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CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Sep. 30, 2014
ASSETS    
Utility Plant $ 4,037.8us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution $ 3,928.3us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution
Less: Accumulated depreciation and amortization 1,213.1us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation 1,168.6us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
Net Utility Plant 2,824.7us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet 2,759.7us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
Non-utility Property (net of accumulated depreciation and amortization, $7.2 and $6.7 at March 31, 2015 and September 30, 2014, respectively) 10.5us-gaap_PropertyPlantAndEquipmentNet 9.2us-gaap_PropertyPlantAndEquipmentNet
Goodwill 946.0us-gaap_Goodwill 937.8us-gaap_Goodwill
Other Property and Investments 60.8lg_OtherInvestmentsOtherProperty 60.0lg_OtherInvestmentsOtherProperty
Total Other Property and Investments 1,017.3lg_OtherPropertyAndInvestments 1,007.0lg_OtherPropertyAndInvestments
Current Assets:    
Cash and cash equivalents 46.9us-gaap_CashAndCashEquivalentsAtCarryingValue 16.1us-gaap_CashAndCashEquivalentsAtCarryingValue
Accounts receivable:    
Utility 303.8us-gaap_AccountsReceivableGrossCurrent 148.2us-gaap_AccountsReceivableGrossCurrent
Other 70.1lg_OtherAccountsReceivable 86.5lg_OtherAccountsReceivable
Allowance for doubtful accounts (18.6)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent (15.9)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
Delayed customer billings 61.7us-gaap_UnbilledReceivablesCurrent 10.8us-gaap_UnbilledReceivablesCurrent
Inventories:    
Natural gas 90.8us-gaap_EnergyRelatedInventoryGasStoredUnderground 245.5us-gaap_EnergyRelatedInventoryGasStoredUnderground
Propane gas 11.9us-gaap_EnergyRelatedInventoryPropaneGas 11.7us-gaap_EnergyRelatedInventoryPropaneGas
Materials and supplies 14.3us-gaap_OtherInventoryPurchasedGoods 13.0us-gaap_OtherInventoryPurchasedGoods
Natural gas receivable 5.9lg_NaturalGasReceivable 7.3lg_NaturalGasReceivable
Derivative instrument assets 3.0us-gaap_DerivativeAssetsCurrent 2.4us-gaap_DerivativeAssetsCurrent
Unamortized purchased gas adjustments 0lg_UnamortizedPurchasedGasAdjustmentsAssets 54.0lg_UnamortizedPurchasedGasAdjustmentsAssets
Regulatory assets 27.4us-gaap_RegulatoryAssetsCurrent 26.8us-gaap_RegulatoryAssetsCurrent
Prepayments and other 20.0us-gaap_PrepaidExpenseAndOtherAssetsCurrent 21.6us-gaap_PrepaidExpenseAndOtherAssetsCurrent
Total Current Assets 637.2us-gaap_AssetsCurrent 628.0us-gaap_AssetsCurrent
Deferred Charges:    
Regulatory assets 632.9us-gaap_RegulatoryAssetsNoncurrent 614.3us-gaap_RegulatoryAssetsNoncurrent
Other 68.6us-gaap_OtherAssetsNoncurrent 65.0us-gaap_OtherAssetsNoncurrent
Total Deferred Charges 701.5us-gaap_DeferredCosts 679.3us-gaap_DeferredCosts
Total Assets 5,180.7us-gaap_Assets 5,074.0us-gaap_Assets
Capitalization:    
Common stock 43.3us-gaap_CommonStockValue 43.2us-gaap_CommonStockValue
Paid-in capital 1,033.3us-gaap_AdditionalPaidInCapital 1,029.4us-gaap_AdditionalPaidInCapital
Retained earnings 539.2us-gaap_RetainedEarningsAccumulatedDeficit 437.5us-gaap_RetainedEarningsAccumulatedDeficit
Accumulated other comprehensive loss (4.2)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (1.7)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Total Common Stock Equity 1,611.6us-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 3,347.9us-gaap_CapitalizationLongtermDebtAndEquity 3,359.4us-gaap_CapitalizationLongtermDebtAndEquity
Current Liabilities:    
Current portion of long-term debt 80.0us-gaap_LongTermDebtCurrent 0us-gaap_LongTermDebtCurrent
Notes payable 247.6us-gaap_NotesPayableCurrent 287.1us-gaap_NotesPayableCurrent
Accounts payable 160.1us-gaap_AccountsPayableCurrent 176.7us-gaap_AccountsPayableCurrent
Advance customer billings 8.4us-gaap_DeferredRevenueCurrent 32.2us-gaap_DeferredRevenueCurrent
Wages and compensation accrued 30.9us-gaap_EmployeeRelatedLiabilitiesCurrent 36.0us-gaap_EmployeeRelatedLiabilitiesCurrent
Dividends payable 20.5us-gaap_DividendsPayableCurrent 19.9us-gaap_DividendsPayableCurrent
Customer deposits 36.2us-gaap_CustomerDepositsCurrent 34.0us-gaap_CustomerDepositsCurrent
Interest accrued 14.9us-gaap_InterestPayableCurrent 15.1us-gaap_InterestPayableCurrent
Unamortized purchased gas adjustments 76.7lg_UnamortizedPurchasedGasAdjustments 22.4lg_UnamortizedPurchasedGasAdjustments
Taxes accrued 85.2us-gaap_TaxesPayableCurrent 63.4us-gaap_TaxesPayableCurrent
Deferred income taxes 0us-gaap_DeferredTaxLiabilitiesCurrent 9.9us-gaap_DeferredTaxLiabilitiesCurrent
Regulatory liabilities 38.5us-gaap_RegulatoryLiabilityCurrent 41.3us-gaap_RegulatoryLiabilityCurrent
Other 54.7us-gaap_OtherLiabilitiesCurrent 47.8us-gaap_OtherLiabilitiesCurrent
Total Current Liabilities 853.7us-gaap_LiabilitiesCurrent 785.8us-gaap_LiabilitiesCurrent
Deferred Credits and Other Liabilities:    
Deferred income taxes 438.7us-gaap_DeferredTaxLiabilitiesNoncurrent 383.8us-gaap_DeferredTaxLiabilitiesNoncurrent
