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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016
Supplemental Cash Flow Information [Abstract] |  |
Cash Paid During the Period for Interest – Net of Amounts Capitalized | $ 51.4 | $ 54.9
Cash Paid During the Period for Income Taxes | 0.4 | 0.5
Noncash Investing and Financing Activities [Abstract] |  |
Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment | 1.2 | (19.5)
Capitalized Asset Retirement Costs | 19.7 | 3.7
AFUDC–Equity | 0.7 | 1.7
ALLETE Common Stock Contributed to the Pension Plans | 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 | 8.0
Long-Term Finance Receivable for Land Inventory [Member] |  |
Noncash Consideration for Land Inventory [Line Items] |  |
Consideration for Land Inventory | $ 0.0 | $ 12.0

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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=98513485&loc=d3e4297-108586 + Details
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