Investment in ATC (Details) $ in Millions		12 Months Ended
	Jan. 29, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 27, 2016
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance		$ 124.5
Cash Investments		5.4	$ 1.6	$ 3.9
Equity Earnings in ATC		18.5	16.3	19.6
Equity Investment Ending Balance		135.6	124.5
Income Statement Data [Abstract]
ALLETE's Equity in Net Income		$ 18.5	16.3	19.6
FERC [Member] | Return on Equity Complaint 2 [Member]
Income Statement Data [Abstract]
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		$ 10.9
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance		124.5	121.1
Cash Investments		5.4	1.6
Equity Earnings in ATC		18.5	16.3	19.6
Distributed ATC Earnings		(12.8)	(14.5)
Equity Investment Ending Balance		135.6	124.5	121.1
Balance Sheet Data [Abstract]
Current Assets		75.8	80.5
Non-Current Assets		4,312.9	3,957.6
Total Assets		4,388.7	4,038.1
Current Liabilities		495.1	330.3
Long-Term Debt		1,865.3	1,800.0
Other Non-Current Liabilities		271.5	245.0
Members' Equity		1,756.8	1,662.8
Total Liabilities and Members' Equity		4,388.7	4,038.1
Income Statement Data [Abstract]
Revenue		650.8	615.8	635.0
Operating Expense		322.5	319.3	307.4
Other Expense		95.5	96.1	88.9
Net Income		232.8	200.4	238.7
ALLETE's Equity in Net Income		$ 18.5	$ 16.3	$ 19.6
Authorized Return on Equity		10.32%			12.20%
Authorized Return on Equity, Including Incentive Adder		10.82%
ATC [Member] | Sensitivity Analysis [Member]
Income Statement Data [Abstract]
Basis Point Reduction on Approved Rate of Return on Common Equity		50
ATC [Member] | Sensitivity Analysis [Member] | After-tax [Member]
Income Statement Data [Abstract]
Annual Effect on Future Equity Earnings in ATC		$ 0.5
ATC [Member] | Subsequent Event [Member]
ALLETE's Investment in ATC [Roll Forward]
Cash Investments	$ 3.1