Pension and postretirement benefit costs 243.9us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent 244.9us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
Asset retirement obligations 101.5us-gaap_AssetRetirementObligationsNoncurrent 99.2us-gaap_AssetRetirementObligationsNoncurrent
Regulatory liabilities 114.3us-gaap_RegulatoryLiabilityNoncurrent 125.8us-gaap_RegulatoryLiabilityNoncurrent
Other 80.7us-gaap_OtherLiabilitiesNoncurrent 75.1us-gaap_OtherLiabilitiesNoncurrent
Total Deferred Credits and Other Liabilities 979.1us-gaap_LiabilitiesNoncurrent 928.8us-gaap_LiabilitiesNoncurrent
Commitments and Contingencies      
Total Capitalization and Liabilities 5,180.7us-gaap_LiabilitiesAndStockholdersEquity 5,074.0us-gaap_LiabilitiesAndStockholdersEquity
Laclede Gas    
ASSETS    
Utility Plant 2,479.8us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
2,403.3us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Less: Accumulated depreciation and amortization 566.2us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
542.3us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Net Utility Plant 1,913.6us-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 56.5lg_OtherInvestmentsOtherProperty
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
55.7lg_OtherInvestmentsOtherProperty
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Other Property and Investments 266.7lg_OtherPropertyAndInvestments
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
265.9lg_OtherPropertyAndInvestments
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Current Assets:    
Cash and cash equivalents 9.3us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
3.7us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Accounts receivable:    
Utility 224.1us-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 (14.0)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
(10.7)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Delayed customer billings 61.7us-gaap_UnbilledReceivablesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
10.8us-gaap_UnbilledReceivablesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Receivables from associated companies 2.3us-gaap_DueFromRelatedPartiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
11.4us-gaap_DueFromRelatedPartiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Inventories:    
Natural gas 60.8us-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 9.2us-gaap_OtherInventoryPurchasedGoods
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
7.8us-gaap_OtherInventoryPurchasedGoods
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Unamortized purchased gas adjustments 0lg_UnamortizedPurchasedGasAdjustmentsAssets
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
54.0lg_UnamortizedPurchasedGasAdjustmentsAssets
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Regulatory assets 17.4us-gaap_RegulatoryAssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
18.0us-gaap_RegulatoryAssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Prepayments and other 16.2us-gaap_PrepaidExpenseAndOtherAssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
15.5us-gaap_PrepaidExpenseAndOtherAssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Current Assets 418.4us-gaap_AssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
443.6us-gaap_AssetsCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Deferred Charges:    
Regulatory assets 545.3us-gaap_RegulatoryAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
523.7us-gaap_RegulatoryAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Other 11.9us-gaap_OtherAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
10.8us-gaap_OtherAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Deferred Charges 557.2us-gaap_DeferredCosts
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
534.5us-gaap_DeferredCosts
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Assets 3,155.9us-gaap_Assets
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
3,105.0us-gaap_Assets
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Capitalization:    
Common stock and Paid-in capital 746.3lg_CommonStockIssuedandAdditionalPaidinCapital
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
744.1lg_CommonStockIssuedandAdditionalPaidinCapital
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Retained earnings 314.7us-gaap_RetainedEarningsAccumulatedDeficit
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
265.6us-gaap_RetainedEarningsAccumulatedDeficit
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Accumulated other comprehensive loss (2.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
(1.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Common Stock Equity 1,058.6us-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 1,866.6us-gaap_CapitalizationLongtermDebtAndEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
1,815.7us-gaap_CapitalizationLongtermDebtAndEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Current Liabilities:    
Notes payable 142.6us-gaap_NotesPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
