XML 34 R40.htm IDEA: XBRL DOCUMENT v2.4.1.9
Stock-Based Compensation (Stock Options Narrative) (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended 0 Months Ended 1 Months Ended
Dec. 31, 2014
Option
Dec. 31, 2013
Dec. 31, 2012
Jun. 16, 2009
Oct. 31, 2014
Jul. 31, 2014
Mar. 31, 2014
Dec. 31, 1998
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of options granted 3ngs_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsNumberofOptionsGranted              
Options granted in period (in shares) 60,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross              
Options granted in period, weighted average exercise price (in dollars per share) $ 30.06us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice              
Weighted average grant date fair value of options granted (in dollars per share) $ 16.56us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue $ 10.30us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue $ 8.22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue          
Total intrinsic value of options exercised $ 48us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 458us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 151us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue          
Proceeds from exercise of stock options 59us-gaap_ProceedsFromStockOptionsExercised 829us-gaap_ProceedsFromStockOptionsExercised 180us-gaap_ProceedsFromStockOptionsExercised          
Unrecognized compensation cost related to stock options 808us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions              
Stock Options                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based compensation expense $ 426us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
$ 198us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
$ 440us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
         
Unrecognized compensation cost related to stock options, weighted average period for recognition 3 years              
Stock Options | 1998 Stock Option Plan                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Award vesting period 3 years              
Award expiration period 10 years              
Stock Options | Common Stock | 1998 Stock Option Plan                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of shares authorized       750,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= ngs_A1998StockOptionPlanMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
      550,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= ngs_A1998StockOptionPlanMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
Additional shares authorized       200,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= ngs_A1998StockOptionPlanMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
       
Number of shares available for grant 186,419us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= ngs_A1998StockOptionPlanMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
             
Non-Executive Employees                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Award vesting period 3 years              
Options granted in period (in shares) 60,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_TitleOfIndividualAxis
= ngs_NonExecutiveEmployeesMember
             
Options granted in period, weighted average exercise price (in dollars per share)         $ 23.30us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_TitleOfIndividualAxis
= ngs_NonExecutiveEmployeesMember
$ 33.36us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_TitleOfIndividualAxis
= ngs_NonExecutiveEmployeesMember
$ 30.41us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_TitleOfIndividualAxis
= ngs_NonExecutiveEmployeesMember