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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		58 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Mar. 31, 2024	Apr. 02, 2023	Jan. 01, 2023	Dec. 31, 2023
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 394.4			$ 743.9
Income (loss) from equity investments	(0.4)	$ (0.3)
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%
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%
Advanced Alloys & Solutions \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 73.7
Advanced Alloys & Solutions \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	(0.4)	(0.5)
Advanced Alloys & Solutions \| Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments		$ 0.2
Advanced Alloys & Solutions \| Related Party \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Other receivables	$ 1.0			$ 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%
Tsingshan Group \| Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%
VSMPO \| VSMPO \| Uniti
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%
ATI Inc. \| Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	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%