Business Segments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) a	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) a	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Business Segments [Line Items]
Operating Revenue	$ 323.3	$ 260.7	$ 643.3	$ 557.2
Fuel and Purchased Power	80.1	83.6	166.1	179.8
Transmission Services	11.3	10.5	26.2	21.3
Cost of Sales	52.3	18.9	83.5	42.4
Operating and Maintenance	85.4	74.3	165.1	148.6
Depreciation and Amortization	41.3	33.9	80.3	66.1
Taxes Other than Income Taxes	13.4	11.3	26.2	22.5
Operating Income	39.5	28.2	95.9	76.5
Interest Expense	(16.2)	(13.5)	(31.3)	(26.3)
Equity Earnings in ATC	4.7	5.2	8.6	10.3
Other Income (Expense)	0.7	1.9	1.8	3.9
Income (Loss) Before Non-Controlling Interest and Income Taxes	28.7	21.8	75.0	64.4
Income Tax Expense (Benefit)	6.4	4.9	12.6	13.7
Net Income (Loss)	22.3	16.9	62.4	50.7
Less: Non-Controlling Interest in Subsidiaries	(0.2)	0.1	0.0	0.4
Net Income (Loss) Attributable to ALLETE	22.5	16.8	62.4	50.3
Total Assets	4,673.0	3,895.6	4,673.0	3,895.6	$ 4,360.8
Property, Plant and Equipment – Net	3,451.5	3,020.4	3,451.5	3,020.4	$ 3,284.8
Accumulated Depreciation	1,380.1	1,288.9	1,380.1	1,288.9
Capital Additions			117.9	341.7
Regulated Operations [Member]
Business Segments [Line Items]
Operating Revenue	230.0	229.6	492.8	493.8
Fuel and Purchased Power	80.1	83.6	166.1	179.8
Transmission Services	11.3	10.5	26.2	21.3
Cost of Sales	1.0	4.3	5.5	13.0
Operating and Maintenance	57.8	63.6	116.5	124.1
Depreciation and Amortization	33.7	29.6	65.8	58.4
Taxes Other than Income Taxes	12.1	10.6	23.7	20.8
Operating Income	34.0	27.4	89.0	76.4
Interest Expense	(13.3)	(11.4)	(26.3)	(22.9)
Equity Earnings in ATC	4.7	5.2	8.6	10.3
Other Income (Expense)	0.7	2.0	1.6	3.8
Income (Loss) Before Non-Controlling Interest and Income Taxes	26.1	23.2	72.9	67.6
Income Tax Expense (Benefit)	2.5	5.7	7.9	16.2
Net Income (Loss)	23.6	17.5	65.0	51.4
Less: Non-Controlling Interest in Subsidiaries	0.0	0.0	0.0	0.0
Net Income (Loss) Attributable to ALLETE	23.6	17.5	65.0	51.4
Total Assets	3,783.2	3,424.3	3,783.2	3,424.3
Property, Plant and Equipment – Net	3,056.9	2,791.7	3,056.9	2,791.7
Accumulated Depreciation	$ 1,294.0	1,221.1	1,294.0	1,221.1
Capital Additions			$ 111.7	335.6
Investments and Other [Member]
Business Segments [Line Items]
Land in Minnesota (in acres) | a	5,000		5,000
Operating Revenue	$ 93.3	31.1	$ 150.5	63.4
Fuel and Purchased Power	0.0	0.0	0.0	0.0
Transmission Services	0.0	0.0	0.0	0.0
Cost of Sales	51.3	14.6	78.0	29.4
Operating and Maintenance	27.6	10.7	48.6	24.5
Depreciation and Amortization	7.6	4.3	14.5	7.7
Taxes Other than Income Taxes	1.3	0.7	2.5	1.7
Operating Income	5.5	0.8	6.9	0.1
Interest Expense	(2.9)	(2.1)	(5.0)	(3.4)
Equity Earnings in ATC	0.0	0.0	0.0	0.0
Other Income (Expense)	0.0	(0.1)	0.2	0.1
Income (Loss) Before Non-Controlling Interest and Income Taxes	2.6	(1.4)	2.1	(3.2)
Income Tax Expense (Benefit)	3.9	(0.8)	4.7	(2.5)
Net Income (Loss)	(1.3)	(0.6)	(2.6)	(0.7)
Less: Non-Controlling Interest in Subsidiaries	(0.2)	0.1	0.0	0.4
Net Income (Loss) Attributable to ALLETE	(1.1)	(0.7)	(2.6)	(1.1)
Total Assets	889.8	471.3	889.8	471.3
Property, Plant and Equipment – Net	394.6	228.7	394.6	228.7
Accumulated Depreciation	$ 86.1	$ 67.8	86.1	67.8
Capital Additions			$ 6.2	$ 6.1