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Inventory - Schedule of Inventory Allowance (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2021 | Dec. 31, 2020
Inventory Valuation Reserves [Roll Forward] |  |
Beginning balance | $ (221) | $ (24)
Accruals | 0 | (251)
Write-offs | 184 | 54
Ending balance | $ (37) | $ (221)

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- Definition Inventory Allowance, Accrual + References No definition available. + Details
Name: | ngs_InventoryAllowanceAccrual
Namespace Prefix: | ngs_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Inventory Valuation Reserves + References No definition available. + Details
Name: | ngs_InventoryValuationReservesRollForward
Namespace Prefix: | ngs_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Inventory Write-Offs + References No definition available. + Details
Name: | ngs_InventoryWriteOffs
Namespace Prefix: | ngs_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of valuation reserve for inventory. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB TOPIC 5.BB) -URI http://asc.fasb.org/extlink&oid=27011343&loc=d3e100047-122729 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_InventoryValuationReserves
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant