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Short-term and Long-term Debt - Schedule of Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Debt Instrument [Line Items] |  |
Total Long-term Debt | $ 1,373.5 us-gaap_LongTermDebt | $ 1,110.2 us-gaap_LongTermDebt
Less: Due Within One Year | 100.7 us-gaap_LongTermDebtCurrent | 27.2 us-gaap_LongTermDebtCurrent
Net Long-Term Debt | 1,272.8 us-gaap_LongTermDebtNoncurrent | 1,083.0 us-gaap_LongTermDebtNoncurrent
6.94% Series Due 2014 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A694SeriesDue2014Member | 18.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A694SeriesDue2014Member
7.70% Series Due 2016 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 20.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A770SeriesDue2016Member | 20.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A770SeriesDue2016Member
1.83% Series Due 2018 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_AlleteBonds183DueApril2018Member | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_AlleteBonds183DueApril2018Member
8.17% Series Due 2019 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 42.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A817SeriesDue2019Member | 42.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A817SeriesDue2019Member
5.28% Series Due 2020 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 35.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A528SeriesDue2020Member | 35.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A528SeriesDue2020Member
4.85% Series Due 2021 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A485SeriesDue2021Member | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A485SeriesDue2021Member
3.02% Series Due 2021 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 60.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds3.02DueSeptember2021Member | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds3.02DueSeptember2021Member
3.40% Series Due 2022 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 75.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds3.40DueJuly2022Member | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds3.40DueJuly2022Member
4.95% Pollution Control Series F Due 2022 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A495PollutionControlSeriesFDue2022Member | 111.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A495PollutionControlSeriesFDue2022Member
6.02% Series Due 2023 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 75.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A602SeriesDue2023Member | 75.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A602SeriesDue2023Member
3.69% Series Due 2024 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 60.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds3.69DueMarch2024Member | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds3.69DueMarch2024Member
4.90% Series Due 2025 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 30.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A490SeriesDue2025Member | 30.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A490SeriesDue2025Member
5.10% Series Due 2025 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 30.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A510SeriesDue2025Member | 30.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A510SeriesDue2025Member
3.20% Series Due 2026 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 75.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_AlleteBonds320DueJuly2026Member | 75.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_AlleteBonds320DueJuly2026Member
5.99% Series Due 2027 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 60.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A599SeriesDue2027Member | 60.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A599SeriesDue2027Member
3.30% Series Due 2028 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 40.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_AlleteBonds330DueOctober2028Member | 40.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_AlleteBonds330DueOctober2028Member
3.74% Series Due 2029 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds3.74DueSeptember2029Member | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds3.74DueSeptember2029Member
5.69% Series Due 2036 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A569SeriesDue2036Member | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A569SeriesDue2036Member
6.00% Series Due 2040 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 35.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A600SeriesDue2040Member | 35.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A600SeriesDue2040Member
5.82% Series Due 2040 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 45.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A582SeriesDue2040Member | 45.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A582SeriesDue2040Member
4.08% Series Due 2042 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 85.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_AlleteBonds408DueJuly2042Member | 85.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_AlleteBonds408DueJuly2042Member
4.21% Series Due 2043 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 60.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_AlleteBonds421DueOctober2043Member | 60.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_AlleteBonds421DueOctober2043Member
4.95% Series Due 2044 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 40.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds4.95DueMarch2044Member | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds4.95DueMarch2044Member
5.05% Series Due 2044 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 40.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds5.05DueJuly2044Member | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds5.05DueJuly2044Member
4.39% Series Due 2044 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds4.39DueSeptember2044Member | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEBonds4.39DueSeptember2044Member
Unsecured Term Loan Variable Rate Due 2015 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 75.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEAmendedTermLoanVariableRateDueAugust2015Member | 75.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_ALLETEAmendedTermLoanVariableRateDueAugust2015Member
Senior Unsecured Notes 5.99% Due 2017 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_SeniorUnsecuredNotes599Due2017Member | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_SeniorUnsecuredNotes599Due2017Member
Variable Demand Revenue Refunding Bonds Series 1997 A Due 2015-2020 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 24.6 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_VariableDemandRevenueRefundingBondsSeries1997AbcDue20132020Member | 24.6 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_VariableDemandRevenueRefundingBondsSeries1997AbcDue20132020Member
Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006 Due 2025 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 27.8 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_IndustrialDevelopmentVariableRateDemandRefundingRevenueBondsSeries2006Due2025Member | 27.8 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_IndustrialDevelopmentVariableRateDemandRefundingRevenueBondsSeries2006Due2025Member
SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A415SeriesDue2028Member | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_A415SeriesDue2028Member
Other Long-Term Debt, 0.08%-7.50% Due 2015-2037 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-term Debt | $ 59.1 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_OtherLongTermDebt1080Due20122037Member | $ 41.8 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = ale_OtherLongTermDebt1080Due20122037Member

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