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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The per share price of the Company's stock at which the contract holder of the freestanding contract has the right to purchase or sell the Company's stock at a future date.

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The number of issuer's shares to which the forward contract is indexed.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Net Income or Loss Available to Common Stockholders plus adjustments resulting from the assumption that dilutive convertible securities were converted, options or warrants were exercised, or that other shares were issued upon the satisfaction of certain conditions.

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The cash inflow from the issuance of common stock, preferred stock, treasury stock, stock options, and other types of equity.

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

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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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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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Earnings Per Share and Common Stock (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Feb. 04, 2015	Sep. 05, 2014	Feb. 26, 2014	Mar. 04, 2014
Earnings Per Share and Common Stock [Abstract]
Antidilutive Shares Excluded from Diluted EPS Computation	0	0
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 39.9	$ 33.5
Average Common Shares	46.9	41.4
Earnings Per Share	$ 0.85us-gaap_EarningsPerShareBasic	$ 0.81us-gaap_EarningsPerShareBasic
Earnings Per Share - Diluted [Abstract]
Net Income to Attributable ALLETE	39.9	33.5
Average Common Shares	47.1	41.6
Earnings Per Share	$ 0.85us-gaap_EarningsPerShareDiluted	$ 0.80us-gaap_EarningsPerShareDiluted
Dilutive Securities (in shares)	0.2	0.2
Proceeds from Issuance of Common Stock - Pension Plan	0	19.5
Common Stock Issued - Pension Plan (in shares)		0.4
Forward Contracts [Member]
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward Contract Indexed to Issuer's Equity, Indexed Shares					2.8
Common Stock Issued - Option Agreement (in shares)			1.4	1.4
Proceeds from Issuance of Common Stock - Option Agreement			65.4	65.0
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share					$ 48.01
Options Held [Member]
Forward Contract Indexed to Issuer's Equity [Line Items]
Common Stock Issued - Option Agreement (in shares)						0.4
Proceeds from Issuance of Common Stock - Option Agreement						$ 20.2