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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		58 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Jun. 30, 2024	Jul. 02, 2023	Jun. 30, 2024	Jul. 02, 2023	Jan. 01, 2023	Dec. 31, 2023
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 425.6		$ 425.6			$ 743.9
Income (loss) from equity investments	(0.4)	$ 0.0	(0.8)	$ (0.3)
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	$ 93.9		$ 93.9
Advanced Alloys & Solutions \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	(0.4)	(0.3)	(0.8)	(0.8)
Advanced Alloys & Solutions \| Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments		$ 0.3		$ 0.5
Advanced Alloys & Solutions \| Related Party \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Other receivables	$ 0.5		$ 0.5			$ 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%