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Short-Term and Long-Term Debt (Details) (USD $)
In Millions, unless otherwise specified
9 Months Ended 0 Months Ended
Sep. 30, 2014
Mar. 04, 2014
Jun. 26, 2014
Sep. 16, 2014
Dec. 31, 2014
Jan. 30, 2014
Dec. 31, 2013
Debt Instrument [Line Items]              
Short-Term Debt         $ 104.4us-gaap_DebtCurrent   $ 27.2us-gaap_DebtCurrent
Lines of Credit, Current         47.5us-gaap_LinesOfCreditCurrent   5.4us-gaap_LinesOfCreditCurrent
Letters of Credit Drawn         3.7ale_LettersofCreditDrawn   0ale_LettersofCreditDrawn
Long-Term Debt         1,272.8us-gaap_LongTermDebtNoncurrent   1,083.0us-gaap_LongTermDebtNoncurrent
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Long-term Debt           23.3ale_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedLongtermDebt  
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt           2.2us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesLongTermDebt  
Long-Term Debt Due Within One Year         100.7us-gaap_LongTermDebtCurrent   27.2us-gaap_LongTermDebtCurrent
Long-Term Debt, 2016 Maturities         24.9us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo    
Long-Term Debt, 2017 Maturities         54.6us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree    
Long-Term Debt, 2018 Maturities         54.8us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour    
Long-Term Debt, 2019 Maturities         46.9us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive    
Long-Term Debt, Thereafter Maturities         1,091.6us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive    
Proceeds from Issuance of First Mortgage Bond 375.0us-gaap_ProceedsFromIssuanceOfFirstMortgageBond            
Lines of Credit [Member]              
Debt Instrument [Line Items]              
Line of Credit Facility, Maximum Borrowing Capacity         408.4us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= ale_ConsolidatedLinesOfCreditMemberMember
  406.4us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= ale_ConsolidatedLinesOfCreditMemberMember
ALLETE Bonds 3.69% Due March2024 [Member]              
Debt Instrument [Line Items]              
Proceeds from Issuance of First Mortgage Bond   60.0us-gaap_ProceedsFromIssuanceOfFirstMortgageBond
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds3.69DueMarch2024Member
         
First Mortgage Bonds, Interest Rate   3.69%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds3.69DueMarch2024Member
         
ALLETE Bonds 4.95% Due March2044 [Member]              
Debt Instrument [Line Items]              
Proceeds from Issuance of First Mortgage Bond   40.0us-gaap_ProceedsFromIssuanceOfFirstMortgageBond
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds4.95DueMarch2044Member
         
First Mortgage Bonds, Interest Rate   4.95%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds4.95DueMarch2044Member
         
ALLETE Bonds 3.40% Due July2022 [Member]              
Debt Instrument [Line Items]              
Proceeds from Issuance of First Mortgage Bond     75.0us-gaap_ProceedsFromIssuanceOfFirstMortgageBond
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds3.40DueJuly2022Member
       
First Mortgage Bonds, Interest Rate     3.40%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds3.40DueJuly2022Member
       
ALLETE Bonds 5.05% Due July2044 [Member]              
Debt Instrument [Line Items]              
Proceeds from Issuance of First Mortgage Bond     40.0us-gaap_ProceedsFromIssuanceOfFirstMortgageBond
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds5.05DueJuly2044Member
       
First Mortgage Bonds, Interest Rate     5.05%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds5.05DueJuly2044Member
       
ALLETE Bonds 3.02% Due September 2021 [Member]              
Debt Instrument [Line Items]              
Proceeds from Issuance of First Mortgage Bond       60.0us-gaap_ProceedsFromIssuanceOfFirstMortgageBond
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds3.02DueSeptember2021Member
     
First Mortgage Bonds, Interest Rate       3.02%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds3.02DueSeptember2021Member
     
ALLETE Bonds 3.74% Due September 2029 [Member]              
Debt Instrument [Line Items]              
Proceeds from Issuance of First Mortgage Bond       50.0us-gaap_ProceedsFromIssuanceOfFirstMortgageBond
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds3.74DueSeptember2029Member
     
First Mortgage Bonds, Interest Rate       3.74%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds3.74DueSeptember2029Member
     
ALLETE Bonds 4.39% Due September 2044 [Member]              
Debt Instrument [Line Items]              
Proceeds from Issuance of First Mortgage Bond       $ 50.0us-gaap_ProceedsFromIssuanceOfFirstMortgageBond
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds4.39DueSeptember2044Member
     
First Mortgage Bonds, Interest Rate       4.39%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ale_ALLETEBonds4.39DueSeptember2044Member