd</File>
    <File>ngs-20141231_cal.xml</File>
    <File>ngs-20141231_def.xml</File>
    <File>ngs-20141231_lab.xml</File>
    <File>ngs-20141231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles />
  <BaseTaxonomies />
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
