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CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Sep. 30, 2014
ASSETS    
Utility Plant $ 3,979.7us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution $ 3,928.3us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution
Less: Accumulated depreciation and amortization 1,195.3us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation 1,168.6us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
Net Utility Plant 2,784.4us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet 2,759.7us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
Non-utility property (net of accumulated depreciation and amortization, $6.9 and $6.7 at December 31, 2014 and September 30, 2014 respectively.) 9.5us-gaap_PropertyPlantAndEquipmentNet 9.2us-gaap_PropertyPlantAndEquipmentNet
Goodwill 937.8us-gaap_Goodwill 937.8us-gaap_Goodwill
Other Property and Investments 60.9lg_OtherInvestmentsOtherProperty 60.0lg_OtherInvestmentsOtherProperty
Total Other Property and Investments 1,008.2lg_OtherPropertyAndInvestments 1,007.0lg_OtherPropertyAndInvestments
Current Assets:    
Cash and cash equivalents 13.4us-gaap_CashAndCashEquivalentsAtCarryingValue 16.1us-gaap_CashAndCashEquivalentsAtCarryingValue
Accounts receivable:    
Utility 355.4us-gaap_AccountsReceivableGrossCurrent 148.2us-gaap_AccountsReceivableGrossCurrent
Other 76.6lg_OtherAccountsReceivable 86.5lg_OtherAccountsReceivable
Allowance for doubtful accounts (12.0)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent (15.9)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
Delayed customer billings 14.6us-gaap_UnbilledReceivablesCurrent 10.8us-gaap_UnbilledReceivablesCurrent
Inventories:    
Natural gas 219.5us-gaap_EnergyRelatedInventoryGasStoredUnderground 245.5us-gaap_EnergyRelatedInventoryGasStoredUnderground
Propane gas 11.9us-gaap_EnergyRelatedInventoryPropaneGas 11.7us-gaap_EnergyRelatedInventoryPropaneGas
Materials and supplies 14.1us-gaap_OtherInventoryPurchasedGoods 13.0us-gaap_OtherInventoryPurchasedGoods
Natural gas receivable 11.1lg_NaturalGasReceivable 7.3lg_NaturalGasReceivable
Derivative instrument assets 4.0us-gaap_DerivativeAssetsCurrent 2.4us-gaap_DerivativeAssetsCurrent
Unamortized purchased gas adjustments 60.6lg_UnamortizedPurchasedGasAdjustmentsAssets 54.0lg_UnamortizedPurchasedGasAdjustmentsAssets
Regulatory assets 87.2us-gaap_RegulatoryAssetsCurrent 80.8us-gaap_RegulatoryAssetsCurrent
Prepayments and other 46.9us-gaap_PrepaidExpenseAndOtherAssetsCurrent 48.4us-gaap_PrepaidExpenseAndOtherAssetsCurrent
Total Current Assets 816.1us-gaap_AssetsCurrent 628.0us-gaap_AssetsCurrent
Deferred Charges:    
Regulatory assets 615.8us-gaap_RegulatoryAssetsNoncurrent 614.3us-gaap_RegulatoryAssetsNoncurrent
Other 64.0us-gaap_OtherAssetsNoncurrent 65.0us-gaap_OtherAssetsNoncurrent
Total Deferred Charges 679.8us-gaap_DeferredCosts 679.3us-gaap_DeferredCosts
Total Assets 5,288.5us-gaap_Assets 5,074.0us-gaap_Assets
Capitalization:    
Common stock 43.3us-gaap_CommonStockValue 43.2us-gaap_CommonStockValue
Paid-in capital 1,031.0us-gaap_AdditionalPaidInCapital 1,029.4us-gaap_AdditionalPaidInCapital
Retained earnings 464.6us-gaap_RetainedEarningsAccumulatedDeficit 437.5us-gaap_RetainedEarningsAccumulatedDeficit
Accumulated other comprehensive loss (5.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (1.7)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Total Common Stock Equity 1,533.5us-gaap_StockholdersEquity 1,508.4us-gaap_StockholdersEquity
Long-term debt (less current portion) 1,736.3us-gaap_LongTermDebtNoncurrent 1,851.0us-gaap_LongTermDebtNoncurrent
Total Capitalization 3,269.8us-gaap_CapitalizationLongtermDebtAndEquity 3,359.4us-gaap_CapitalizationLongtermDebtAndEquity
Current Liabilities:    
Current portion of long-term debt 114.7us-gaap_LongTermDebtCurrent 0us-gaap_LongTermDebtCurrent
Notes payable 397.5us-gaap_NotesPayableCurrent 287.1us-gaap_NotesPayableCurrent
Accounts payable 221.1us-gaap_AccountsPayableCurrent 176.7us-gaap_AccountsPayableCurrent
Advance customer billings 30.9us-gaap_DeferredRevenueCurrent 32.2us-gaap_DeferredRevenueCurrent
Wages and compensation accrued 24.7us-gaap_EmployeeRelatedLiabilitiesCurrent 36.0us-gaap_EmployeeRelatedLiabilitiesCurrent
Dividends payable 20.9us-gaap_DividendsPayableCurrent 19.9us-gaap_DividendsPayableCurrent
Customer deposits 35.8us-gaap_CustomerDepositsCurrent 34.0us-gaap_CustomerDepositsCurrent
Interest accrued 19.9us-gaap_InterestPayableCurrent 15.1us-gaap_InterestPayableCurrent
Unamortized purchase gas adjustment 22.1lg_UnamortizedPurchasedGasAdjustments 22.4lg_UnamortizedPurchasedGasAdjustments
Taxes accrued 50.8us-gaap_TaxesPayableCurrent 63.4us-gaap_TaxesPayableCurrent
Regulatory liabilities 75.9us-gaap_RegulatoryLiabilityCurrent 63.7us-gaap_RegulatoryLiabilityCurrent
Deferred income taxes 21.1us-gaap_DeferredTaxLiabilitiesCurrent 9.9us-gaap_DeferredTaxLiabilitiesCurrent
Other 122.8us-gaap_OtherLiabilitiesCurrent 89.1us-gaap_OtherLiabilitiesCurrent
Total Current Liabilities 1,082.3us-gaap_LiabilitiesCurrent 785.8us-gaap_LiabilitiesCurrent
Deferred Credits and Other Liabilities:    
Deferred income taxes 394.6us-gaap_DeferredTaxLiabilitiesNoncurrent 383.8us-gaap_DeferredTaxLiabilitiesNoncurrent
Pension and postretirement benefit costs 241.7us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent 244.9us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
Asset retirement obligations 100.3us-gaap_AssetRetirementObligationsNoncurrent 99.2us-gaap_AssetRetirementObligationsNoncurrent
Regulatory liabilities 122.9us-gaap_RegulatoryLiabilityNoncurrent 125.8us-gaap_RegulatoryLiabilityNoncurrent
