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Short-Term and Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified | 0 Months Ended | 6 Months Ended |  |  | 0 Months Ended |  | 0 Months Ended |  | 0 Months Ended |  | 0 Months Ended |  | 0 Months Ended |  |
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Jun. 27, 2014 | Jun. 30, 2014 | Jan. 30, 2014 | Dec. 31, 2013 | Dec. 31, 2013 ALLETE Bonds 4.86% Series Due 2013 [Member] | Mar. 04, 2014 ALLETE Bonds 3.69% Due March2024 [Member] | Mar. 04, 2014 ALLETE Bonds 4.95% Due March2044 [Member] | Jun. 26, 2014 ALLETE Bonds 3.40% Due July2022 [Member] | Jun. 26, 2014 ALLETE Bonds 3.40% Due July2022 [Member] | Jun. 26, 2014 ALLETE Bonds 5.05% Due July2044 [Member] | Jun. 26, 2014 ALLETE Bonds 5.05% Due July2044 [Member] | Jun. 27, 2014 ALLETE Bonds 3.02% Due September 2021 [Member] | Jun. 27, 2014 ALLETE Bonds 3.02% Due September 2021 [Member] | Jun. 27, 2014 ALLETE Bonds 3.74% Due September 2029 [Member] | Jun. 27, 2014 ALLETE Bonds 3.74% Due September 2029 [Member] | Jun. 27, 2014 ALLETE Bonds 4.39% Due September 2044 [Member] | Jun. 27, 2014 ALLETE Bonds 4.39% Due September 2044 [Member] | Jul. 01, 2014 Subsequent Event [Member] 4.95% Pollution Control Series F Due 2022 [Member]
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-Term Debt Outstanding |  | $ 10.8 |  | $ 27.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-Term Debt Outstanding, Subsequently Matured |  | 10.8 |  | 27.2 | 18.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt Outstanding |  | 1,316.8 |  | 1,083.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt |  |  | 23.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt |  |  | 2.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt |  | 1,327.6 |  | 1,110.2 |  |  |  |  |  |  |  |  |  |  |  |  |  | 111.0
Proceeds from Issuance of First Mortgage Bonds |  | 215.0 |  |  |  | 60.0 | 40.0 | 75.0 |  | 40.0 |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  | 3.69% | 4.95% |  | 3.40% |  | 5.05% |  |  |  |  |  |  |
Expected Proceeds from Future Issuance of First Mortgage Bonds | $ 160.0 |  |  |  |  |  |  |  |  |  |  | $ 60.0 |  | $ 50.0 |  | $ 50.0 |  |
Future Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  |  |  |  |  |  |  |  | 3.02% |  | 3.74% |  | 4.39% |

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