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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		10 Months Ended	54 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2018	Sep. 30, 2022	Dec. 31, 2021
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 274.0		$ 274.0				$ 687.7
Sale of noncontrolling interests			0.9	$ 0.0
Income (loss) from equity investments	10.0	$ 0.0	10.4	0.0
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Tariff refunds receivable	19.7		19.7
Allegheny & Tsingshan Stainless \| Import Duties
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments			9.9
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1.8		1.8
Sale of noncontrolling interests			$ 0.9
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Sale of noncontrolling interests					$ 12.0
Allegheny & Tsingshan Stainless \| Forecast
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%
Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	$ 5.5		$ 5.5
Advanced Alloys & Solutions [Member] \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	61.1		61.1
Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	9.5	(0.4)	9.2	(0.9)
Due from Affiliates	11.9		11.9				2.9
Advanced Alloys & Solutions [Member] \| Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	0.5	$ 0.4	1.2	$ 0.9
Prepaid expenses and other current assets \| Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	0.5		0.5				0.7
Other Noncurrent Assets [Member] \| Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	$ 11.4		$ 11.4				$ 2.2
China Baowu Steel Group Corporation Limited \| Advanced Alloys & Solutions [Member] \| 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 [Member] \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%		60.00%