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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($) $ in Millions |  | 9 Months Ended
--- | --- | ---
Sep. 22, 2016 | Sep. 30, 2016 | Sep. 30, 2015
Supplemental Cash Flow Information [Abstract] |  |  |
Cash Paid During the Period for Interest – Net of Amounts Capitalized |  | $ 54.9 | $ 46.6
Cash Paid During the Period for Income Taxes |  | 0.5 | 0.1
Noncash Investing and Financing Activities [Abstract] |  |  |
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment |  | (19.5) | (26.8)
Capitalized Asset Retirement Costs |  | 3.7 | 7.8
AFUDC–Equity |  | 1.7 | 2.6
Contingent Consideration |  | 0.0 | 35.7
Noncash Consideration for Land Inventory [Line Items] |  |  |
Consideration for Land Inventory | $ 21.0 |  |
ALLETE Common Stock Received for Land Inventory [Member] |  |  |
Noncash Consideration for Land Inventory [Line Items] |  |  |
Consideration for Land Inventory |  | 8.0 | 0.0
Long-Term Finance Receivable for Land Inventory [Member] |  |  |
Noncash Consideration for Land Inventory [Line Items] |  |  |
Consideration for Land Inventory |  | $ 12.0 | $ 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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