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Regulatory Matters - Regulatory Assets and Liabilities (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets and Liabilities Currently Earning a Return No regulatory assets or liabilities are currently earning a return.  
Current Regulatory Assets $ 15.6us-gaap_RegulatoryAssetsCurrent $ 16.3us-gaap_RegulatoryAssetsCurrent
Non-Current Regulatory Assets 358.0us-gaap_RegulatoryAssetsNoncurrent 357.3us-gaap_RegulatoryAssetsNoncurrent
Total Regulatory Assets 373.6us-gaap_RegulatoryAssets 373.6us-gaap_RegulatoryAssets
Non-Current Regulatory Liabilities 101.2us-gaap_RegulatoryLiabilityNoncurrent 94.2us-gaap_RegulatoryLiabilityNoncurrent
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 13.5us-gaap_RegulatoryLiabilityNoncurrent
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_DeferredIncomeTaxChargesMember		 13.4us-gaap_RegulatoryLiabilityNoncurrent
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_DeferredIncomeTaxChargesMember
Plant Removal Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 22.0us-gaap_RegulatoryLiabilityNoncurrent
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_RemovalCostsMember		 22.8us-gaap_RegulatoryLiabilityNoncurrent
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_RemovalCostsMember
Wholesale and Retail Contra AFUDC [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 46.9us-gaap_RegulatoryLiabilityNoncurrent
/ us-gaap_RegulatoryLiabilityAxis
= ale_WholesaleAndRetailContraAfudcMember		 42.9us-gaap_RegulatoryLiabilityNoncurrent
/ us-gaap_RegulatoryLiabilityAxis
= ale_WholesaleAndRetailContraAfudcMember
Deferred Fuel [Member]    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Assets 15.6us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredFuelCostsMember		 [1] 16.3us-gaap_RegulatoryAssetsCurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredFuelCostsMember		 [1]
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 220.0us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_PensionAndOtherPostretirementPlansCostsMember		 [2] 223.9us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_PensionAndOtherPostretirementPlansCostsMember		 [2]
Non-Current Regulatory Liabilities 2.9us-gaap_RegulatoryLiabilityNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_PensionAndOtherPostretirementPlansCostsMember		 [2] 3.5us-gaap_RegulatoryLiabilityNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_PensionAndOtherPostretirementPlansCostsMember		 [2]
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 46.8us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredIncomeTaxChargesMember		 46.6us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredIncomeTaxChargesMember
Asset Retirement Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 18.6us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_AssetRetirementObligationCostsMember		 17.8us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_AssetRetirementObligationCostsMember
Cost Recovery Riders [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 63.4us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= ale_CurrentCostRecoveryRiderMember		 [3] 59.7us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= ale_CurrentCostRecoveryRiderMember		 [3]
PPACA Income Tax Deferral [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 5.0us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= ale_MedicarePartDRegulatoryAssetMember		 5.0us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= ale_MedicarePartDRegulatoryAssetMember
Other    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 4.2us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember		 4.3us-gaap_RegulatoryAssetsNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember
Non-Current Regulatory Liabilities $ 15.9us-gaap_RegulatoryLiabilityNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember		 $ 11.6us-gaap_RegulatoryLiabilityNoncurrent
/ us-gaap_RegulatoryAssetAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember
[1] Current regulatory assets are included in Prepayments and Other on the Consolidated Balance Sheet.
[2] Defined benefit pension and other postretirement items included in our Regulated Operations, which are otherwise required to be recognized in accumulated other comprehensive income, are recognized as regulatory assets or regulatory liabilities on the Consolidated Balance Sheet. (See Note 15. Pension and Other Postretirement Benefit Plans.)
[3] The cost recovery rider regulatory assets are due to capital expenditures related to our Bison Wind Energy Center, investment in CapX2020 projects, and the Boswell Unit 4 environmental upgrade and are recognized in accordance with the accounting standards for alternative revenue programs.