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Regulatory Assets and Liabilities - Liability is Equal to Percent of Asset Cost (Detail)
 12 Months Ended
Dec. 31, 2014
Orwell [Member] | Mains [Member]  
Asset Retirement Obligations [Line Items]  
Regulatory Actions Estimated Useful Life Percentage Of Asset Cost Estimated Liability 15.00%egas_RegulatoryActionsEstimatedUsefulLifePercentageOfAssetCostEstimatedLiability
/ dei_LegalEntityAxis
= egas_OrwellMember
/ us-gaap_RegulatoryAssetAxis
= egas_MainsMember
Orwell [Member] | Meter And Regulator Stations [Member]  
Asset Retirement Obligations [Line Items]  
Regulatory Actions Estimated Useful Life Percentage Of Asset Cost Estimated Liability 10.00%egas_RegulatoryActionsEstimatedUsefulLifePercentageOfAssetCostEstimatedLiability
/ dei_LegalEntityAxis
= egas_OrwellMember
/ us-gaap_RegulatoryAssetAxis
= egas_MeterAndRegulatorStationsMember
Orwell [Member] | Service Lines [Member]  
Asset Retirement Obligations [Line Items]  
Regulatory Actions Estimated Useful Life Percentage Of Asset Cost Estimated Liability 75.00%egas_RegulatoryActionsEstimatedUsefulLifePercentageOfAssetCostEstimatedLiability
/ dei_LegalEntityAxis
= egas_OrwellMember
/ us-gaap_RegulatoryAssetAxis
= egas_ServiceLinesMember
Brainard [Member] | Mains [Member]  
Asset Retirement Obligations [Line Items]  
Regulatory Actions Estimated Useful Life Percentage Of Asset Cost Estimated Liability 20.00%egas_RegulatoryActionsEstimatedUsefulLifePercentageOfAssetCostEstimatedLiability
/ dei_LegalEntityAxis
= egas_BrainardMember
/ us-gaap_RegulatoryAssetAxis
= egas_MainsMember