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Investment in ATC (Details) $ in Millions		3 Months Ended		6 Months Ended
	Jul. 29, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2015				$ 124.5
Cash Investments				1.6	$ 0.8
Equity in ATC Earnings		$ 4.1	$ 4.7	8.9	$ 8.6
Equity Investment Balance as of June 30, 2016		$ 129.0		$ 129.0
ATC [Member]
Investment in ATC [Line Items]
Ownership Percentage		8.00%		8.00%
Expected Additional Investment in 2016		$ 2.7		$ 2.7
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2015				124.5
Cash Investments				1.6
Equity in ATC Earnings				8.9
Distributed ATC Earnings				(6.0)
Equity Investment Balance as of June 30, 2016		$ 129.0		$ 129.0
Authorized Return on Equity		12.20%		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
After-tax [Member] | Sensitivity Analysis [Member] | ATC [Member]
ALLETE's Investment in ATC [Roll Forward]
Annual Effect on Future Equity Earnings in ATC				$ 0.5
Pre-tax [Member] | Sensitivity Analysis [Member] | ATC [Member]
ALLETE's Investment in ATC [Roll Forward]
Annual Effect on Future Equity Earnings in ATC				$ 0.9

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