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Commitments and Contingencies (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Commitments and Contingencies Disclosure [Abstract]      
401(k) plan, minimum eligibility age 18 years    
401(k) plan, requisite service period 6 months    
401(k) plan, maximum annual contribution per employee, percent 100.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent    
401(k) plan, employer's matching contribution, annual vesting percentage 20.00%us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage    
401(k) plan, employer contribution, ultimate vesting percentage 100.00%ngs_DefinedContributionPlanEmployerContributionUltimateVestingPercentage    
401(k) plan, employer's matching contribution, vesting term 6 years    
401(k) plan, employer contribution amount $ 296ngs_DefinedContributionPlanEmployerContributionAmount $ 294ngs_DefinedContributionPlanEmployerContributionAmount $ 283ngs_DefinedContributionPlanEmployerContributionAmount
Operating leases, maximum term 5 years    
Operating leases, rent expense $ 380us-gaap_LeaseAndRentalExpense $ 358us-gaap_LeaseAndRentalExpense $ 453us-gaap_LeaseAndRentalExpense