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Summary of Significant Accounting Policies (Concentration Risk) (Details) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 10.00%us-gaap_ConcentrationRiskPercentage1 |  |
EOG | Sales and Rental Income | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 15.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = ngs_SalesRevenueGoodsNetAndRentalIncomeMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = ngs_EogResourcesIncMember | 18.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = ngs_SalesRevenueGoodsNetAndRentalIncomeMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = ngs_EogResourcesIncMember | 32.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = ngs_SalesRevenueGoodsNetAndRentalIncomeMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = ngs_EogResourcesIncMember
EOG | Accounts Receivable | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 20.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_AccountsReceivableMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = ngs_EogResourcesIncMember | 18.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_AccountsReceivableMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = ngs_EogResourcesIncMember |
Promatcon | Accounts Receivable | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 18.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_AccountsReceivableMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = ngs_PromatconMember |  |
Oxy | Accounts Receivable | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 14.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_AccountsReceivableMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = ngs_OxyMember |  |
Devon | Sales and Rental Income | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 18.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = ngs_SalesRevenueGoodsNetAndRentalIncomeMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = ngs_DevonEnergyProductionIncMember | 15.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = ngs_SalesRevenueGoodsNetAndRentalIncomeMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = ngs_DevonEnergyProductionIncMember | 12.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = ngs_SalesRevenueGoodsNetAndRentalIncomeMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = ngs_DevonEnergyProductionIncMember
Devon | Accounts Receivable | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage |  | 14.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_AccountsReceivableMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = ngs_DevonEnergyProductionIncMember |

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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592 + Details
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