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Property, Plant and Equipment (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Property, Plant and Equipment [Line Items]    
Total Property, Plant and Equipment - Net $ 3,286.4us-gaap_PropertyPlantAndEquipmentNet $ 2,576.5us-gaap_PropertyPlantAndEquipmentNet
Asset Retirement Obligation [Roll Forward]    
Beginning Obligation 81.8us-gaap_AssetRetirementObligation [1] 77.9us-gaap_AssetRetirementObligation
Accretion 5.5us-gaap_AssetRetirementObligationAccretionExpense 4.6us-gaap_AssetRetirementObligationAccretionExpense
Liabilities recognized 23.0us-gaap_AssetRetirementObligationLiabilitiesIncurred [1]  
Liabilities settled (0.5)us-gaap_AssetRetirementObligationLiabilitiesSettled  
Revisions in estimated cash flows (0.6)us-gaap_AssetRetirementObligationRevisionOfEstimate (0.7)us-gaap_AssetRetirementObligationRevisionOfEstimate
Ending Obligation 109.2us-gaap_AssetRetirementObligation [1] 81.8us-gaap_AssetRetirementObligation [1]
Regulated Utility [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment 3,903.3us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember		 3,380.0us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
Construction Work in Progress 355.4us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember		 303.9us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
Accumulated Depreciation (1,260.2)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember		 (1,181.7)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
Property, Plant and Equipment - Net 2,998.5us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember		 2,502.2us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
Regulated Utility [Member] | Generation [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 5 years  
Regulated Utility [Member] | Generation [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 50 years  
Regulated Utility [Member] | Transmission [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 44 years  
Regulated Utility [Member] | Transmission [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 67 years  
Regulated Utility [Member] | Distribution [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 18 years  
Regulated Utility [Member] | Distribution [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 65 years  
Investments and Other Operations [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment 357.8us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember		 [2] 131.3us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember		 [2]
Construction Work in Progress 5.9us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember		 3.4us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
Accumulated Depreciation (75.8)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember		 (60.4)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
Property, Plant and Equipment - Net $ 287.9us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember		 $ 74.3us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
Investments and Other Operations [Member] | Investments and Other Operations [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 5 years  
Investments and Other Operations [Member] | Investments and Other Operations [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 44 years  
[1] The increase in 2014 is primarily related to BNI Coal for coal mining expansion and ALLETE Clean Energy due to wind energy facilities acquisitions.
[2] Includes primarily ALLETE Clean Energy, BNI Coal and a small amount of non-rate base generation.