Investment in ATC (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended	0 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 29, 2015
Investment in ATC [Abstract]
Equity Method Investment, Ownership Percentage	8.00%
Equity Method Investment, Approved Return on Common Equity	12.20%
Equity Method Investment, Total Expected Investment for 2015	$ 1.9
Subsequent Event [Line Items]
Cash Investments	3.9	3.1
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance	114.6us-gaap_EquityMethodInvestments	107.3us-gaap_EquityMethodInvestments
Cash Investments	3.9	3.1
Equity Earnings in ATC	19.6	20.3	19.4
Distributed ATC Earnings	(17.0)	(16.1)
Equity Investment Ending Balance	121.1us-gaap_EquityMethodInvestments	114.6us-gaap_EquityMethodInvestments	107.3us-gaap_EquityMethodInvestments
Balance Sheet Data [Abstract]
Current Assets	66.4	80.7
Non-Current Assets	3,728.7	3,509.5
Total Assets	3,795.1	3,590.2
Current Liabilities	313.1	381.4
Long-Term Debt	1,701.0	1,550.0
Other Non-Current Liabilities	163.8	126.2
Members' Equity	1,617.2	1,532.6
Total Liabilities and Members' Equity	3,795.1	3,590.2
Income Statement Data [Abstract]
Revenue	635.0	626.3	603.2
Operating Expense	307.4	295.1	281.0
Other Expense	88.9	83.6	84.8
Net Income	238.7	247.6	237.4
ALLETE's Equity in Net Income	19.6	20.3	19.4
Subsequent Event [Member]
Subsequent Event [Line Items]
Cash Investments				0.4
ALLETE's Investment in ATC [Roll Forward]
Cash Investments				$ 0.4

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