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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($) $ in Millions	3 Months Ended
	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 decrease in accounts payable in the current period related to capital additions recognized in prior periods.

+ References

No definition available.

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- References

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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

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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

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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

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- Definition

Value of shares issued during the period to an employee benefit plan, such as a defined contribution or defined benefit plan.

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