Other 76.9us-gaap_OtherLiabilitiesNoncurrent 75.1us-gaap_OtherLiabilitiesNoncurrent
Total Deferred Credits and Other Liabilities 936.4us-gaap_LiabilitiesNoncurrent 928.8us-gaap_LiabilitiesNoncurrent
Commitments and Contingencies      
Total Capitalization and Liabilities 5,288.5us-gaap_LiabilitiesAndStockholdersEquity 5,074.0us-gaap_LiabilitiesAndStockholdersEquity
Laclede Gas    
ASSETS    
Utility Plant 2,439.5us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
2,403.3us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Less: Accumulated depreciation and amortization 558.2us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
542.3us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Net Utility Plant 1,881.3us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
1,861.0us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Goodwill 210.2us-gaap_Goodwill
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
210.2us-gaap_Goodwill
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Other Property and Investments 56.6lg_OtherInvestmentsOtherProperty
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
55.7lg_OtherInvestmentsOtherProperty
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Other Property and Investments 266.8lg_OtherPropertyAndInvestments
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
265.9lg_OtherPropertyAndInvestments
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Current Assets:    
Cash and cash equivalents 5.9us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
3.7us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Accounts receivable:    
Utility 262.0us-gaap_AccountsReceivableGrossCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
111.1us-gaap_AccountsReceivableGrossCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Other 19.4lg_OtherAccountsReceivable
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
19.2lg_OtherAccountsReceivable
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Allowance for doubtful accounts (7.7)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
(10.7)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Delayed customer billings 14.6us-gaap_UnbilledReceivablesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
10.8us-gaap_UnbilledReceivablesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Receivables from associated companies 4.0us-gaap_DueFromRelatedPartiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
11.4us-gaap_DueFromRelatedPartiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Inventories:    
Natural gas 167.5us-gaap_EnergyRelatedInventoryGasStoredUnderground
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
191.1us-gaap_EnergyRelatedInventoryGasStoredUnderground
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Propane gas 12.0us-gaap_EnergyRelatedInventoryPropaneGas
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
11.7us-gaap_EnergyRelatedInventoryPropaneGas
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Materials and supplies 9.0us-gaap_OtherInventoryPurchasedGoods
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
7.8us-gaap_OtherInventoryPurchasedGoods
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Unamortized purchased gas adjustments 60.6lg_UnamortizedPurchasedGasAdjustmentsAssets
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
54.0lg_UnamortizedPurchasedGasAdjustmentsAssets
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Regulatory assets 78.6us-gaap_RegulatoryAssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
72.0us-gaap_RegulatoryAssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Prepayments and other 32.4us-gaap_PrepaidExpenseAndOtherAssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
33.5us-gaap_PrepaidExpenseAndOtherAssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Current Assets 579.7us-gaap_AssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
443.6us-gaap_AssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Deferred Charges:    
Regulatory assets 526.9us-gaap_RegulatoryAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
523.7us-gaap_RegulatoryAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Other 8.7us-gaap_OtherAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
10.8us-gaap_OtherAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Deferred Charges 535.6us-gaap_DeferredCosts
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
534.5us-gaap_DeferredCosts
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Assets 3,263.4us-gaap_Assets
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
3,105.0us-gaap_Assets
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Capitalization:    
Common stock 0.1us-gaap_CommonStockValue
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
0.1us-gaap_CommonStockValue
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Paid-in capital 745.3us-gaap_AdditionalPaidInCapital
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
744.0us-gaap_AdditionalPaidInCapital
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Retained earnings 284.7us-gaap_RetainedEarningsAccumulatedDeficit
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
265.6us-gaap_RetainedEarningsAccumulatedDeficit
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Accumulated other comprehensive loss (2.7)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
(1.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Common Stock Equity 1,027.4us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
1,007.8us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Long-term debt (less current portion) 808.0us-gaap_LongTermDebtNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
