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Amount, after allowance for credit loss, of right to consideration from customer for product sold and service rendered in normal course of business, classified as current.

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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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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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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Joint Ventures (Details) - USD ($) $ in Millions	10 Months Ended	12 Months Ended			58 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Dec. 30, 2018	Dec. 29, 2024	Dec. 31, 2023	Jan. 01, 2023	Jan. 01, 2023
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents		$ 721.2	$ 743.9
Income (loss) from equity method investments		(1.0)	(1.6)	$ 12.6
Accounts receivable, net		709.2	625.0
Sale to noncontrolling interests		0.0	0.0	6.4
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		100.9
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%
Income (loss) from equity method investments		$ (1.0)	$ (1.8)	9.1
Business Exit Costs				(1.8)
Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage			50.00%
Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Revenue from related parties			$ 4.9	45.0
AA&S \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments		(1.0)	(1.8)	8.2
AA&S \| Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments			0.2	4.4
AA&S \| Related Party \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from affiliates		$ 0.6	$ 1.5
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%
VSMPO \| VSMPO \| Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity			50.00%
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			5.5	$ 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