Operations and Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013	Jan. 30, 2014	Dec. 31, 2013
Inventories [Abstract]
Fuel	$ 16.8				$ 13.1
Materials and Supplies	54.8				46.2
Total Inventories	71.6				59.3
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause	18.5				23.0
Restricted Cash	2.4	[1]			0	[1]
Other	8.4				12.1
Total Prepayments and Other Current Assets	29.3				35.1
Other Non-Current Assets [Abstract]
Restricted Cash	4.9				5.4
Other Current Liabilites [Abstract]
Customer Deposits	21.2				26.0
Power Purchase Agreements	12.3	[2]		12.4	0	[2]
Other	26.9				26.6
Total Other Current Liabilities	60.4				52.6
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation	94.6				81.8
Power Purchase Agreements	90.5	[2]		99.4	0	[2]
Other	46.5				45.4
Total Other Non-Current Liabilities	231.6				127.2
Supplemental Statement of Cash Flows Information [Abstract]
Cash Paid During the Period for Interest - Net of Amounts Capitalized	39.4		36.0
Cash Paid During the Period for Income Taxes	2.8		0.6
Noncash Investing and Financing Activities [Abstract]
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment	(6.5)		(10.0)
Capitalized Asset Retirement Costs	0.6		1.9
AFUDC - Equity	5.9		3.4
ALLETE Common Stock Contributed to the Pension Plan	$ 19.5		$ 0

[1] Restricted Cash related to ALLETE Clean Energy’s wind energy facilities operating expense and capital distribution reserve requirements.

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