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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 30, 2025	Mar. 31, 2024	Dec. 29, 2024
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 475.8		$ 721.2
Income (loss) from equity investments	0.0	$ (0.4)
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1.0
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%
AA&S | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 71.3
AA&S | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments		$ (0.4)
AA&S | Related Party | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Other receivables	$ 0.5		$ 0.6
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. | Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%
ATI Inc. | AA&S | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%

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