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Joint Ventures (Details) - USD ($) $ in Millions | 10 Months Ended | 12 Months Ended | 58 Months Ended
--- | --- | --- | ---
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 |

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