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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Supplemental Cash Flow Information [Abstract]      
Cash Paid During the Period for Interest – Net of Amounts Capitalized $ 64.5 $ 68.2 $ 59.0
Cash Paid During the Period for Income Taxes 0.4 0.5 0.4
Remeasurement of Deferred Income Taxes Resulting from the TCJA [Abstract]      
Increase to Regulatory Assets 80.9 0.0 0.0
Decrease in Investment in ATC (27.9) 0.0 0.0
Decrease to Deferred Income Taxes (353.6) 0.0 0.0
Increase to Regulatory Liabilities 393.6 0.0 0.0
Noncash Investing and Financing Activities [Abstract]      
Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment 67.2 22.0 40.6
Capitalized Asset Retirement Costs (15.6) 3.7 12.4
Camp Ripley Solar Financing 0.0 15.0 0.0
AFUDC–Equity 1.2 2.1 3.3
ALLETE Common Stock Contributed to the Defined Benefit Pension Plan 13.5 0.0 0.0
Contingent Consideration 0.0 0.0 35.7
Noncash Consideration for Land Inventory [Line Items]      
Consideration for Land Inventory   21.0  
ALLETE Common Stock Received for Land Inventory [Member]      
Noncash Consideration for Land Inventory [Line Items]      
Consideration for Land Inventory 0.0 8.0 0.0
Long-Term Finance Receivable for Land Inventory [Member]      
Noncash Consideration for Land Inventory [Line Items]      
Consideration for Land Inventory $ 0.0 $ 12.0 $ 0.0