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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, 2017 | Mar. 31, 2016
Supplemental Cash Flow Information [Abstract] |  |
Cash Paid During the Period for Interest – Net of Amounts Capitalized | $ 18.9 | $ 19.6
Noncash Investing and Financing Activities [Abstract] |  |
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment | 3.5 | 25.6
Capitalized Asset Retirement Costs | 19.3 | 3.5
AFUDC–Equity | 0.2 | 0.9
ALLETE Common Stock Contributed to the Pension Plan | $ 13.5 | $ 0.0

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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=66902209&loc=d3e3602-108585 + Details
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- Definition The increase (decrease) during the reporting period in the carrying amount of asset retirement obligations. + 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=66902209&loc=d3e3602-108585 + Details
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- Definition The amount of cash paid for interest during the period net of cash paid for interest that is capitalized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586 + Details
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