Per Share Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 29, 2025	Jun. 30, 2024	Jun. 29, 2025	Jun. 30, 2024	Sep. 03, 2024
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income attributable to ATI	$ 100.7	$ 81.9	$ 197.7	$ 148.0
Net income attributable to ATI after assumed conversions	$ 100.7	$ 84.1	$ 197.7	$ 152.3
Denominator for basic net income per common share – weighted average shares	139,800	124,400	140,700	125,300
Share-based compensation	3,300	3,100	3,000	2,800
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions	143,100	146,300	143,700	146,900
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.72	$ 0.66	$ 1.40	$ 1.18
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.70	$ 0.58	$ 1.38	$ 1.04
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0	0	0	0
Purchase of treasury stock	$ (250.0)		$ (320.0)	$ (150.0)
Stock repurchase plan, authorized amount					$ 700.0
Treasury Stock, Shares, Acquired	3,200		4,400	3,400
Share Repurchase Program, Utilized, Amount	$ 430.0		$ 430.0
Purchase of treasury stock	(252.6)	$ 0.0	(322.8)	$ (151.2)
3.5% Convertible Senior Notes due 2025
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible Senior Notes- Effect on numerator	$ 0.0	$ 2.2	$ 0.0	$ 4.3
Convertible Senior Notes- Effect on denominator	0	18,800	0	18,800
3.5% Convertible Senior Notes due 2025 | Convertible Debt
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		3.50%		3.50%
Interest rate		3.50%		3.50%
Treasury Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Purchase of treasury stock	$ (252.6)	$ 0.0	$ (322.8)	$ (151.2)