Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Earnings Per Share Reconciliation [Abstract]
Income (loss) attributable to ATI	$ (101.2)	$ 10.0
Redeemable noncontrolling interest	0.0	(0.1)
Income (loss) available to ATI after assumed conversions	$ (101.2)	$ 9.9
Denominator for basic net income (loss) per common share – weighted average shares	107.3	107.2
Share-based compensation	0.0	0.8
Denominator for diluted net income (loss) per common share – adjusted weighted average shares assuming conversions	107.3	108.0
Basic income (loss) from continuing operations attributable to ATI per common share (in dollars per share)	$ (0.94)	$ 0.09
Diluted income (loss) from continuing operations attributable to ATI per common share (in dollars per share)	$ (0.94)	$ 0.09
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount	0.8	0.0

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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