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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Supplemental Cash Flow Information [Abstract]    
Cash Paid During the Period for Interest – Net of Amounts Capitalized $ 32.5 $ 32.9
Cash Paid During the Period for Income Taxes 0.3 0.4
Noncash Investing and Financing Activities [Abstract]    
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment (0.4) (24.4)
Capitalized Asset Retirement Costs 19.7 2.3
AFUDC–Equity 0.4 1.2
ALLETE Common Stock Contributed to the Pension Plans $ 13.5 $ 0.0