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COMMITMENTS AND CONTINGENCIES - Contingencies (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
site
Aug. 30, 2014
Dec. 31, 2013
Resident
Loss Contingencies [Line Items]      
Number of former manufactured gas plant (MGP) sites in Missouri 3lg_NumberOfFormerManufacturedGasPlantSitesInMissouri    
Number of former manufactured gas plant (MGP) sites in Shrewsbury, Missouri 1lg_NumberOfFormerManufacturedGasPlantSitesInShrewsburyMissouri    
Number of former manufactured gas plant (MGP) sites in the City of St. Louis 2lg_NumberOfFormerManufacturedGasPlantSitesInCityOfStLouis    
Minimum cost estimate for a one-time contractual transfer of risk   $ 8.1us-gaap_LossContingencyRangeOfPossibleLossMinimum  
Maximum cost estimate for a one-time contractual transfer of risk   $ 39.3us-gaap_LossContingencyRangeOfPossibleLossMaximum  
Alagasco      
Loss Contingencies [Line Items]      
Number of former manufactured gas plant (MGP) 9lg_SiteContingencyNumberofFormerManufacturedGasPlant
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
   
Number of former manufactured gas distribution 5lg_SiteContingencyNumberofFormerManufacturedGasDistribution
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
   
Previous Ownership | Missouri Gas Energy (MGE)      
Loss Contingencies [Line Items]      
Number of former manufactured gas plant (MGP) 7lg_SiteContingencyNumberofFormerManufacturedGasPlant
/ us-gaap_EnvironmentalRemediationSiteAxis
= lg_PreviousOwnershipMember
/ dei_LegalEntityAxis
= lg_MissouriGasEnergyMgeMember
   
Previous Ownership | Laclede Gas      
Loss Contingencies [Line Items]      
Number of former manufactured gas plant (MGP) 19lg_SiteContingencyNumberofFormerManufacturedGasPlant
/ us-gaap_EnvironmentalRemediationSiteAxis
= lg_PreviousOwnershipMember
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
   
Current Ownership | Alagasco      
Loss Contingencies [Line Items]      
Number of former manufactured gas plant (MGP) 4lg_SiteContingencyNumberofFormerManufacturedGasPlant
/ us-gaap_EnvironmentalRemediationSiteAxis
= lg_CurrentOwnershipMember
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
   
Number of former manufactured gas distribution 1lg_SiteContingencyNumberofFormerManufacturedGasDistribution
/ us-gaap_EnvironmentalRemediationSiteAxis
= lg_CurrentOwnershipMember
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
   
Removal Action, Phase I | 35th Avenue Superfund Site      
Loss Contingencies [Line Items]      
Number of residents at high levels of contamination     50lg_SiteContingencyNumberofResidentsContaminated
/ us-gaap_EnvironmentalRemediationContingencyAxis
= lg_RemovalActionPhaseIMember
/ us-gaap_EnvironmentalRemediationSiteAxis
= lg_A35thAvenueSuperfundSiteMember
Removal Action, Phase II | 35th Avenue Superfund Site      
Loss Contingencies [Line Items]      
Number of residents at high levels of contamination   30lg_SiteContingencyNumberofResidentsContaminated
/ us-gaap_EnvironmentalRemediationContingencyAxis
= lg_RemovalActionPhaseIIMember
/ us-gaap_EnvironmentalRemediationSiteAxis
= lg_A35thAvenueSuperfundSiteMember