v3.26.1
Per Share Information - Schedule of Basic and Diluted Income Per Common Share (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended 6 Months Ended
Jun. 28, 2026
Jun. 29, 2025
Jun. 28, 2026
Jun. 29, 2025
Numerator for basic income per common share –        
Net income attributable to ATI $ 151.0 $ 100.7 $ 269.2 $ 197.7
Denominator:        
Denominator for basic net income per common share – weighted average shares 136.3 139.8 136.5  
Effect of dilutive securities:        
Share-based compensation 2.0 3.3 2.0 3.0
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions 138.3 143.1 138.5 143.7
Basic net income attributable to ATI per common share (in dollars per share) $ 1.11 $ 0.72 $ 1.97 $ 1.40
Diluted net income attributable to ATI per common share (in dollars per share) $ 1.09 $ 0.70 $ 1.94 $ 1.38