Investment in ATC (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Jul. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Investment in ATC [Abstract]
Ownership Percentage		8.00%		8.00%
Approved Return on Common Equity		12.20%		12.20%
Cash Investments				$ 0.8
Equity Method Investment, Expected Additional Investment		$ 0.7		0.7
Subsequent Event [Line Items]
Cash Investments				0.8
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2014				121.1
Cash Investments				0.8
Equity Earnings in ATC		4.7	$ 5.2	8.6	$ 10.3
Distributed ATC Earnings				(6.3)
Equity Investment Balance as of June 30, 2015		124.2		124.2
ATC Summarized Financial Data [Abstract]
Revenue		165.2	160.0	317.6	323.3
Operating Expense		80.3	74.4	160.3	153.0
Other Expense		24.3	21.9	48.7	43.5
Net Income		60.6	63.7	108.6	126.8
ALLETE's Equity in Net Income		$ 4.7	$ 5.2	$ 8.6	$ 10.3
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