Per Share Information (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share Reconciliation [Abstract]
Income (loss) from continuing operations attributable to ATI	$ 10.0	$ (18.1)
Redeemable noncontrolling interest	(0.1)us-gaap_TemporaryEquityOtherChanges	0us-gaap_TemporaryEquityOtherChanges
Income (loss) from continuing operations available to ATI after assumed conversions	$ 9.9	$ (18.1)
Denominator for basic net income (loss) per common share-weighted average shares	107,200,000	107,000,000
Share-based compensation	800,000	0
4.25% Convertible Notes due 2014	0	0
Denominator for diluted net income (loss) per common share – adjusted weighted average shares assuming conversions	108,000,000	107,000,000
Basic income (loss) from continuing operations attributable to ATI per common share (in dollars per share)	$ 0.09	$ (0.17)
Diluted income (loss) from continuing operations attributable to ATI per common share (in dollars per share)	$ 0.09	$ (0.17)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount	0	10,000,000

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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Amount of income (loss) from continuing operations attributable to the parent. Also defined as revenue less expenses and taxes from ongoing operations before extraordinary items but after deduction of those portions of income or loss from continuing operations that are allocable to noncontrolling interests.

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

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The amount of net income (loss) derived from continuing operations during 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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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of convertible debt securities using the if-converted method.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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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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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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