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COMMITMENTS AND CONTINGENCIES - Commitments (Details) (USD $)
In Millions, unless otherwise specified
 6 Months Ended
Mar. 31, 2015
Long-term Purchase Commitment [Line Items]  
Minimum total payments required for natural gas contracts $ 1,595.0us-gaap_LongTermPurchaseCommitmentAmount
Laclede Gas  
Long-term Purchase Commitment [Line Items]  
Minimum total payments required for natural gas contracts 750.4us-gaap_LongTermPurchaseCommitmentAmount
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Alagasco  
Long-term Purchase Commitment [Line Items]  
Minimum total payments required for natural gas contracts $ 521.5us-gaap_LongTermPurchaseCommitmentAmount
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember