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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		10 Months Ended	45 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2018	Dec. 31, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1,006.8		$ 1,006.8				$ 645.9
Income (loss) from equity investments	0.4	$ (1.0)	0.4	$ (6.6)
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1.4		$ 1.4
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Sales to noncontrolling interests					$ 12.0
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	$ 0.0	0.0	$ 0.0	(2.4)
Allegheny & Tsingshan Stainless | Forecast
Schedule of Equity Method Investments [Line Items]
Sales to 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	54.9		54.9
Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	0.3	(1.6)	(0.6)	(10.1)
Due from Affiliates	4.8		4.8				14.0
Advanced Alloys & Solutions [Member] | Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	0.1	$ 0.6	1.0	$ 1.1
Prepaid expenses and other current assets | Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	0.8		0.8				0.5
Other Noncurrent Assets [Member] | Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	$ 4.0		$ 4.0				$ 13.5
Allegheny Technologies Inc | Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%		51.00%
Allegheny Technologies 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%
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%

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