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COMMITMENTS AND CONTINGENCIES - Commitments (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Dec. 31, 2014
Long-term Purchase Commitment [Line Items]  
Minimum total payments required for natural gas contracts $ 1,607.9us-gaap_LongTermPurchaseCommitmentAmount
Laclede Gas  
Long-term Purchase Commitment [Line Items]  
Minimum total payments required for natural gas contracts 783.4us-gaap_LongTermPurchaseCommitmentAmount
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Alagasco  
Long-term Purchase Commitment [Line Items]  
Minimum total payments required for natural gas contracts $ 533.1us-gaap_LongTermPurchaseCommitmentAmount
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember