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Stock-Based Compensation (Restricted Stock Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 20, 2014	Jan. 23, 2014	Jun. 03, 2014
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted	92,572us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Share-based compensation expense	$ 2,818us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 1,485us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 978us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Total unrecognized compensation expense	$ 608us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Unrecognized compensation cost related to stock options, weighted average period for recognition	1 year
Chief Financial Officer | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted				15,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_TitleOfIndividualAxis = us-gaap_ChiefFinancialOfficerMember
Vice President of Technical Services | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted				15,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_TitleOfIndividualAxis = ngs_VicePresidentofTechnicalServicesMember
Executive Officer | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period				1 year
Director | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted				13,152us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember
Award vesting period				1 year
Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period					1 year
Chief Executive Officer | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted					49,420us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_TitleOfIndividualAxis = us-gaap_ChiefExecutiveOfficerMember
2009 Restricted Stock/Units Plan | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares authorized						500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_PlanNameAxis = ngs_TwoThousandNineRestrictedStockUnitsPlanMember
Number of shares authorized						800,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_PlanNameAxis = ngs_TwoThousandNineRestrictedStockUnitsPlanMember
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	423,717ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberofRemainingSharesAuthorized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_PlanNameAxis = ngs_TwoThousandNineRestrictedStockUnitsPlanMember

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- Definition

Share-Based Compensation Arrangement By Share-Based Payment Award, Number of Remaining Shares Authorized

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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-Publisher FASB

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-Name Staff Accounting Bulletin (SAB)

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Number of additional shares authorized for issuance under an established share-based compensation plan.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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