Operations and Significant Accounting Policies - Reclassifications (Details) - USD ($) $ in Millions	12 Months Ended
	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