Debt - Schedule of Interest Expense On the 2025 Convertible Notes (Details) - ATI Convertible Senior Notes, 3.5%, Due 2025 - Convertible Debt - USD ($) $ in Millions	12 Months Ended
	Dec. 29, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Contractual coupon rate	$ 7.2	$ 10.2
Amortization of debt issuance costs	1.3	1.9
Total interest expense	$ 8.5	$ 12.1