238.6us-gaap_NotesPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Notes payable – associated companies 10.4us-gaap_NotesPayableRelatedPartiesClassifiedCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
0us-gaap_NotesPayableRelatedPartiesClassifiedCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Accounts payable 77.3us-gaap_AccountsPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
70.1us-gaap_AccountsPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Accounts payable – associated companies 5.7us-gaap_AccountsPayableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
6.0us-gaap_AccountsPayableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Advance customer billings 0us-gaap_DeferredRevenueCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
15.5us-gaap_DeferredRevenueCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Wages and compensation accrued 26.9us-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 8.1us-gaap_InterestPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
8.1us-gaap_InterestPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Unamortized purchased gas adjustments 39.1lg_UnamortizedPurchasedGasAdjustments
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
0lg_UnamortizedPurchasedGasAdjustments
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Taxes accrued 79.2us-gaap_TaxesPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
43.9us-gaap_TaxesPayableCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Other 36.3us-gaap_OtherLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
41.9us-gaap_OtherLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Current Liabilities 460.7us-gaap_LiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
488.2us-gaap_LiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Deferred Credits and Other Liabilities:    
Deferred income taxes 424.3us-gaap_DeferredTaxLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
399.8us-gaap_DeferredTaxLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Pension and postretirement benefit costs 212.4us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
215.3us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Asset retirement obligations 72.9us-gaap_AssetRetirementObligationsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
71.2us-gaap_AssetRetirementObligationsNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Regulatory liabilities 71.7us-gaap_RegulatoryLiabilityNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
72.1us-gaap_RegulatoryLiabilityNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Other 47.3us-gaap_OtherLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
42.7us-gaap_OtherLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Total Deferred Credits and Other Liabilities 828.6us-gaap_LiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
801.1us-gaap_LiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Commitments and Contingencies      
Total Capitalization and Liabilities 3,155.9us-gaap_LiabilitiesAndStockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
3,105.0us-gaap_LiabilitiesAndStockholdersEquity
/ dei_LegalEntityAxis
= lg_LacledeGasCompanyMember
Alagasco    
ASSETS    
Utility Plant 1,558.0us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
1,525.1us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Less: Accumulated depreciation and amortization 646.9us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
626.4us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Net Utility Plant 911.1us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
898.7us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Current Assets:    
Cash and cash equivalents 35.4us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
5.6us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Accounts receivable:    
Utility 79.6us-gaap_AccountsReceivableGrossCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
39.0us-gaap_AccountsReceivableGrossCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Other 5.2lg_OtherAccountsReceivable
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
5.1lg_OtherAccountsReceivable
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Allowance for doubtful accounts (4.6)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
(5.1)us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Inventories:    
Natural gas 27.6us-gaap_EnergyRelatedInventoryGasStoredUnderground
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
48.0us-gaap_EnergyRelatedInventoryGasStoredUnderground
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Materials and supplies 4.9us-gaap_OtherInventoryPurchasedGoods
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
5.1us-gaap_OtherInventoryPurchasedGoods
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Regulatory assets 10.0us-gaap_RegulatoryAssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
8.8us-gaap_RegulatoryAssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Deferred income taxes 0us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
