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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Supplemental Cash Flow Information [Abstract]      
Cash Paid During the Period for Interest - Net of Amounts Capitalized $ 42.7 $ 43.2 $ 35.7
Cash Received During the Period for Income Taxes 0 [1] (11.4) [1] (54.2) [1]
Noncash Investing and Financing Activities [Abstract]      
Increase in Accounts Payable for Capital Additions to Property, Plant and Equipment 20.2 5.9 7.5
Capitalized Asset Retirement Costs 17.1 0.3 2.8
AFUDC - Equity 5.1 2.5 4.2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
ALLETE Common Stock Contributed to Pension Plan 0 (20.0) 0
Pension [Member]
      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
ALLETE Common Stock Contributed to Pension Plan   $ 20.0  
[1] Due to bonus depreciation provisions in 2009 and 2010 federal legislation, NOLs were generated which resulted in little or no estimated tax payments, and refunds were received from NOL carrybacks against prior years’ taxable income.