Joint Ventures (Details) - USD ($) $ in Millions	Mar. 29, 2026	Dec. 28, 2025
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 401.7	$ 416.7
Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 75.1
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%