Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Cash Flow Information [Abstract]
Cash Paid During the Period for Interest - Net of Amounts Capitalized	$ 51.3us-gaap_InterestPaidNet	$ 47.5us-gaap_InterestPaidNet	$ 42.7us-gaap_InterestPaidNet
Cash Paid During the Period for Income Taxes	5.1us-gaap_IncomeTaxesPaidNet	0.5us-gaap_IncomeTaxesPaidNet	0us-gaap_IncomeTaxesPaidNet
Noncash Investing and Financing Activities [Abstract]
Increase in Accounts Payable for Capital Additions to Property, Plant and Equipment	21.7	8.3	20.2
Capitalized Asset Retirement Costs	22.4	(0.7)	17.1
AFUDC - Equity	7.8	4.6	5.1
ALLETE Common Stock Contributed to the Defined Benefit Pension Plan	$ 19.5	$ 0	$ 0

Net Increase or Decrease in the asset retirement obligation during the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

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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

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-Section 50

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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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

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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)

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The amount of cash paid for interest during the period net of cash paid for interest that is capitalized.

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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.

+ References

No definition available.

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