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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Supplemental Cash Flow Information [Abstract] |  |  |  |
Cash Paid During the Period for Interest – Net of Amounts Capitalized |  | $ 66.0 | $ 64.5 | $ 68.2
Remeasurement of Deferred Income Taxes Resulting from the TCJA [Abstract] |  |  |  |
Increase to Regulatory Assets |  | 0.0 | 80.9 | 0.0
Decrease in Investment in ATC |  | 0.0 | (27.9) | 0.0
Increase (Decrease) in Deferred Income Taxes |  | 0.0 | (353.6) | 0.0
Increase to Regulatory Liabilities |  | 0.0 | 393.6 | 0.0
Noncash Investing and Financing Activities [Abstract] |  |  |  |
Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment |  | (0.1) | 67.2 | (22.0)
Reclassification of Property, Plant and Equipment to Inventory | [1] | 46.3 | 0.0 | 0.0
Capitalized Asset Retirement Costs |  | 14.2 | (15.6) | 3.7
Camp Ripley Solar Financing |  | 0.0 | 0.0 | 15.0
AFUDC–Equity |  | 1.2 | 1.2 | 2.1
ALLETE Common Stock Contributed to the Defined Benefit Pension Plan |  | 0.0 | 13.5 | 0.0
ALLETE Common Stock Received for Land Inventory [Member] |  |  |  |
Noncash Consideration for Land Inventory [Line Items] |  |  |  |
Consideration for Land Inventory |  | 0.0 | 0.0 | 8.0
Long-Term Finance Receivable for Land Inventory [Member] |  |  |  |
Noncash Consideration for Land Inventory [Line Items] |  |  |  |
Consideration for Land Inventory |  | $ 0.0 | $ 0.0 | $ 12.0
[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 $46.3 million reclassification from Property, Plant and Equipment – Net to Inventories – Net for project costs incurred in the prior year. On the Consolidated Statement of Cash Flows, the sale of the wind energy facility in the fourth quarter of 2018 resulted in Operating Activities – Inventories increasing by $46.3 million in 2018 due to the project costs incurred in the prior year.

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