Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss) attributable to ATI	$ (7.9)	$ 21.1
Net income (loss) attributable to ATI after assumed conversions	$ (7.9)	$ 23.4
Denominator for basic net income (loss) per common share – weighted average shares	126.8	126.1
Share-based compensation	0.0	0.4
Denominator for diluted net income (loss) per common share – adjusted weighted average shares and assumed conversions	126.8	146.4
Basic net (loss) income attributable to ATI per common share (in dollars per share)	$ (0.06)	$ 0.17
Diluted net (loss) income attributable to ATI per common share (in dollars per share)	$ (0.06)	$ 0.16
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	25.7	0.0
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible Senior Notes- Effect on numerator	$ 0.0	$ 2.3
Convertible Senior Notes- Effect on denominator	0.0	19.9
Interest rate	4.75%		4.75%
Allegheny Technologies 3.5% Convertible Senior Notes due 2025
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible Senior Notes- Effect on numerator	$ 0.0	$ 0.0
Convertible Senior Notes- Effect on denominator	0.0	0.0

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