Summary of Significant Accounting Policies - New and Pending Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 29, 2024	Dec. 31, 2023	Jan. 01, 2023	Jan. 02, 2022
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest	$ 1,955.2	$ 1,480.5	$ 1,157.2	$ 832.7
Additional Paid-In Capital
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest	1,943.9	1,697.1	1,668.1	1,596.7
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest	$ 64.3	$ (70.1)	$ (480.9)	$ (777.7)
ATI Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		3.50%