Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earnings Per Share Reconciliation [Abstract]
Net income (loss)	$ (38.2)	$ (1,572.6)	$ 252.5
Numerator for diluted net income (loss) per common share - Income (loss) attributable to ATI after assumed conversions	$ (38.2)	$ (1,572.6)	$ 265.3
Denominator for basic net income (loss) per common share - weighted average shares (shares)	127.1	126.5	125.8
Effect of dilutive securities: Share-based compensation	0.0	0.0	0.8
Denominator for diluted net income (loss) per common share—adjusted weighted average shares and assumed conversions	127.1	126.5	146.5
Basic income (loss) attributable to ATI per common share (in dollars per share)	$ (0.30)	$ (12.43)	$ 2.01
Diluted income (loss) attributable to ATI per common share (in dollars per share)	$ (0.30)	$ (12.43)	$ 1.81
Anti-dilutive securities excluded from computation of earnings per share amount	25.6	22.8	0.0
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member]
Earnings Per Share Reconciliation [Abstract]
Effect of dilutive securities: Convertible Senior Notes	$ 0.0	$ 0.0	$ 12.8
Effect of dilutive securities: Convertible Senior Notes	0.0	0.0	19.9
Debt instrument interest rate stated percentage	4.75%	4.75%
Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025 [Member]
Earnings Per Share Reconciliation [Abstract]
Effect of dilutive securities: Convertible Senior Notes	$ 0.0	$ 0.0	$ 0.0
Effect of dilutive securities: Convertible Senior Notes	0.0	0.0	0.0
Convertible Debt [Member] \| Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member]
Earnings Per Share Reconciliation [Abstract]
Debt instrument interest rate stated percentage	4.75%
Convertible Debt [Member] \| Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025 [Member]
Earnings Per Share Reconciliation [Abstract]
Debt instrument interest rate stated percentage	3.50%	3.50%

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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