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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.)

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