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
	Mar. 29, 2026	Mar. 30, 2025
Numerator for basic income per common share –
Net income attributable to ATI	$ 118.2	$ 97.0
Denominator:
Denominator for basic net income per common share – weighted average shares	136.7	141.7
Effect of dilutive securities:
Share-based compensation	1.9	2.5
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions	138.6	144.2
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.86	$ 0.68
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.85	$ 0.67