Business Segments (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($) a	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) a	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Business Segments [Line Items]
Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments									During the year ended December 31, 2015, management updated our reportable segment presentation to reflect the manner in which we operate, assess, and allocate resources after our recent acquisitions. We now present three reportable segments, Regulated Operations, ALLETE Clean Energy, and U.S. Water Services. Prior period amounts have been revised to conform with the current business segment presentation.
Number of Reportable Segments									3
Operating Revenue	$ 380.6	$ 462.5	$ 323.3	$ 320.0	$ 290.7	$ 288.9	$ 260.7	$ 296.5	$ 1,486.4	$ 1,136.8	$ 1,018.4
Net Income (Loss) Attributable to Parent	18.3	$ 60.4	$ 22.5	$ 39.9	32.9	$ 41.6	$ 16.8	$ 33.5	141.1	124.8	104.7
Depreciation and Amortization									170.0	135.7	116.6
Impairment of Real Estate									36.3	0.0	0.0
Interest Expense									64.9	54.8	50.3
Equity Earnings in ATC									16.3	19.6	20.3
Income Tax Expense (Benefit)									25.3	36.7	28.7
Assets	4,907.1				4,360.8				4,907.1	4,360.8
Capital Expenditures									$ 251.8	604.3
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments									3
Operating Revenue									$ 991.2	1,003.5	925.5
Net Income (Loss) Attributable to Parent									131.6	123.0	103.6
Depreciation and Amortization									135.1	118.0	110.2
Interest Expense									53.9	49.2	44.4
Equity Earnings in ATC									16.3	19.6	20.3
Income Tax Expense (Benefit)									24.4	39.0	35.1
Assets	3,862.6				3,709.6				3,862.6	3,709.6
Capital Expenditures									224.4	583.5
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue									262.1	33.2	0.0
Net Income (Loss) Attributable to Parent									29.9	3.3	(3.4)
Depreciation and Amortization									18.7	10.1	0.0
Interest Expense									3.3	0.8	0.0
Income Tax Expense (Benefit)									21.0	2.3	(2.3)
Assets	504.6				291.9				504.6	291.9
Capital Expenditures									8.6	4.2
U.S. Water Services [Member]
Business Segments [Line Items]
Operating Revenue									119.8	0.0	0.0
Net Income (Loss) Attributable to Parent									0.9	0.0	0.0
Depreciation and Amortization									7.3	0.0	0.0
Interest Expense									1.4	0.0	0.0
Income Tax Expense (Benefit)									0.9	0.0	0.0
Assets	$ 258.3				0.0				258.3	0.0
Capital Expenditures									$ 2.9	0.0
Corporate and Other [Member]
Business Segments [Line Items]
Number of Operating Segments									2
Land in Minnesota (Acres) | a	5,000								5,000
Operating Revenue									$ 113.3	100.1	92.9
Net Income (Loss) Attributable to Parent									(21.3)	(1.5)	4.5
Depreciation and Amortization									8.9	7.6	6.4
Impairment of Real Estate									36.3	0.0	0.0
Interest Expense									8.6	7.1	8.2
Income Tax Expense (Benefit)									(21.0)	(4.6)	(4.1)
Assets	$ 281.6				$ 359.3				281.6	359.3
Capital Expenditures									15.9	16.6
Eliminations [Member]
Business Segments [Line Items]
Interest Expense									$ (2.3)	$ (2.3)	$ (2.3)