Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Supplemental Cash Flow Elements [Abstract]
Cash Paid During the Period for Interest - Net of Amounts Capitalized	$ 46.6	$ 39.4
Cash Paid During the Period for Income Taxes	0.1	2.8
Noncash Investing and Financing Activities [Abstract]
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment	(26.8)	(6.5)
Capitalized Asset Retirement Costs	7.8	0.6
AFUDC–Equity	2.6	5.9
ALLETE Common Stock Contributed to the Pension Plan	0.0	19.5
Contingent Consideration	$ 35.7	$ 0.0