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Short-Term and Long-Term Debt (Details) $ in Millions | Sep. 24, 2015 USD ($) | Aug. 25, 2015 USD ($) | Sep. 30, 2015 USD ($) | Jul. 01, 2015 USD ($) | Dec. 31, 2014 USD ($)
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Short-Term and Long-Term Debt [Line Items] |  |  |  |  |
Short-Term Debt Outstanding |  |  | $ 49.1 |  | $ 104.4
Long-Term Debt Outstanding |  |  | $ 1,549.0 |  | $ 1,272.8
Required Indebtedness to Total Capitalization Ratio |  |  | 0.65 |  |
Proceeds from Issuance of First Mortgage Bonds | $ 100.0 |  |  |  |
ALLETE Term Loan Variable Rate Due 2017 [Member] |  |  |  |  |
Short-Term and Long-Term Debt [Line Items] |  |  |  |  |
Principal Amount |  | $ 125.0 |  |  |
Variable Rate Basis |  | LIBOR |  |  |
Basis Spread on Variable Rate |  | 0.625% |  |  |
Debt Refinanced |  | $ 75.0 |  |  |
Required Indebtedness to Total Capitalization Ratio |  | 0.65 |  |  |
Cross-Default Amount |  | $ 35.0 |  |  |
ALLETE Bonds 2.80% Due September 2020 [Member] |  |  |  |  |
Short-Term and Long-Term Debt [Line Items] |  |  |  |  |
Proceeds from Issuance of First Mortgage Bonds | $ 40.0 |  |  |  |
Interest Rate | 2.80% |  |  |  |
ALLETE Bonds 3.86% Due September 2030 [Member] |  |  |  |  |
Short-Term and Long-Term Debt [Line Items] |  |  |  |  |
Proceeds from Issuance of First Mortgage Bonds | $ 60.0 |  |  |  |
Interest Rate | 3.86% |  |  |  |
Armenia Mountain [Member] |  |  |  |  |
Short-Term and Long-Term Debt [Line Items] |  |  |  |  |
Business Combination, Long-Term Debt Assumed Including Current Portion |  |  |  | $ 60.9 |
Business Combination, Current Portion of Long-Term Debt Assumed |  |  |  | $ 5.9 |

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