Operations and Significant Accounting Policies - Reclassifications (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2014	Jun. 30, 2014	Dec. 31, 2014
Changes in Regulatory and Other Non-Current Liabilities to Amortization of Power Purchase Agreements [Member]
Reclassifications [Line Items]
Reclassification Adjustment		$ 5.6
Operating Activities [Member]
Reclassifications [Line Items]
Reclassification Adjustment		0.0
Investing Activities [Member]
Reclassifications [Line Items]
Reclassification Adjustment		0.0
Financing Activities [Member]
Reclassifications [Line Items]
Reclassification Adjustment		0.0
Operating and Maintenance Expenses to Cost of Sales [Member]
Reclassifications [Line Items]
Reclassification Adjustment	$ 18.9	42.4
Operating and Maintenance Expenses to Transmission Services [Member]
Reclassifications [Line Items]
Reclassification Adjustment	10.5	21.3
Operating and Maintenance Expenses to Taxes Other than Income Taxes [Member]
Reclassifications [Line Items]
Reclassification Adjustment	$ 11.3	22.5
Operating Income [Member]
Reclassifications [Line Items]
Reclassification Adjustment		0.0
Net Income [Member]
Reclassifications [Line Items]
Reclassification Adjustment		0.0
Net Income Attributable to ALLETE [Member]
Reclassifications [Line Items]
Reclassification Adjustment		$ 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