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v3.20.2

Related Party (Details) - Corporate Joint Venture - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2019 | Mar. 31, 2017
Related Party Transaction [Line Items] |  |
Purchases from joint venture | $ 1.0 |
Genis Holdings, LLC | N-G, LLC |  |
Related Party Transaction [Line Items] |  |
Percent ownership |  | 50.00%

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- Definition Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 272 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055 + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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- Details
Name: | dei_LegalEntityAxis=ngs_GenisHoldingsLLCMember
Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Balance Type: |
Period Type: |