Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Supplemental Cash Flow Information [Abstract]
Cash Paid During the Period for Interest – Net of Amounts Capitalized	$ 51.4	$ 54.9
Cash Paid During the Period for Income Taxes	0.4	0.5
Noncash Investing and Financing Activities [Abstract]
Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment	1.2	(19.5)
Capitalized Asset Retirement Costs	19.7	3.7
AFUDC–Equity	0.7	1.7
ALLETE Common Stock Contributed to the Pension Plans	13.5	0.0
ALLETE Common Stock Received for Land Inventory [Member]
Noncash Consideration for Land Inventory [Line Items]
Consideration for Land Inventory	0.0	8.0
Long-Term Finance Receivable for Land Inventory [Member]
Noncash Consideration for Land Inventory [Line Items]
Consideration for Land Inventory	$ 0.0	$ 12.0