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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Supplemental Cash Flow Information [Abstract]      
Cash Paid During the Period for Interest – Net of Amounts Capitalized $ 59.0 $ 51.3 $ 47.5
Cash Paid During the Period for Income Taxes 0.4 5.1 0.5
Noncash Investing and Financing Activities [Abstract]      
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment (40.6)    
Increase in Accounts Payable for Capital Additions to Property, Plant and Equipment   21.7 8.3
Capitalized Asset Retirement Costs 12.4 22.4 (0.7)
AFUDC–Equity 3.3 7.8 4.6
ALLETE Common Stock Contributed to the Defined Benefit Pension Plan 0.0 19.5 0.0
Noncash or Part Noncash Acquisition, Value of Liabilities Assumed $ 35.7 $ 0.0 $ 0.0