Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental Cash Flow Information [Abstract]
Cash Paid During the Period for Interest – Net of Amounts Capitalized	$ 68.2	$ 59.0	$ 51.3
Cash Paid During the Period for Income Taxes	0.5	0.4	5.1
Noncash Investing and Financing Activities [Abstract]
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment	(22.0)	(40.6)
Increase in Accounts Payable for Capital Additions to Property, Plant and Equipment			21.7
Capitalized Asset Retirement Costs	3.7	12.4	22.4
Camp Ripley Solar Project Financing	15.0	0.0	0.0
AFUDC–Equity	2.1	3.3	7.8
ALLETE Common Stock Contributed to the Defined Benefit Pension Plan	0.0	0.0	19.5
Contingent Consideration	0.0	35.7	0.0
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	8.0	0.0	0.0
Long-Term Finance Receivable for Land Inventory [Member]
Noncash Consideration for Land Inventory [Line Items]
Consideration for Land Inventory	$ 12.0	$ 0.0	$ 0.0