Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Earnings Per Share Reconciliation [Abstract]
Net income attributable to ATI	$ 21.1	$ 15.0
4.75% Convertible Senor Notes due 2022	2.3	0.0
Net income attributable to ATI after assumed conversions	$ 23.4	$ 15.0
Denominator for basic net income per common share – weighted average shares	126.1	125.4
Share-based compensation	0.4	0.7
Effective of dilutive securities: 4.75% Convertible Senior Notes due 2022	19.9	0.0
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions	146.4	126.1
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.17	$ 0.12
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.16	$ 0.12
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0.0	19.9
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Interest rate	4.75%		4.75%

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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