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Investment in ATC (Details) $ in Millions		3 Months Ended		9 Months Ended
	Oct. 28, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 27, 2016
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2015				$ 124.5
Cash Investments				3.5	$ 1.2
Equity in ATC Earnings		$ 6.1	$ 5.5	15.0	$ 14.1
Equity Investment Balance as of September 30, 2016		$ 133.8		$ 133.8
ATC [Member]
Investment in ATC [Line Items]
Ownership Percentage		8.00%		8.00%
Expected Additional Investment in 2016		$ 0.0		$ 0.0
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2015				124.5
Cash Investments				3.5
Equity in ATC Earnings				15.0
Distributed ATC Earnings				(9.2)
Equity Investment Balance as of September 30, 2016		$ 133.8		$ 133.8
Authorized Return on Equity		10.82%		10.82%		12.20%
ATC [Member] | Subsequent Event [Member]
ALLETE's Investment in ATC [Roll Forward]
Cash Investments	$ 1.9
Sensitivity Analysis [Member] | ATC [Member]
ALLETE's Investment in ATC [Roll Forward]
Basis Point Reduction on Approved Rate of Return on Common Equity		50		50
Sensitivity Analysis [Member] | After-tax [Member] | ATC [Member]
ALLETE's Investment in ATC [Roll Forward]
Annual Effect on Future Equity Earnings in ATC				$ 0.5
Sensitivity Analysis [Member] | Pre-tax [Member] | ATC [Member]
ALLETE's Investment in ATC [Roll Forward]
Annual Effect on Future Equity Earnings in ATC				$ 0.9
FERC [Member] | Return on Equity Complaint 2 [Member]
ALLETE's Investment in ATC [Roll Forward]
Proposed Return on Equity		9.70%		9.70%

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