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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Joint Ventures (Details) - USD ($) $ in Millions	10 Months Ended	12 Months Ended			49 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Mar. 31, 2022
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents		$ 687.7	$ 645.9
Income (loss) from equity method investments		0.1	(7.0)	$ (10.7)
Undistributed earnings of investees accounted for under equity method		13.8
Non-cash joint venture impairment charge		0.0	2.4	11.4
Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents		100.2
Next Gen Alloys LLC [Member]
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%
Impairment of long-lived assets held-for-use				14.2
Income (loss) from equity method investments		$ (0.9)	(10.6)	(19.3)
Revenue from related parties		0.0	18.4	14.6
Related party accounts receivable		$ 0.0	0.0
Due from affiliates, allowance for credit loss				4.3
Non-cash joint venture impairment charge				11.4
Business Exit Costs			4.8
Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage		50.00%
Revenue from related parties		$ 45.8	36.7	31.3
Related party accounts receivable		6.1	1.4
Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from affiliates		5.5
Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments		(0.9)	(8.2)	(12.2)
Due from affiliates		2.9	14.0
Advanced Alloys & Solutions [Member] \| Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments		1.0	1.2	1.5
Prepaid expenses and other current assets [Member] \| Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from affiliates		0.7	0.5
Other Noncurrent Assets [Member] \| Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from affiliates		$ 2.2	$ 13.5
Allegheny Technologies Inc \| Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage		60.00%
Allegheny Technologies Inc \| Next Gen Alloys LLC [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage		51.00%
Allegheny Technologies Inc \| China Baowu Steel Group Corporation Limited [Member] \| Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity		40.00%
Property, Plant and Equipment \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments				$ (7.1)
Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage		50.00%
Sale to Noncontrolling Interests	$ 12.0
Forecast \| Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Sale to Noncontrolling Interests					$ 17.5