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Investment in ATC (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Oct. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Investment in ATC [Abstract]
Ownership Percentage		8.00%		8.00%
Cash Investments				$ 1.2	$ 3.1
Equity Method Investment, Expected Additional Investment		$ 0.0		0.0
Subsequent Event [Line Items]
Cash Investments				1.2	3.1
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2014				121.1
Cash Investments				1.2	3.1
Equity Earnings in ATC		5.5	$ 5.3	14.1	15.6
Distributed ATC Earnings				(10.4)
Equity Investment Balance as of September 30, 2015		126.0		126.0
ATC Summarized Financial Data [Abstract]
Revenue		164.5	163.6	482.0	487.0
Operating Expense		78.0	76.6	238.3	229.6
Other Expense		23.1	21.4	71.7	65.1
Net Income		63.4	65.6	172.0	192.3
ALLETE's Equity in Net Income		$ 5.5	$ 5.3	$ 14.1	$ 15.6
Subsequent Event [Member]
Investment in ATC [Abstract]
Cash Investments	$ 0.4
Subsequent Event [Line Items]
Cash Investments	0.4
ALLETE's Investment in ATC [Roll Forward]
Cash Investments	$ 0.4

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