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Amount of consolidated retained earnings that represent undistributed earnings of 50% or less owned by persons accounted for by the equity method

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Amount of receivables arising from transactions with related parties due within one year or the normal operating cycle, if longer.

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Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Includes, but is not limited to, one-time termination benefits, termination of an operating lease or other contract, consolidating or closing facilities, and relocating employees, and termination benefits associated with an ongoing benefit arrangement. Excludes expenses associated with special or contractual termination benefits, a discontinued operation or an asset retirement obligation.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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For an unclassified balance sheet, amount of receivables due from an entity that is affiliated with the reporting entity by means of direct or indirect ownership.

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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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Amount of income (loss) for proportionate share of equity method investee's income (loss).

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Decrease in noncontrolling interest (for example, but not limited to, redeeming or purchasing the interests of noncontrolling shareholders, issuance of shares (interests) by the non-wholly owned subsidiary to the parent entity for other than cash, and a buyback of shares (interest) by the non-wholly owned subsidiary from the noncontrolling interests).

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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Amount of cash inflow from a noncontrolling interest. Includes, but is not limited to, purchase of additional shares or other increase in noncontrolling interest ownership.

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Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended	10 Months Ended	12 Months Ended			58 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2018	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2022
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 584.0		$ 584.0	$ 687.7		$ 584.0
Income (loss) from equity method investments			12.6	0.1	$ (7.0)
Undistributed earnings of investees accounted for under equity method	7.8		7.8			7.8
Sale to noncontrolling interests			6.4	0.0	0.0
Sales of subsidiary shares to noncontrolling interest			0.9
Noncontrolling Interests
Schedule of Equity Method Investments [Line Items]
Sales of subsidiary shares to noncontrolling interest			0.9
Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	53.5		53.5			53.5
Next Gen Alloys LLC [Member]
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 1.2		$ 1.2			$ 1.2
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%		50.00%			50.00%
Income (loss) from equity method investments			$ 9.1	(0.9)	(10.6)
Revenue from related parties			0.0	0.0	18.4
Business Exit Costs			$ (1.8)		4.8
Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%		50.00%			50.00%
Revenue from related parties			$ 45.0	45.8	36.7
Related party accounts receivable	$ 4.5		4.5	6.1		$ 4.5
Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments			8.2	(0.9)	(8.2)
Due from affiliates	3.2		3.2	2.9		3.2
Advanced Alloys & Solutions [Member] \| Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments			4.4	1.0	$ 1.2
Prepaid expenses and other current assets [Member] \| Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from affiliates	0.4		0.4	0.7		0.4
Other Noncurrent Assets [Member] \| Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from affiliates	$ 2.8		$ 2.8	$ 2.2		$ 2.8
Allegheny Technologies Inc \| Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%		60.00%			60.00%
Allegheny Technologies Inc \| Next Gen Alloys LLC [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%		51.00%			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%		40.00%			40.00%
VSMPO \| VSMPO \| Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%		50.00%			50.00%
Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%		50.00%			50.00%
Sale to noncontrolling interests	$ 5.5	$ 12.0				$ 17.5
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income from Tariff Refunds			$ 19.7
Allegheny & Tsingshan Stainless \| Import Duties
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments			$ 9.9