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Stock-Based Compensation (Summary of Unvested Stock Options) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Employee Service Share-based Compensation, Nonvested Awards, Stock Options, Outstanding [Roll Forward] |  |  |
Unvested, beginning of period (in shares) | 63,082ngs_EmployeeServiceShareBasedCompensationNonvestedAwardsOutstandingNumberStockOptions |  |
Granted (in shares) | 60,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross |  |
Vested (in shares) | (28,915)ngs_EmployeeServiceShareBasedCompensationVestedInPeriodStockOptions |  |
Cancelled/Forfeited (in shares) | (2,333)ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsNonvestedOptionsCancelledOrForfeitedNumberOfShares |  |
Unvested, end of period (in shares) | 91,834ngs_EmployeeServiceShareBasedCompensationNonvestedAwardsOutstandingNumberStockOptions | 63,082ngs_EmployeeServiceShareBasedCompensationNonvestedAwardsOutstandingNumberStockOptions |
Employee Service Share-based Compensation, Nonvested Awards, Stock Options, Weighted Average Grant Date Fair Value [Roll Forward] |  |  |
Unvested, weighted average grant date fair value, beginning of period (in dollars per share) | $ 9.87ngs_EmployeeServiceSharebasedCompensationNonvestedAwardsOutstandingStockOptionsWeightedAverageGrantDateFairValue |  |
Granted, weighted average exercise price (in dollars per share) | $ 16.56us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 10.30us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 8.22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Vested, weighted average exercise price (in dollars per share) | $ 9.80ngs_EmployeeServiceSharebasedCompensationNonvestedAwardsVestedinPeriodStockOptionsWeightedAverageGrantDateFairValue |  |
Cancelled/Forfeited, weighted average exercise price (in dollars per share) | $ 12.87ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsNonvestedOptionsCancelledOrForfeitedWeightedAverageGrantDateFairValue |  |
Unvested, weighted average grant date fair value, end of period (in dollars per share) | $ 14.19ngs_EmployeeServiceSharebasedCompensationNonvestedAwardsOutstandingStockOptionsWeightedAverageGrantDateFairValue | $ 9.87ngs_EmployeeServiceSharebasedCompensationNonvestedAwardsOutstandingStockOptionsWeightedAverageGrantDateFairValue |

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- Definition Employee Service Share-based Compensation, Nonvested Awards, Vested in Period, Stock Options, Weighted Average Grant Date Fair Value + References No definition available. + Details
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- Definition Employee Service Share-based Compensation, Vested in Period, Stock Options + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Cancelled or Forfeited, Number of Shares + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Cancelled or Forfeited, Weighted Average Grant Date Fair Value + References No definition available. + Details
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- Definition Gross number of share options (or share units) granted during the period. + References No definition available. + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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