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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)

X

- 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

X

- 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

X

- 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

X

- 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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant