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Investment in ATC (Details) $ in Millions		3 Months Ended
	Apr. 28, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Sep. 27, 2016
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2016		$ 135.6
Cash Investments		3.1	$ 1.2
Equity in ATC Earnings		6.1	$ 4.8
Equity Investment Balance as of March 31, 2017		$ 140.2
FERC [Member] | Return on Equity Complaint 2 [Member]
ALLETE's Investment in ATC [Roll Forward]
Proposed Return on Equity		9.70%
Proposed Return on Equity, Including Incentive Adder		10.20%
ATC [Member]
Investment in ATC [Line Items]
Ownership Percentage		8.00%
Expected Additional Investment in 2017		$ 5.9
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2016		135.6
Cash Investments		3.1
Equity in ATC Earnings		6.1
Distributed ATC Earnings		(4.6)
Equity Investment Balance as of March 31, 2017		$ 140.2
Authorized Return on Equity		10.32%		12.20%
Authorized Return on Equity, Including Incentive Adder		10.82%
ATC [Member] | Sensitivity Analysis [Member]
ALLETE's Investment in ATC [Roll Forward]
Basis Point Reduction on Approved Rate of Return on Common Equity		50
ATC [Member] | Sensitivity Analysis [Member] | After-tax [Member]
ALLETE's Investment in ATC [Roll Forward]
Annual Effect on Future Equity Earnings in ATC		$ 0.5
ATC [Member] | Subsequent Event [Member]
ALLETE's Investment in ATC [Roll Forward]
Cash Investments	$ 1.9

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