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Investment in ATC (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		0 Months Ended	3 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jul. 30, 2014 Subsequent Event [Member]	Jun. 30, 2014 Sensitivity Analysis [Member]
Sensitivity Analysis [Line Items]
Basis Point Reduction on Approved Rate of Return on Common Equity						50
Effect on Future Earnings, Amount						$ 0.5
Ownership Percentage	8.00%		8.00%
Approved Return on Common Equity	12.20%		12.20%
Equity Method Investment, Expected Additional Investment	2.7		2.7
Equity Method Investment, Proposed Return on Common Equity	9.15%		9.15%
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2013			114.6
Cash Investments			2.3		0.8
Equity Earnings in ATC	5.2	5.0	10.3	10.2
Distributed ATC Earnings			(8.4)
Equity Investment Balance as of June 30, 2014	118.8		118.8
ATC Summarized Financial Data [Abstract]
Revenue	160.0	152.1	323.3	303.9
Operating Expense	74.4	69.9	153.0	139.7
Other Expense	21.9	20.9	43.5	42.4
Net Income	63.7	61.3	126.8	121.8
ALLETE's Equity in Net Income	5.2	5.0	10.3	10.2
Subsequent Event [Line Items]
Cash Investments			$ 2.3		$ 0.8

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