Joint Ventures (Details) - USD ($) $ in Millions	Sep. 28, 2025	Dec. 29, 2024
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 372.2	$ 721.2
AA&S | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 99.1
China Baowu Steel Group Corporation Limited | AA&S | China Baowu Steel Group Corporation Limited | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	40.00%
ATI Inc. | AA&S | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%