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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Sep. 30, 2014
Sep. 30, 2013
Related Party Transaction [Line Items]        
Number of operating segments 3us-gaap_NumberOfOperatingSegments      
Accrued unbilled revenues $ 102.2us-gaap_AccountsReceivableNetCurrent   $ 29.4us-gaap_AccountsReceivableNetCurrent  
Gross Receipts taxes recorded in regulated gas distribution operating revenues 27.7lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues 19.9lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues    
Goodwill 937.8us-gaap_Goodwill   937.8us-gaap_Goodwill  
Stockholders' Equity 1,533.5us-gaap_StockholdersEquity 1,066.4us-gaap_StockholdersEquity 1,508.4us-gaap_StockholdersEquity 1,046.3us-gaap_StockholdersEquity
Laclede Gas        
Related Party Transaction [Line Items]        
Gross Receipts taxes recorded in regulated gas distribution operating revenues 21.9lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
19.9lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
   
Goodwill 210.2us-gaap_Goodwill
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
  210.2us-gaap_Goodwill
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
 
Accruals for capital expenditures 2.5us-gaap_CapitalExpendituresIncurredButNotYetPaid
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
  3.0us-gaap_CapitalExpendituresIncurredButNotYetPaid
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
 
Stockholders' Equity 1,027.4us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
996.0us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
1,007.8us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
973.9us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Alagasco        
Related Party Transaction [Line Items]        
Gross Receipts taxes recorded in regulated gas distribution operating revenues 5.8lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
6.4lg_GrossReceiptsTaxAmountsIncludedOperatingRevenues
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
   
Accruals for capital expenditures 4.8us-gaap_CapitalExpendituresIncurredButNotYetPaid
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
  5.0us-gaap_CapitalExpendituresIncurredButNotYetPaid
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
 
Stockholders' Equity 860.2us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
384.6us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
849.6us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
364.8us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Missouri Gas Energy (MGE)        
Related Party Transaction [Line Items]        
Goodwill 210.2us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= lg_MissouriGasEnergyMgeMember
  210.2us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= lg_MissouriGasEnergyMgeMember
 
Missouri Gas Energy (MGE) | Laclede Gas        
Related Party Transaction [Line Items]        
Goodwill 210.2us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= lg_MissouriGasEnergyMgeMember
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
  210.2us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= lg_MissouriGasEnergyMgeMember
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
 
Alagasco        
Related Party Transaction [Line Items]        
Goodwill 727.6us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= lg_AlabamaGasCorporationMember
  727.6us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= lg_AlabamaGasCorporationMember
 
Alagasco | Laclede Gas        
Related Party Transaction [Line Items]        
Goodwill 0us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= lg_AlabamaGasCorporationMember
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
  0us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= lg_AlabamaGasCorporationMember
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
 
Affiliated Entity | Gas Utility        
Related Party Transaction [Line Items]        
Related party costs and expenses 0.3us-gaap_CostsAndExpensesRelatedParty
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember
/ us-gaap_StatementBusinessSegmentsAxis
= lg_GasUtilityMember
0.3us-gaap_CostsAndExpensesRelatedParty
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember
/ us-gaap_StatementBusinessSegmentsAxis
= lg_GasUtilityMember
   
Regulated Operation | Affiliated Entity | Gas Utility        
Related Party Transaction [Line Items]        
Revenue from related parties 25.3us-gaap_RevenueFromRelatedParties
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember
/ us-gaap_StatementBusinessSegmentsAxis
= lg_GasUtilityMember
19.5us-gaap_RevenueFromRelatedParties
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember
/ us-gaap_StatementBusinessSegmentsAxis
= lg_GasUtilityMember
   
Unregulated Operation | Affiliated Entity | Gas Marketing        
Related Party Transaction [Line Items]        
Revenue from related parties 1.0us-gaap_RevenueFromRelatedParties
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember
/ us-gaap_StatementBusinessSegmentsAxis
= lg_GasMarketingMember
0.1us-gaap_RevenueFromRelatedParties
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember
/ us-gaap_StatementBusinessSegmentsAxis
= lg_GasMarketingMember
   
Common Stock Issued        
Related Party Transaction [Line Items]        
Stockholders' Equity 43.3us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
32.8us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
43.2us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
32.7us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
Common Stock Issued | Laclede Gas        
Related Party Transaction [Line Items]        
Stockholders' Equity 0.1us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
0.1us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
0.1us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
0.1us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
Common Stock Issued | Alagasco        
Related Party Transaction [Line Items]        
Stockholders' Equity 0us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
0us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
0us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
0us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
Common Stock Issued | Restatement Adjustment | Alagasco        
Related Party Transaction [Line Items]        
Stockholders' Equity 31.7us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
/ us-gaap_StatementScenarioAxis
= us-gaap_RestatementAdjustmentMember
     
Paid-in Capital        
Related Party Transaction [Line Items]        
Stockholders' Equity 1,031.0us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
594.8us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
1,029.4us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
594.3us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
Paid-in Capital | Laclede Gas        
Related Party Transaction [Line Items]        
Stockholders' Equity 745.3us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
739.3us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
744.0us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
738.1us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
Paid-in Capital | Alagasco        
Related Party Transaction [Line Items]        
Stockholders' Equity 503.9us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
34.5us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
503.9us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
34.5us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
Paid-in Capital | Restatement Adjustment | Alagasco        
Related Party Transaction [Line Items]        
Stockholders' Equity $ 31.7us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
/ us-gaap_StatementScenarioAxis
= us-gaap_RestatementAdjustmentMember