Business Segments (Details) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015 USD ($) a	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) a	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Business Segments [Line Items]
Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments				During the quarter ended September 30, 2015, management updated our reportable segment presentation to reflect the manner in which we operate, assess, and allocate resources after our recent acquisitions. We will 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		$ 462.5	$ 288.9	$ 1,105.8	$ 846.1
Net Income (Loss) Attributable to ALLETE		$ 60.4	41.6	$ 122.8	91.9
Equity Method Investment, Ownership Percentage		8.00%		8.00%
Interest Expense		$ 17.7	13.2	$ 49.0	39.5
Total Assets		4,931.8	4,067.9	$ 4,931.8	4,067.9	$ 4,360.8
Regulated Operations [Member] [Member]
Business Segments [Line Items]
Number of Operating Segments				3
Operating Revenue		250.2	255.8	$ 743.0	749.6
Net Income (Loss) Attributable to ALLETE	[1]	$ 43.8	40.9	$ 108.1	91.6
Equity Method Investment, Ownership Percentage		8.00%		8.00%
Total Assets		$ 3,836.3	3,519.9	$ 3,836.3	3,519.9
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue		151.1	6.8	197.5	22.6
Net Income (Loss) Attributable to ALLETE		13.2	0.5	18.7	1.2
Total Assets		565.7	184.4	565.7	184.4
U.S. Water Services [Member]
Business Segments [Line Items]
Operating Revenue		36.1	0.0	86.0	0.0
Net Income (Loss) Attributable to ALLETE		1.0	0.0	1.5	0.0
Total Assets		$ 257.8	0.0	$ 257.8	0.0
Corporate and Other [Member]
Business Segments [Line Items]
Number of Operating Segments				2
Land in Minnesota (in acres) | a		5,000		5,000
Operating Revenue		$ 25.1	26.3	$ 79.3	73.9
Net Income (Loss) Attributable to ALLETE	[1]	2.4	0.2	(5.5)	(0.9)
Total Assets		$ 272.0	$ 363.6	272.0	$ 363.6
Related Party [Member] | Regulated Operations [Member] [Member]
Business Segments [Line Items]
Interest Expense				0.3
Related Party [Member] | Corporate and Other [Member]
Business Segments [Line Items]
Interest Income				$ 1.8

[1] During the third quarter of 2015, the Company entered into an intercompany loan agreement, and allocated long-term debt to ALLETE Transmission Holdings, which owns approximately 8 percent of ATC, to better reflect the capital requirements of ALLETE Transmission Holdings and our investment in ATC. ALLETE Transmission Holdings recognized interest expense of approximately $0.3 million after-tax in the third quarter of 2015 which is reflected in our Regulated Operations segment. Our Corporate and Other segment recognized interest income of the same amount. The amounts are eliminated in consolidation. Prior period segment results have been revised to conform to the current presentation as if the intercompany loan existed as of January 1, 2014.