Per Share Information - Schedule of Basic and Diluted Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 28, 2025	Dec. 29, 2024	Dec. 31, 2023	Sep. 29, 2024
Numerator for basic net income per common share -
Net income attributable to ATI	$ 404.3	$ 367.8	$ 410.8
Numerator for diluted net income per common share -
Net income attributable to ATI after assumed conversions	$ 404.3	$ 373.7	$ 421.4
Denominator:
Denominator for basic net income per common share—weighted average shares (in shares)	138.6	130.4	128.1
Effect of dilutive securities:
Share-based compensation (in shares)	3.2	3.2	3.1
Denominator for diluted net income per common share—adjusted weighted average shares and assumed conversions	141.8	146.6	150.0
Basic net income attributable to ATI per common share (in dollars per share)	$ 2.92	$ 2.82	$ 3.21
Diluted net income attributable to ATI per common share (in dollars per share)	$ 2.85	$ 2.55	$ 2.81
3.5% Convertible Senior Notes due 2025
Numerator for basic net income per common share -
Effect of dilutive securities: convertible senior notes	$ 0.0	$ 5.9	$ 10.6
Effect of dilutive securities:
Convertible Senior Notes- Effect on denominator (in shares)	0.0	13.0	18.8
3.5% Convertible Senior Notes due 2025 | Convertible Debt
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Interest rate				3.50%