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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Supplemental Cash Flow Information [Abstract] |  |  |
Cash Paid During the Period for Interest – Net of Amounts Capitalized | $ 59.0 | $ 51.3 | $ 47.5
Cash Paid During the Period for Income Taxes | 0.4 | 5.1 | 0.5
Noncash Investing and Financing Activities [Abstract] |  |  |
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment | (40.6) |  |
Increase in Accounts Payable for Capital Additions to Property, Plant and Equipment |  | 21.7 | 8.3
Capitalized Asset Retirement Costs | 12.4 | 22.4 | (0.7)
AFUDC–Equity | 3.3 | 7.8 | 4.6
ALLETE Common Stock Contributed to the Defined Benefit Pension Plan | 0.0 | 19.5 | 0.0
Noncash or Part Noncash Acquisition, Value of Liabilities Assumed | $ 35.7 | $ 0.0 | $ 0.0

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- Definition Future cash outflow to pay for purchases of fixed assets that have occurred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 + Details
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- Definition The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income, net of any cash received during the current period as refunds for the overpayment of taxes. + 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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