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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Gross Receipts Taxes (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
Mar. 31, 2014
Schedule of Gross Receipts Taxes [Line Items]        
Gross Receipts taxes recorded in regulated gas distribution operating revenues $ 44.7lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues $ 34.3lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues $ 72.4lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues $ 54.3lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
Laclede Gas        
Schedule of Gross Receipts Taxes [Line Items]        
Gross Receipts taxes recorded in regulated gas distribution operating revenues 33.9lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember		 34.3lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember		 55.8lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember		 54.3lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Alagasco        
Schedule of Gross Receipts Taxes [Line Items]        
Gross Receipts taxes recorded in regulated gas distribution operating revenues $ 10.8lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember		 $ 13.0lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember		 $ 16.6lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember		 $ 19.4lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember