Investment in ATC (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 29, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance		$ 121.1
Cash Investments		1.6	$ 3.9	$ 3.1
Equity Earnings in ATC		16.3	19.6	20.3
Equity Investment Ending Balance		124.5	121.1
Income Statement Data [Abstract]
ALLETE's Equity in Net Income		$ 16.3	19.6	20.3
ATC [Member]
Investment in ATC [Line Items]
Ownership Percentage		8.00%
Expected Additional Investment in 2016		$ 6.2
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance		121.1	114.6
Cash Investments		1.6	3.9
Equity Earnings in ATC		16.3	19.6	20.3
Distributed ATC Earnings		(14.5)	(17.0)
Equity Investment Ending Balance		124.5	121.1	114.6
Balance Sheet Data [Abstract]
Current Assets		80.5	66.4
Non-Current Assets		3,957.6	3,728.7
Total Assets		4,038.1	3,795.1
Current Liabilities		330.3	313.1
Long-Term Debt		1,800.0	1,701.0
Other Non-Current Liabilities		245.0	163.8
Members' Equity		1,662.8	1,617.2
Total Liabilities and Members' Equity		4,038.1	3,795.1
Income Statement Data [Abstract]
Revenue		615.8	635.0	626.3
Operating Expense		319.3	307.4	295.1
Other Expense		96.1	88.9	83.6
Net Income		200.4	238.7	247.6
ALLETE's Equity in Net Income		$ 16.3	$ 19.6	$ 20.3
ATC [Member] | Subsequent Event [Member]
ALLETE's Investment in ATC [Roll Forward]
Cash Investments	$ 1.2