Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Business Segments [Line Items]
Operating Revenue	$ 296.5	$ 263.8
Fuel and Purchased Power Expense	96.2	86.5
Operating and Maintenance Expense	119.8	104.7
Depreciation Expense	32.2	28.2
Operating Income (Loss)	48.3	44.4
Interest Expense	(12.8)	(12.3)
Equity Earnings in ATC	5.1	5.2
Other Income	2.0	2.7
Income (Loss) Before Non-Controlling Interest and Income Taxes	42.6	40.0
Income Tax Expense (Benefit)	8.8	7.5
Net Income (Loss)	33.8	32.5
Less: Non-Controlling Interest in Subsidiaries	0.3	0
Net Income (Loss) Attributable to ALLETE	33.5	32.5
Total Assets	3,749.2	3,251.3	3,476.8
Property, Plant and Equipment - Net	2,905.1	2,366.7	2,576.5
Accumulated Depreciation	1,266.7	1,179.8
Capital Additions	195.2	43.1
Regulated Operations [Member]
Business Segments [Line Items]
Operating Revenue	264.2	241.4
Fuel and Purchased Power Expense	96.2	86.5
Operating and Maintenance Expense	90.2	82.2
Depreciation Expense	28.8	26.8
Operating Income (Loss)	49.0	45.9
Interest Expense	(11.5)	(10.7)
Equity Earnings in ATC	5.1	5.2
Other Income	1.8	1.1
Income (Loss) Before Non-Controlling Interest and Income Taxes	44.4	41.5
Income Tax Expense (Benefit)	10.5	9.4
Net Income (Loss)	33.9	32.1
Less: Non-Controlling Interest in Subsidiaries	0	0
Net Income (Loss) Attributable to ALLETE	33.9	32.1
Total Assets	3,335.5	2,967.9
Property, Plant and Equipment - Net	2,671.3	2,300.8
Accumulated Depreciation	1,202.9	1,121.8
Capital Additions	193.5	42.6
Investments and Other [Member]
Business Segments [Line Items]
Land in Minnesota (in acres)	5,000
Operating Revenue	32.3	22.4
Fuel and Purchased Power Expense	0	0
Operating and Maintenance Expense	29.6	22.5
Depreciation Expense	3.4	1.4
Operating Income (Loss)	(0.7)	(1.5)
Interest Expense	(1.3)	(1.6)
Equity Earnings in ATC	0	0
Other Income	0.2	1.6
Income (Loss) Before Non-Controlling Interest and Income Taxes	(1.8)	(1.5)
Income Tax Expense (Benefit)	(1.7)	(1.9)
Net Income (Loss)	(0.1)	0.4
Less: Non-Controlling Interest in Subsidiaries	0.3	0
Net Income (Loss) Attributable to ALLETE	(0.4)	0.4
Total Assets	413.7	283.4
Property, Plant and Equipment - Net	233.8	65.9
Accumulated Depreciation	63.8	58.0
Capital Additions	$ 1.7	$ 0.5