2.3us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Prepayments and other 3.3us-gaap_PrepaidExpenseAndOtherAssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
1.6us-gaap_PrepaidExpenseAndOtherAssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Current Assets 161.4us-gaap_AssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
110.4us-gaap_AssetsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Deferred Charges:    
Regulatory assets 87.6us-gaap_RegulatoryAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
90.6us-gaap_RegulatoryAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Deferred income taxes 249.1us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
277.8us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Other 50.4us-gaap_OtherAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
47.1us-gaap_OtherAssetsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Deferred Charges 387.1us-gaap_DeferredCosts
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
415.5us-gaap_DeferredCosts
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Assets 1,459.6us-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 506.3us-gaap_AdditionalPaidInCapital
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
503.9us-gaap_AdditionalPaidInCapital
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Retained earnings 402.6us-gaap_RetainedEarningsAccumulatedDeficit
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
345.7us-gaap_RetainedEarningsAccumulatedDeficit
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Common Stock Equity 908.9us-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 1,043.9us-gaap_CapitalizationLongtermDebtAndEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
1,099.4us-gaap_CapitalizationLongtermDebtAndEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Current Liabilities:    
Current portion of long-term debt 80.0us-gaap_LongTermDebtCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
0us-gaap_LongTermDebtCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Notes payable 25.0us-gaap_NotesPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
16.0us-gaap_NotesPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Accounts payable 37.9us-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 8.4us-gaap_DeferredRevenueCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
16.7us-gaap_DeferredRevenueCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Wages and compensation accrued 4.0us-gaap_EmployeeRelatedLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
5.7us-gaap_EmployeeRelatedLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Customer deposits 21.0us-gaap_CustomerDepositsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
19.1us-gaap_CustomerDepositsCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Interest accrued 3.6us-gaap_InterestPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
3.9us-gaap_InterestPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Unamortized purchased gas adjustments 37.6lg_UnamortizedPurchasedGasAdjustments
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
22.4lg_UnamortizedPurchasedGasAdjustments
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Taxes accrued 23.0us-gaap_TaxesPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
30.0us-gaap_TaxesPayableCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Regulatory liabilities 37.9us-gaap_RegulatoryLiabilityCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
40.7us-gaap_RegulatoryLiabilityCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Other 11.1us-gaap_OtherLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
6.8us-gaap_OtherLiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Current Liabilities 290.7us-gaap_LiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
195.9us-gaap_LiabilitiesCurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Deferred Credits and Other Liabilities:    
Pension and postretirement benefit costs 31.5us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
29.6us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Asset retirement obligations 28.4us-gaap_AssetRetirementObligationsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
27.7us-gaap_AssetRetirementObligationsNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Regulatory liabilities 42.8us-gaap_RegulatoryLiabilityNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
53.7us-gaap_RegulatoryLiabilityNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Other 22.3us-gaap_OtherLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
18.3us-gaap_OtherLiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Deferred Credits and Other Liabilities 125.0us-gaap_LiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
129.3us-gaap_LiabilitiesNoncurrent
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
Total Capitalization and Liabilities $ 1,459.6us-gaap_LiabilitiesAndStockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember
$ 1,424.6us-gaap_LiabilitiesAndStockholdersEquity
/ dei_LegalEntityAxis
= lg_AlabamaGasCorporationMember