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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Supplemental Cash Flow Information [Abstract] |  |  |
Cash Paid for Interest – Net of Amounts Capitalized |  | $ 19.3 | $ 18.9
Noncash Investing and Financing Activities [Abstract] |  |  |
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment |  | (48.1) | (3.5)
Reclassification of Property, Plant and Equipment to Inventory | [1] | 46.9 | 0.0
Capitalized Asset Retirement Costs |  | 0.8 | 19.3
AFUDC–Equity |  | 0.3 | 0.2
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 value from Property, Plant and Equipment – Net to Inventory – Net as ALLETE Clean Energy will no longer own and operate the facility upon completion.

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- Definition The increase (decrease) during the reporting period in the capitalized cost of equity (based on assumed rate of return) and / or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments. As a result of this capitalization, net income is increased. This element pertains to certain regulated industries, such as public utilities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 + Details
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