v3.26.1
Joint Ventures (Details) - USD ($)
$ in Millions
Jun. 28, 2026
Dec. 28, 2025
Schedule of Equity Method Investments [Line Items]    
Cash and cash equivalents $ 783.0 $ 416.7
Shanghai STAL Precision Stainless Steel Co Ltd    
Schedule of Equity Method Investments [Line Items]    
Cash and cash equivalents $ 102.3 $ 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%