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Operations and Significant Accounting Policies - Reclassifications (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2014 | Sep. 30, 2014 | Dec. 31, 2014
Operating and Maintenance Expenses to Cost of Sales [Member] |  |  |
Reclassifications [Line Items] |  |  |
Reclassification Adjustment | $ 16.8 | $ 59.2 |
Operating and Maintenance Expenses to Transmission Services [Member] |  |  |
Reclassifications [Line Items] |  |  |
Reclassification Adjustment | 11.9 | 33.2 |
Operating and Maintenance Expenses to Taxes Other than Income Taxes [Member] |  |  |
Reclassifications [Line Items] |  |  |
Reclassification Adjustment | 11.4 | 33.9 |
Operating Income [Member] |  |  |
Reclassifications [Line Items] |  |  |
Reclassification Adjustment | 0.0 | 0.0 |
Net Income [Member] |  |  |
Reclassifications [Line Items] |  |  |
Reclassification Adjustment | 0.0 | 0.0 |
Net Income Attributable to ALLETE [Member] |  |  |
Reclassifications [Line Items] |  |  |
Reclassification Adjustment | $ 0.0 | $ 0.0 |
Property, Plant and Equipment - Net to Goodwill and Intangible Assets - Net [Member] |  |  |
Reclassifications [Line Items] |  |  |
Reclassification Adjustment |  |  | $ 1.6
Other Non-Current Assets to Goodwill and Intangible Assets - Net [Member] |  |  |
Reclassifications [Line Items] |  |  |
Reclassification Adjustment |  |  | 3.2
Total Assets [Member] |  |  |
Reclassifications [Line Items] |  |  |
Reclassification Adjustment |  |  | $ 0.0

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