807.9us-gaap_LongTermDebtNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Capitalization 1,835.4us-gaap_CapitalizationLongtermDebtAndEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
1,815.7us-gaap_CapitalizationLongtermDebtAndEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Current Liabilities:    
Notes payable 319.5us-gaap_NotesPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
238.6us-gaap_NotesPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Accounts payable 107.3us-gaap_AccountsPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
70.1us-gaap_AccountsPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Accounts payable – associated companies 9.8us-gaap_AccountsPayableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
6.0us-gaap_AccountsPayableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Advance customer billings 13.2us-gaap_DeferredRevenueCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
15.5us-gaap_DeferredRevenueCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Wages and compensation accrued 19.5us-gaap_EmployeeRelatedLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
30.3us-gaap_EmployeeRelatedLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Dividends payable 19.9us-gaap_DividendsPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
19.0us-gaap_DividendsPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Customer deposits 15.2us-gaap_CustomerDepositsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
14.8us-gaap_CustomerDepositsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Interest accrued 9.7us-gaap_InterestPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
8.1us-gaap_InterestPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Taxes accrued 34.5us-gaap_TaxesPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
43.9us-gaap_TaxesPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Regulatory liabilities 0.6us-gaap_RegulatoryLiabilityCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
0.6us-gaap_RegulatoryLiabilityCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Deferred income taxes 20.9us-gaap_DeferredTaxLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
11.3us-gaap_DeferredTaxLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Other 51.9us-gaap_OtherLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
30.6us-gaap_OtherLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Current Liabilities 621.4us-gaap_LiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
488.2us-gaap_LiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Deferred Credits and Other Liabilities:    
Deferred income taxes 406.5us-gaap_DeferredTaxLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
399.8us-gaap_DeferredTaxLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Pension and postretirement benefit costs 211.1us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
215.3us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Asset retirement obligations 72.1us-gaap_AssetRetirementObligationsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
71.2us-gaap_AssetRetirementObligationsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Regulatory liabilities 70.9us-gaap_RegulatoryLiabilityNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
72.1us-gaap_RegulatoryLiabilityNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Other 46.0us-gaap_OtherLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
42.7us-gaap_OtherLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Deferred Credits and Other Liabilities 806.6us-gaap_LiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
801.1us-gaap_LiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Commitments and Contingencies      
Total Capitalization and Liabilities 3,263.4us-gaap_LiabilitiesAndStockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
3,105.0us-gaap_LiabilitiesAndStockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Alagasco    
ASSETS    
Utility Plant 1,540.1us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
1,525.1us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Less: Accumulated depreciation and amortization 637.0us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
626.4us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Net Utility Plant 903.1us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
898.7us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Current Assets:    
Cash and cash equivalents 3.3us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
5.6us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Accounts receivable:    
Utility 93.4us-gaap_AccountsReceivableGrossCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
39.0us-gaap_AccountsReceivableGrossCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Other 5.8lg_OtherAccountsReceivable
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
5.1lg_OtherAccountsReceivable
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Allowance for doubtful accounts (4.3)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
(5.1)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Inventories:    
Natural gas 43.6us-gaap_EnergyRelatedInventoryGasStoredUnderground
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
48.0us-gaap_EnergyRelatedInventoryGasStoredUnderground
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Materials and supplies 5.0us-gaap_OtherInventoryPurchasedGoods
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
5.1us-gaap_OtherInventoryPurchasedGoods
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Regulatory assets 8.6us-gaap_RegulatoryAssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
8.8us-gaap_RegulatoryAssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Deferred income taxes 2.3us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
