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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		58 Months Ended
	Sep. 29, 2024	Oct. 01, 2023	Sep. 29, 2024	Oct. 01, 2023	Jan. 01, 2023	Dec. 31, 2023
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 406.6		$ 406.6			$ 743.9
Income (loss) from equity investments	(0.2)	$ (0.7)	(1.0)	$ (1.0)
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1.0		$ 1.0
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Allegheny & Tsingshan Stainless | Tsingshan Group
Schedule of Equity Method Investments [Line Items]
Sale of noncontrolling interests					$ 17.5
Uniti
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Advanced Alloys & Solutions | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 102.3		$ 102.3
Advanced Alloys & Solutions | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	(0.2)	(0.5)	(1.0)	(1.3)
Advanced Alloys & Solutions | Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments		$ (0.2)		$ 0.3
Advanced Alloys & Solutions | Related Party | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Other receivables	$ 0.2		$ 0.2			$ 1.5
China Baowu Steel Group Corporation Limited | Advanced Alloys & Solutions | 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%
Tsingshan Group | Tsingshan Group | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%		50.00%
VSMPO | VSMPO | Uniti
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%		50.00%
ATI Inc. | Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%		51.00%
ATI Inc. | Advanced Alloys & Solutions | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%		60.00%

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