Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Segments [Line Items]
Operating Revenue									$ 1,136.8	$ 1,018.4	$ 961.2
Fuel and Purchased Power Expense									356.1	334.8	308.7
Operating and Maintenance Expense									456.2	412.9	397.1
Depreciation Expense									135.7	116.6	100.2
Operating Income (Loss)	51.5	60.8	28.2	48.3	46.9	38.4	24.4	44.4	188.8	154.1	155.2
Interest Expense									(54.8)	(50.3)	(45.5)
Equity Earnings in ATC									19.6	20.3	19.4
Other Income									8.6	9.3	6.0
Income (Loss) Before Non-Controlling Interest and Income Taxes									162.2	133.4	135.1
Income Tax Expense (Benefit)									36.7	28.7	38.0
Net Income (Loss)									125.5	104.7	97.1
Less: Non-Controlling Interest in Subsidiaries									0.7	0	0
Net Income (Loss) Attributable to ALLETE	32.9	41.6	16.8	33.5	33.0	25.2	14.0	32.5	124.8	104.7	97.1
Total Assets	4,360.8				3,476.8				4,360.8	3,476.8	3,253.4
Capital Additions									604.3	339.5	432.2
Regulated Operations [Member]
Business Segments [Line Items]
Operating Revenue									1,003.5	925.5	874.4
Fuel and Purchased Power Expense									356.1	334.8	308.7
Operating and Maintenance Expense									345.6	322.4	310.0
Depreciation Expense									118.0	110.2	93.9
Operating Income (Loss)									183.8	158.1	161.8
Interest Expense									(46.9)	(42.1)	(39.8)
Equity Earnings in ATC									19.6	20.3	19.4
Other Income									7.8	4.7	5.1
Income (Loss) Before Non-Controlling Interest and Income Taxes									164.3	141.0	146.5
Income Tax Expense (Benefit)									39.9	36.1	50.4
Net Income (Loss)									124.4	104.9	96.1
Less: Non-Controlling Interest in Subsidiaries									0	0	0
Net Income (Loss) Attributable to ALLETE									124.4	104.9	96.1
Total Assets	3,709.6				3,160.8				3,709.6	3,160.8	2,962.4
Capital Additions									583.5	326.3	418.2
Investments and Other [Member]
Business Segments [Line Items]
Land in Minnesota (in acres)	5,000								5,000
Operating Revenue									133.3	92.9	86.8
Fuel and Purchased Power Expense									0	0	0
Operating and Maintenance Expense									110.6	90.5	87.1
Depreciation Expense									17.7	6.4	6.3
Operating Income (Loss)									5.0	(4.0)	(6.6)
Interest Expense									(7.9)	(8.2)	(5.7)
Equity Earnings in ATC									0	0	0
Other Income									0.8	4.6	0.9
Income (Loss) Before Non-Controlling Interest and Income Taxes									(2.1)	(7.6)	(11.4)
Income Tax Expense (Benefit)									(3.2)	(7.4)	(12.4)
Net Income (Loss)									1.1	(0.2)	1.0
Less: Non-Controlling Interest in Subsidiaries									0.7	0	0
Net Income (Loss) Attributable to ALLETE									0.4	(0.2)	1.0
Total Assets	651.2				316.0				651.2	316.0	291.0
Capital Additions									$ 20.8	$ 13.2	$ 14.0