Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($) $ in Millions		9 Months Ended
	Sep. 22, 2016	Sep. 30, 2016	Sep. 30, 2015
Supplemental Cash Flow Information [Abstract]
Cash Paid During the Period for Interest – Net of Amounts Capitalized		$ 54.9	$ 46.6
Cash Paid During the Period for Income Taxes		0.5	0.1
Noncash Investing and Financing Activities [Abstract]
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment		(19.5)	(26.8)
Capitalized Asset Retirement Costs		3.7	7.8
AFUDC–Equity		1.7	2.6
Contingent Consideration		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		8.0	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