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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		10 Months Ended	58 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2018	Dec. 31, 2022
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 267.1	$ 584.0		$ 267.1			$ 584.0
Sale of noncontrolling interests				0.0	$ 0.9
Income (loss) from equity investments	0.0		$ 10.0	(0.3)	10.4
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Tariff refunds receivable			19.7		19.7
Allegheny & Tsingshan Stainless \| A&T Stainless
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	$ 1.1			$ 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%			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%			50.00%
Advanced Alloys & Solutions \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 70.5			$ 70.5
Advanced Alloys & Solutions \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	(0.3)		9.5	(0.8)	9.2
Advanced Alloys & Solutions \| Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	0.3		$ 0.5	0.5	$ 1.2
Advanced Alloys & Solutions \| Related Party \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Other receivables	2.4	3.2		2.4			3.2
Prepaid expenses and other current assets \| Advanced Alloys & Solutions \| Related Party \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Other receivables	0.5	0.4		0.5			0.4
Other Noncurrent Assets [Member] \| Advanced Alloys & Solutions \| Related Party \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Other receivables	$ 1.9	$ 2.8		$ 1.9			$ 2.8
China Baowu Steel Group Corporation Limited \| Advanced Alloys & Solutions \| China Baowu Steel Group Corporation Limited \| Shanghai STAL Precision Stainless Steel Co Ltd
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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%