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Stock-Based Compensation (Summary of Unvested Stock Options) (Details) (USD $)
 12 Months Ended
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