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Other Long-term Liabilities (Details) - Paint and Coatings - USD ($) $ in Thousands | Jul. 30, 2008 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Other Long-term Liabilities |  |  |
Long-term purchase commitment, amount | $ 4,800 |  |
Cash received from supplier as discount toward future purchases | 300 |  |
Liability related to deferred discount | $ 300 | $ 57 | $ 92

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- Definition Long-term Purchase Commitment, Cash Received as Discount toward Future Purchases + References No definition available. + Details
Name: | ngs_LongTermPurchaseCommitmentCashReceivedAsDiscountTowardFuturePurchases
Namespace Prefix: | ngs_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Long-term Purchase Commitment, Liability Related to Deferred Discount + References No definition available. + Details
Name: | ngs_LongTermPurchaseCommitmentLiabilityRelatedToDeferredDiscount
Namespace Prefix: | ngs_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Other Liabilities [Line Items] + References No definition available. + Details
Name: | ngs_OtherLiabilitiesLineItems
Namespace Prefix: | ngs_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The minimum amount the entity agreed to spend under the long-term purchase commitment. + References No definition available. + Details
Name: | us-gaap_LongTermPurchaseCommitmentAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=us-gaap_InventoriesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |