Investment in ATC (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		1 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Apr. 30, 2014 Subsequent Event [Member]
Investment in ATC [Abstract]
Ownership Percentage	8.00%
Approved Return on Common Equity	12.20%
Equity Method Investment, Expected Additional Investment	$ 3.4
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2013	114.6
Cash Investments	1.2		1.2
Equity Earnings in ATC	5.1	5.2
Distributed ATC Earnings	(4.3)
Equity Investment Balance as of March 31, 2014	116.6
ATC Summarized Financial Data [Abstract]
Revenue	163.3	151.8
Operating Expense	78.6	69.8
Other Expense	21.6	21.5
Net Income	63.1	60.5
ALLETE's Equity in Net Income	5.1	5.2
Subsequent Event [Line Items]
Cash Investments	$ 1.2		$ 1.2