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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Jun. 29, 2025	Jun. 30, 2024	Jun. 29, 2025	Jun. 30, 2024	Dec. 29, 2024
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 319.6		$ 319.6		$ 721.2
Other	0.0	$ (0.4)	0.0	$ (0.8)
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	1.0		1.0
AA&S \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 86.7		$ 86.7
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%		40.00%
ATI Inc. \| Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%		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%		60.00%