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Property and Equipment (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 18,192,000us-gaap_PropertyPlantAndEquipmentGross	$ 17,121,000us-gaap_PropertyPlantAndEquipmentGross
Less accumulated depreciation	(10,830,000)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	(9,692,000)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment
Property and equipment, net	7,362,000us-gaap_PropertyPlantAndEquipmentNet	7,429,000us-gaap_PropertyPlantAndEquipmentNet
Depreciation	21,400,000us-gaap_Depreciation	18,000,000us-gaap_Depreciation	15,600,000us-gaap_Depreciation
Land and building
Property, Plant and Equipment [Line Items]
Property and equipment, gross	5,636,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = ngs_LandAndBuildingExcludingLeaseholdImprovementsMember	5,636,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = ngs_LandAndBuildingExcludingLeaseholdImprovementsMember
Useful lives	30 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	793,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LeaseholdImprovementsMember	770,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LeaseholdImprovementsMember
Useful lives	39 years
Office equipment and furniture
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,376,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = ngs_OfficeEquipmentAndFurnitureMember	1,434,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = ngs_OfficeEquipmentAndFurnitureMember
Useful lives	5 years
Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	573,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ComputerSoftwareIntangibleAssetMember	573,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ComputerSoftwareIntangibleAssetMember
Useful lives	5 years
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,847,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_MachineryAndEquipmentMember	2,635,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_MachineryAndEquipmentMember
Useful lives	7 years
Vehicles
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 6,967,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_VehiclesMember	$ 6,073,000us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_VehiclesMember
Useful lives	3 years

X

- Definition

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.14)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 14

-Article 5

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X

- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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X

- Definition

Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.13)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.13)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 13

-Subparagraph a

-Article 5



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 8

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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

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