Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Business Segments [Line Items]
Operating Revenue	$ 320.0	$ 296.5
Fuel and Purchased Power	86.0	96.2
Transmission Services	14.9	10.8
Cost of Sales	31.2	23.5
Operating and Maintenance	79.7	74.3
Depreciation and Amortization	39.0	32.2
Taxes Other than Income Taxes	12.8	11.2
Operating Income (Loss)	56.4	48.3
Interest Expense	(15.1)	(12.8)
Equity Earnings in ATC	3.9	5.1
Other Income	1.1	2.0
Income (Loss) Before Non-Controlling Interest and Income Taxes	46.3	42.6
Income Tax Expense (Benefit)	6.2	8.8
Net Income (Loss)	40.1	33.8
Less: Non-Controlling Interest in Subsidiaries	0.2	0.3
Net Income (Loss) Attributable to ALLETE	39.9	33.5
Total Assets	4,580.1	3,749.2	4,360.8
Property, Plant and Equipment - Net	3,319.2	2,905.1	3,284.8
Accumulated Depreciation	1,367.0	1,266.7
Capital Additions	57.3	195.2
Regulated Operations [Member]
Business Segments [Line Items]
Operating Revenue	262.8	264.2
Fuel and Purchased Power	86.0	96.2
Transmission Services	14.9	10.8
Cost of Sales	4.5	8.7
Operating and Maintenance	58.7	60.5
Depreciation and Amortization	32.1	28.8
Taxes Other than Income Taxes	11.6	10.2
Operating Income (Loss)	55.0	49.0
Interest Expense	(13.0)	(11.5)
Equity Earnings in ATC	3.9	5.1
Other Income	0.9	1.8
Income (Loss) Before Non-Controlling Interest and Income Taxes	46.8	44.4
Income Tax Expense (Benefit)	5.4	10.5
Net Income (Loss)	41.4	33.9
Less: Non-Controlling Interest in Subsidiaries	0	0
Net Income (Loss) Attributable to ALLETE	41.4	33.9
Total Assets	3,748.4	3,335.5
Property, Plant and Equipment - Net	3,025.3	2,671.3
Accumulated Depreciation	1,286.9	1,202.9
Capital Additions	54.9	193.5
Investments and Other [Member]
Business Segments [Line Items]
Land in Minnesota (in acres)	5,000
Operating Revenue	57.2	32.3
Fuel and Purchased Power	0	0
Transmission Services	0	0
Cost of Sales	26.7	14.8
Operating and Maintenance	21.0	13.8
Depreciation and Amortization	6.9	3.4
Taxes Other than Income Taxes	1.2	1.0
Operating Income (Loss)	1.4	(0.7)
Interest Expense	(2.1)	(1.3)
Equity Earnings in ATC	0	0
Other Income	0.2	0.2
Income (Loss) Before Non-Controlling Interest and Income Taxes	(0.5)	(1.8)
Income Tax Expense (Benefit)	0.8	(1.7)
Net Income (Loss)	(1.3)	(0.1)
Less: Non-Controlling Interest in Subsidiaries	0.2	0.3
Net Income (Loss) Attributable to ALLETE	(1.5)	(0.4)
Total Assets	831.7	413.7
Property, Plant and Equipment - Net	293.9	233.8
Accumulated Depreciation	80.1	63.8
Capital Additions	$ 2.4	$ 1.7