Per Share Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 28, 2025	Sep. 29, 2024	Sep. 28, 2025	Sep. 29, 2024	Sep. 03, 2024
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income attributable to ATI	$ 110.0	$ 82.7	$ 307.7	$ 230.7
Net income attributable to ATI after assumed conversions	$ 110.0	$ 84.4	$ 307.7	$ 236.7
Denominator for basic net income per common share – weighted average shares	137,000	128,700	139,500	126,500
Share-based compensation	3,500	3,700	3,100	3,100
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions	140,500	146,800	142,600	146,900
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.80	$ 0.64	$ 2.21	$ 1.82
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.78	$ 0.57	$ 2.16	$ 1.61
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0	0	0	0
Purchase of treasury stock	$ (150.0)	$ (40.0)	$ (470.0)	$ (190.0)
Stock repurchase plan, authorized amount					$ 700.0
Treasury Stock, Shares, Acquired	2,000	700	6,400	4,100
Share Repurchase Program, Utilized, Amount	$ 580.0		$ 580.0
Purchase of treasury stock	(151.5)	$ (38.8)	(474.3)	$ (190.0)
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	$ 1.7	$ 0.0	$ 6.0
Convertible Senior Notes- Effect on denominator	0	14,400	0	17,300
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	$ (151.5)	$ (38.8)	$ (474.3)	$ (190.0)