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Operations and Significant Accounting Policies - Reclassifications (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Operating and Maintenance Expense to Cost of Sales [Member] |  |
Schedule of Prior Period Reclassifications [Line Items] |  |
Prior Period Reclassification | $ 77.9 | $ 71.2
Operating and Maintenance Expense to Transmission Services [Member] |  |
Schedule of Prior Period Reclassifications [Line Items] |  |
Prior Period Reclassification | 45.6 | 32.3
Operating and Maintenance Expense to Taxes Other than Income Taxes [Member] |  |
Schedule of Prior Period Reclassifications [Line Items] |  |
Prior Period Reclassification | 45.6 | 41.7
Operating Income [Member] |  |
Schedule of Prior Period Reclassifications [Line Items] |  |
Prior Period Reclassification | 0.0 | 0.0
Net Income [Member] |  |
Schedule of Prior Period Reclassifications [Line Items] |  |
Prior Period Reclassification | 0.0 | 0.0
Net Income Attributable to ALLETE [Member] |  |
Schedule of Prior Period Reclassifications [Line Items] |  |
Prior Period Reclassification | 0.0 | $ 0.0
Property, Plant and Equipment - Net to Goodwill and Intangible Assets - Net [Member] |  |
Schedule of Prior Period Reclassifications [Line Items] |  |
Prior Period Reclassification | 1.6 |
Other Non-Current Assets to Goodwill and Intangible Assets - Net [Member] |  |
Schedule of Prior Period Reclassifications [Line Items] |  |
Prior Period Reclassification | 3.2 |
Total Assets [Member] |  |
Schedule of Prior Period Reclassifications [Line Items] |  |
Prior Period Reclassification | $ 0.0 |

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