Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2024	Apr. 02, 2023	Dec. 31, 2023	Nov. 30, 2023
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income attributable to ATI	$ 66.1	$ 84.5
Net income attributable to ATI after assumed conversions	$ 68.2	$ 87.1
Denominator for basic net income per common share – weighted average shares	126.2	128.5
Share-based compensation	2.5	2.8
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions	147.5	150.1
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.52	$ 0.66
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.46	$ 0.58
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0.0	0.0
Purchase of treasury stock	$ (150.0)	$ (10.1)
Stock repurchase plan, authorized amount				$ 150.0
Treasury Stock, Shares, Acquired	3.4	0.2
Purchase of treasury stock	$ (151.2)	$ (10.1)
Treasury Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Purchase of treasury stock	(151.2)	(10.1)
ATI Inc. 3.5% Convertible Senior Notes due 2025
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible Senior Notes- Effect on numerator	$ 2.1	$ 2.6
Convertible Senior Notes- Effect on denominator	18.8	18.8
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Interest rate	3.50%		3.50%