Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 29, 2024	Oct. 01, 2023	Sep. 29, 2024	Oct. 01, 2023	Sep. 03, 2024	Dec. 31, 2023
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income attributable to ATI	$ 82.7	$ 90.2	$ 230.7	$ 265.1
Net income attributable to ATI after assumed conversions	$ 84.4	$ 92.9	$ 236.7	$ 273.0
Denominator for basic net income per common share – weighted average shares	128.7	128.1	126.5	128.4
Share-based compensation	3.7	3.3	3.1	2.9
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions	146.8	150.2	146.9	150.1
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.64	$ 0.70	$ 1.82	$ 2.06
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.57	$ 0.62	$ 1.61	$ 1.82
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0.0	0.0	0.0	0.0
Purchase of treasury stock	$ (40.0)	$ (45.0)	$ (190.0)	$ (55.1)
Stock repurchase plan, authorized amount					$ 700.0
Treasury Stock, Shares, Acquired	0.7	1.0	4.1	1.2
Share Repurchase Program, Utilized, Amount	$ 40.0		$ 40.0
Purchase of treasury stock	(38.8)	$ (45.4)	(190.0)	$ (55.5)
Treasury Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Purchase of treasury stock	(38.8)	(45.4)	(190.0)	(55.5)
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	$ 1.7	$ 2.7	$ 6.0	$ 7.9
Convertible Senior Notes- Effect on denominator	14.4	18.8	17.3	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%