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Per Share Information (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended
Mar. 30, 2025
Mar. 31, 2024
Sep. 03, 2024
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Net income attributable to ATI $ 97.0 $ 66.1  
Net income attributable to ATI after assumed conversions $ 97.0 $ 68.2  
Denominator for basic net income per common share – weighted average shares 141.7 126.2  
Share-based compensation 2.5 2.5  
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions 144.2 147.5  
Basic net income attributable to ATI per common share (in dollars per share) $ 0.68 $ 0.52  
Diluted net income attributable to ATI per common share (in dollars per share) $ 0.67 $ 0.46  
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount 0.0 0.0  
Purchase of treasury stock $ (70.0) $ (150.0)  
Stock repurchase plan, authorized amount     $ 700.0
Treasury Stock, Shares, Acquired 1.2 3.4  
Share Repurchase Program, Utilized, Amount $ 180.0    
Purchase of treasury stock (70.2) $ (151.2)  
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 $ 2.1  
Convertible Senior Notes- Effect on denominator 0.0 18.8  
3.5% Convertible Senior Notes due 2025 | Convertible Debt      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage   3.50%  
Interest rate   3.50%  
Treasury Stock      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Purchase of treasury stock $ (70.2) $ (151.2)