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Summary of Significant Accounting Policies (Narrative) (Details) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
segment
Dec. 31, 2013
Dec. 31, 2012
Accounting Policies [Abstract]                      
Cash equivalents, qualification, maximum original maturity of short-term investments                 3 months    
Customer deposit requirements, large custom contracts, maximum percentage                 50.00%ngs_CustomerDepositsDepositRequirementsLargeCustomContractsMaximumPercentage    
Allowance for doubtful accounts $ 507,000us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent       $ 436,000us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent       $ 507,000us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent $ 436,000us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent  
Inventory valuation reserves, deductions                 225,000ngs_InventoryValuationReservesDeductions 128,000ngs_InventoryValuationReservesDeductions  
Raw materials 25,984,000us-gaap_InventoryRawMaterialsNetOfReserves       20,524,000us-gaap_InventoryRawMaterialsNetOfReserves       25,984,000us-gaap_InventoryRawMaterialsNetOfReserves 20,524,000us-gaap_InventoryRawMaterialsNetOfReserves  
Work in process 6,640,000us-gaap_InventoryWorkInProcessNetOfReserves       6,308,000us-gaap_InventoryWorkInProcessNetOfReserves       6,640,000us-gaap_InventoryWorkInProcessNetOfReserves 6,308,000us-gaap_InventoryWorkInProcessNetOfReserves  
Inventory, net 32,624,000us-gaap_InventoryNet       26,832,000us-gaap_InventoryNet       32,624,000us-gaap_InventoryNet 26,832,000us-gaap_InventoryNet  
Product warranty reserves 0us-gaap_ProductWarrantyAccrual       28,000us-gaap_ProductWarrantyAccrual       0us-gaap_ProductWarrantyAccrual 28,000us-gaap_ProductWarrantyAccrual  
Anti-dilutive securities                 0us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 0us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 0us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Impairment of goodwill                 0us-gaap_GoodwillImpairmentLoss 0us-gaap_GoodwillImpairmentLoss  
Numerator:                      
Net income 4,004,000us-gaap_NetIncomeLoss 3,883,000us-gaap_NetIncomeLoss 3,385,000us-gaap_NetIncomeLoss 2,856,000us-gaap_NetIncomeLoss 3,158,000us-gaap_NetIncomeLoss 3,394,000us-gaap_NetIncomeLoss 3,844,000us-gaap_NetIncomeLoss 3,994,000us-gaap_NetIncomeLoss 14,128,000us-gaap_NetIncomeLoss 14,390,000us-gaap_NetIncomeLoss 12,685,000us-gaap_NetIncomeLoss
Denominator for basic net income per common share:                      
Weighted average common shares outstanding (in shares)                 12,434,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 12,324,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 12,220,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Denominator for diluted net income per share:                      
Weighted average common shares outstanding (in shares)                 12,434,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 12,324,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 12,220,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Dilutive effect of stock options and restricted shares (in shares)                 287,000us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 226,000us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 100,000us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements
Weighted average common shares outstanding, Diluted (in shares)                 12,721,000us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 12,550,000us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 12,320,000us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Earnings per common share:                      
Basic (in dollars per share) $ 0.32us-gaap_EarningsPerShareBasic $ 0.31us-gaap_EarningsPerShareBasic $ 0.27us-gaap_EarningsPerShareBasic $ 0.23us-gaap_EarningsPerShareBasic $ 0.25us-gaap_EarningsPerShareBasic $ 0.28us-gaap_EarningsPerShareBasic $ 0.31us-gaap_EarningsPerShareBasic $ 0.33us-gaap_EarningsPerShareBasic $ 1.14us-gaap_EarningsPerShareBasic $ 1.17us-gaap_EarningsPerShareBasic $ 1.04us-gaap_EarningsPerShareBasic
Diluted (in dollars per share) $ 0.32us-gaap_EarningsPerShareDiluted $ 0.30us-gaap_EarningsPerShareDiluted $ 0.27us-gaap_EarningsPerShareDiluted $ 0.23us-gaap_EarningsPerShareDiluted $ 0.25us-gaap_EarningsPerShareDiluted $ 0.27us-gaap_EarningsPerShareDiluted $ 0.31us-gaap_EarningsPerShareDiluted $ 0.32us-gaap_EarningsPerShareDiluted $ 1.11us-gaap_EarningsPerShareDiluted $ 1.15us-gaap_EarningsPerShareDiluted $ 1.03us-gaap_EarningsPerShareDiluted
Uncertain tax positions $ 0us-gaap_LiabilityForUncertainTaxPositionsCurrent       $ 0us-gaap_LiabilityForUncertainTaxPositionsCurrent       $ 0us-gaap_LiabilityForUncertainTaxPositionsCurrent $ 0us-gaap_LiabilityForUncertainTaxPositionsCurrent  
Number of business segments                 1us-gaap_NumberOfOperatingSegments