2.3us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Prepayments and other 2.3us-gaap_PrepaidExpenseAndOtherAssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
1.6us-gaap_PrepaidExpenseAndOtherAssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Current Assets 160.0us-gaap_AssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
110.4us-gaap_AssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Deferred Charges:    
Regulatory assets 88.9us-gaap_RegulatoryAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
90.6us-gaap_RegulatoryAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Deferred income taxes 272.9us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
277.8us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Other 47.2us-gaap_OtherAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
47.1us-gaap_OtherAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Deferred Charges 409.0us-gaap_DeferredCosts
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
415.5us-gaap_DeferredCosts
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Assets 1,472.1us-gaap_Assets
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
1,424.6us-gaap_Assets
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Capitalization:    
Common stock 0us-gaap_CommonStockValue
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
0us-gaap_CommonStockValue
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Paid-in capital 503.9us-gaap_AdditionalPaidInCapital
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
503.9us-gaap_AdditionalPaidInCapital
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Retained earnings 356.3us-gaap_RetainedEarningsAccumulatedDeficit
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
345.7us-gaap_RetainedEarningsAccumulatedDeficit
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Common Stock Equity 860.2us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
849.6us-gaap_StockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Long-term debt (less current portion) 135.0us-gaap_LongTermDebtNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
249.8us-gaap_LongTermDebtNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Capitalization 995.2us-gaap_CapitalizationLongtermDebtAndEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
1,099.4us-gaap_CapitalizationLongtermDebtAndEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Current Liabilities:    
Current portion of long-term debt 114.7us-gaap_LongTermDebtCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
0us-gaap_LongTermDebtCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Notes payable 21.5us-gaap_NotesPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
16.0us-gaap_NotesPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Accounts payable 51.6us-gaap_AccountsPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
34.2us-gaap_AccountsPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Accounts payable – associated companies 1.2us-gaap_AccountsPayableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
0.4us-gaap_AccountsPayableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Advance customer billings 38.3us-gaap_DeferredRevenueCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
35.8us-gaap_DeferredRevenueCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Wages and compensation accrued 5.2us-gaap_EmployeeRelatedLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
5.7us-gaap_EmployeeRelatedLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Interest accrued 3.5us-gaap_InterestPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
3.9us-gaap_InterestPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Taxes accrued 28.4us-gaap_TaxesPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
30.0us-gaap_TaxesPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Regulatory liabilities 75.3us-gaap_RegulatoryLiabilityCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
63.1us-gaap_RegulatoryLiabilityCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Deferred income taxes 1.6us-gaap_DeferredTaxLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
0us-gaap_DeferredTaxLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Other 6.8us-gaap_OtherLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
6.8us-gaap_OtherLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Current Liabilities 348.1us-gaap_LiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
195.9us-gaap_LiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Deferred Credits and Other Liabilities:    
Pension and postretirement benefit costs 30.6us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
29.6us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Asset retirement obligations 28.1us-gaap_AssetRetirementObligationsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
27.7us-gaap_AssetRetirementObligationsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Regulatory liabilities 51.9us-gaap_RegulatoryLiabilityNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
53.7us-gaap_RegulatoryLiabilityNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Other 18.2us-gaap_OtherLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
18.3us-gaap_OtherLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Deferred Credits and Other Liabilities 128.8us-gaap_LiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
129.3us-gaap_LiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Capitalization and Liabilities $ 1,472.1us-gaap_LiabilitiesAndStockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
$ 1,424.6us-gaap_LiabilitiesAndStockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember