Summary of Significant Accounting Policies - New and Pending Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 31, 2023	Jan. 01, 2023	Jan. 02, 2022	Jan. 03, 2021
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest	$ 1,480.5	$ 1,157.2	$ 832.7	$ 641.4
Additional Paid-In Capital
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest	1,697.1	1,668.1	1,596.7	1,625.5
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest	$ (70.1)	$ (480.9)	$ (777.7)	106.5
Accounting Standards Update 2020-06 | Additional Paid-In Capital
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest				(49.8)
Accounting Standards Update 2020-06 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest				4.4
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%	3.50%	3.50%
ATI Inc., Convertible Senior Notes, 3.5%, Due 2025 | Convertible Debt [Member] | Accounting Standards Update 2020-06
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest				$ (45.4)