Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2023	Jan. 01, 2023	Jan. 02, 2022	Nov. 30, 2023	Apr. 28, 2023	Feb. 02, 2022
Earnings Per Share Reconciliation [Abstract]
Net income (loss)	$ 410.8	$ 323.5	$ 184.6
Numerator for diluted net income (loss) per common share - Income (loss) attributable to ATI after assumed conversions	$ 421.4	$ 337.0	$ 200.8
Denominator for basic net income (loss) per common share - weighted average shares (shares)	128.1	127.5	127.1
Effect of dilutive securities: Share-based compensation	3.1	2.1	1.0
Denominator for diluted net income per common share—adjusted weighted average shares and assumed conversions	150.0	151.2	152.7
Basic income (loss) attributable to ATI per common share (in dollars per share)	$ 3.21	$ 2.54	$ 1.45
Diluted income (loss) attributable to ATI per common share (in dollars per share)	$ 2.81	$ 2.23	$ 1.32
Anti-dilutive securities excluded from computation of earnings per share amount	0.0	0.0	0.0
Authorized amount				$ 150.0	$ 75.0	$ 150.0
Purchase of treasury stock	$ (85.2)	$ (139.9)	$ 0.0
Treasury stock, shares, acquired	2.0	5.2
Purchase of treasury stock	$ (85.8)	$ (139.9)
ATI Convertible Senior Notes, 4.75%, Due 2022 [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			4.75%
Earnings Per Share Reconciliation [Abstract]
Effect of dilutive securities: Convertible Senior Notes	$ 0.0	$ 2.2	$ 4.4
Effect of dilutive securities: Convertible Senior Notes	0.0	2.8	5.8
ATI Convertible Senior Notes, 3.5%, Due 2025 [Member]
Earnings Per Share Reconciliation [Abstract]
Effect of dilutive securities: Convertible Senior Notes	$ 10.6	$ 11.3	$ 11.8
Effect of dilutive securities: Convertible Senior Notes	18.8	18.8	18.8
Treasury Stock
Earnings Per Share Reconciliation [Abstract]
Purchase of treasury stock	$ (85.8)	$ (139.9)
Convertible Debt [Member] | ATI Convertible Senior Notes, 3.5%, Due 2025 [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.50%	3.50%	3.50%