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Stock-Based Compensation (Summary of Unvested Stock Options) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Shares:
Unvested, beginning of period (in shares)	91,834us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares
Granted (in shares)	0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Vested (in shares)	(35,164)us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares
Canceled/Forfeited (in shares)	(1,500)ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsNonvestedOptionsCancelledOrForfeitedNumberOfShares
Unvested, end of period (in shares)	55,170us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares
Weighted Average Grant Date Fair Value:
Unvested, weighted average grant date fair value, beginning of period (in USD per share)	$ 14.19us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue
Granted, weighted average grant date fair value (in USD per share)	$ 0.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Vested, weighted average grant date fair value (in USD per share)	$ 12.88us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue
Canceled/Forfeited, weighted average grant date fair value (in USD per share)	$ 16.79ngs_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsNonvestedOptionsCanceledorForfeitedWeightedAverageGrantDateFairValue
Unvested, weighted average grant date fair value, end of period (in USD per share)	$ 14.95us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue
Unrecognized compensation cost related to awards, weighted average period for recognition	12 months
Stock Options
Weighted Average Grant Date Fair Value:
Unrecognized compensation cost related to stock options	$ 693,517us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember
Unrecognized compensation cost related to awards, weighted average period for recognition	1 year 9 months 31 days
Share-based compensation expense	$ 114,002us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember	$ 56,790us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 1

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Unrecognized cost of unvested options awarded to employees as compensation.

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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Number of non-vested options outstanding.

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Weighted average grant-date fair value of non-vested options outstanding.

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Number of options vested.

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Weighted average grant-date fair value of options vested.

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