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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($) $ in Millions | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017
Supplemental Cash Flow Information [Abstract] |  |  |
Cash Paid for Interest – Net of Amounts Capitalized |  | $ 33.0 | $ 32.5
Noncash Investing and Financing Activities [Abstract] |  |  |
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment |  | (39.4) | (0.4)
Reclassification of Property, Plant and Equipment to Inventory | [1] | 46.3 | 0.0
Capitalized Asset Retirement Costs |  | 20.8 | 19.7
AFUDC–Equity |  | 0.5 | 0.4
ALLETE Common Stock Contributed to the Pension Plans |  | $ 0.0 | $ 13.5
[1] | On February 28, 2018, Montana-Dakota Utilities exercised its option to purchase the Thunder Spirit II wind energy facility upon completion, resulting in a reclassification of the project costs from Property, Plant and Equipment – Net to Inventories – Net as ALLETE Clean Energy will not own the facility upon completion.

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