Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jul. 02, 2023	Jun. 30, 2024	Jul. 02, 2023	Dec. 31, 2023	Nov. 30, 2023
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income attributable to ATI	$ 81.9	$ 90.4	$ 148.0	$ 174.9
Net income attributable to ATI after assumed conversions	$ 84.1	$ 93.0	$ 152.3	$ 180.1
Denominator for basic net income per common share – weighted average shares	124.4	128.5	125.3	128.5
Share-based compensation	3.1	2.8	2.8	2.8
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions	146.3	150.1	146.9	150.1
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.66	$ 0.70	$ 1.18	$ 1.36
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.58	$ 0.62	$ 1.04	$ 1.20
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0.0	0.0	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	$ 2.2	$ 2.6	$ 4.3	$ 5.2
Convertible Senior Notes- Effect on denominator	18.8	18.8	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%		3.50%