Investments (Details) (USD $) In Millions	9 Months Ended	12 Months Ended
Sep. 30, 2011	Dec. 31, 2010
Investments [Abstract]
ALLETE Properties	$ 93.0	$ 94.0
Available-for-sale Securities	30.5	25.2
Other	6.0	6.8
Total Investments	129.5	126.0
ALLETE Properties [Rollforward]
Land Held-for-sale Beginning Balance (January 1, 2011 and 2010, respectively)	86.0	74.9
Deeds to Collateralized Property	1.9	9.9
Capitalized Improvements and Other	0.1	1.2
Cost of Real Estate Sold	(0.3)	0
Land Held-for-sale Ending Balance	87.7	86.0
Long-Term Finance Receivables (net of allowances of $0.6 and $0.8)	2.0	3.7
Other	3.3	4.3
Total Real Estate Assets	93.0	94.0
Long-Term Finance Receivables (net of allowances of $0.6 and $0.8)	2.0	3.7
Long-Term Finance Receivables Allowance [Roll Forward]
Beginning Balance as of December 31, 2010	0.8
Reduction to Reserve	(0.2)
Ending Balance as of September 30, 2011	$ 0.6	$ 0.8

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

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Land held for sale recorded at cost.

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The allowance for loan and lease losses represents the reserve to cover probable credit losses related to specifically identified loans and leases, as well as probable credit losses inherent in the remainder of the loan portfolio as of the balance sheet date. For banks, include currently required allocated transfer risk reserves. Include carryover of or adjustments to the allowance for loan losses in connection with business combinations determined to be appropriate.

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Reflects the aggregate carrying amount of all categories of loans and leases held in portfolio, net of unearned income and the allowance for losses on loans and leases.

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Other investments not otherwise specified in the taxonomy.

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Other long-term investments not otherwise specified in the taxonomy, not including investments in marketable securities.

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Amount of real estate investments acquired through foreclosure during the period. Title through foreclosure occurs in a legal proceeding in which a secured creditor obtains title typically by bidding the amount of debt owed to it after the borrower has defaulted on the debt instrument.

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The carrying amount of real estate sold during the period.

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Gross amount of real estate as of the balance sheet date.

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Amount of improvements made to real estate investments during the period.

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