Operations and Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013	Jan. 30, 2014	Dec. 31, 2013
Operations and Significant Accounting Policies [Abstract]
Restricted Cash and Cash Equivalents, Noncurrent	$ 5.3
Escrow Deposit					5.4
Inventories [Abstract]
Fuel	16.3				13.1
Materials and Supplies	52.6				46.2
Total Inventories	68.9				59.3
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause	22.1				23.0
Other	11.3				12.1
Total Prepayments and Other Current Assets	33.4				35.1
Other Current Liabilites [Abstract]
Customer Deposits	24.1				26.0
Power Purchase Agreement	13.1	[1]		12.4	0	[1]
Other	24.0				26.6
Total Other Current Liabilities	61.2				52.6
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation	89.2				81.8
Power Purchase Agreement	96.4	[1]		99.4	0	[1]
Other	43.7				45.4
Total Other Non-Current Liabilities	229.3				127.2
Supplemental Statement of Cash Flows Information [Abstract]
Cash Paid During the Period for Interest - Net of Amounts Capitalized	12.4		12.0
Cash Paid During the Period for Income Taxes	0.2		0.5
Noncash Investing and Financing Activities [Abstract]
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment	(22.6)		(26.2)
Capitalized Asset Retirement Costs	0.6		1.9
AFUDC - Equity	1.8		1.1
ALLETE Common Stock Contributed to the Pension Plan	$ 19.5		$ 0

[1] Power Purchase Agreements were acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisition on January 30, 2014 (see Note 4. Acquisitions).