Joint Ventures (Details) - USD ($) $ in Millions	Dec. 28, 2025	Dec. 29, 2024
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 416.7	$ 721.2
Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 97.6
Allegheny Technologies Inc | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%
Allegheny Technologies Inc | Shanghai STAL Precision Stainless Steel Co Ltd | China Baowu Steel Group Corporation Limited
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	40.00%