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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended			10 Months Ended	58 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2018	Dec. 31, 2022
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 196.2	$ 584.0			$ 584.0
Sale of noncontrolling interests	0.0		$ 0.9
Income (loss) from equity investments	(0.3)		0.4
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1.1
Sale of noncontrolling interests			0.9
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		5.5		$ 12.0	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	$ 52.9
Advanced Alloys & Solutions \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	(0.5)		(0.3)
Due from Affiliates	2.9	3.2			3.2
Advanced Alloys & Solutions \| Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	0.2		$ 0.7
Prepaid expenses and other current assets \| Advanced Alloys & Solutions \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	0.5	0.4			0.4
Other Noncurrent Assets [Member] \| Advanced Alloys & Solutions \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	$ 2.4	$ 2.8			$ 2.8
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%