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Commitments and Contingencies (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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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

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- Definition Defined Contribution Plan, Eligibility Age, Minimum + References No definition available. + Details
Name: | ngs_DefinedContributionPlanEligibilityAgeMinimum
Namespace Prefix: | ngs_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Defined Contribution Plan, Employer Contribution Amount + References No definition available. + Details
Name: | ngs_DefinedContributionPlanEmployerContributionAmount
Namespace Prefix: | ngs_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Defined Contribution Plan, Employer Contribution, Ultimate Vesting Percentage + References No definition available. + Details
Name: | ngs_DefinedContributionPlanEmployerContributionUltimateVestingPercentage
Namespace Prefix: | ngs_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Defined Contribution Plan, Employers Matching Contribution, Vesting Term + References No definition available. + Details
Name: | ngs_DefinedContributionPlanEmployersMatchingContributionVestingTerm
Namespace Prefix: | ngs_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Defined Contribution Plan, Requisite Service Period + References No definition available. + Details
Name: | ngs_DefinedContributionPlanRequisiteServicePeriod
Namespace Prefix: | ngs_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Operating Leases of Lessee, Maximum Term + References No definition available. + Details
Name: | ngs_OperatingLeasesOfLesseeMaximumTerm
Namespace Prefix: | ngs_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of employer's matching contributions to a defined contribution plan that vests in a given year. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line. + References No definition available. + Details
Name: | us-gaap_LeaseAndRentalExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration