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Joint Ventures (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 58 Months Ended |
--- | --- | --- | --- | ---
Sep. 29, 2024 | Oct. 01, 2023 | Sep. 29, 2024 | Oct. 01, 2023 | Jan. 01, 2023 | Dec. 31, 2023
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Cash and cash equivalents | $ 406.6 |  | $ 406.6 |  |  | $ 743.9
Income (loss) from equity investments | (0.2) | $ (0.7) | (1.0) | $ (1.0) |  |
Next Gen Alloys LLC |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Cash and cash equivalents | $ 1.0 |  | $ 1.0 |  |  |
Allegheny & Tsingshan Stainless |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  | 50.00% |  |  |
Allegheny & Tsingshan Stainless | Tsingshan Group |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Sale of noncontrolling interests |  |  |  |  | $ 17.5 |
Uniti |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  | 50.00% |  |  |
Advanced Alloys & Solutions | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Cash and cash equivalents | $ 102.3 |  | $ 102.3 |  |  |
Advanced Alloys & Solutions | Allegheny & Tsingshan Stainless |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Income (loss) from equity investments | (0.2) | (0.5) | (1.0) | (1.3) |  |
Advanced Alloys & Solutions | Uniti |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Income (loss) from equity investments |  | $ (0.2) |  | $ 0.3 |  |
Advanced Alloys & Solutions | Related Party | Allegheny & Tsingshan Stainless |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Other receivables | $ 0.2 |  | $ 0.2 |  |  | $ 1.5
China Baowu Steel Group Corporation Limited | Advanced Alloys & Solutions | 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% |  |  |
ATI Inc. | Next Gen Alloys LLC |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Joint venture ownership percentage | 51.00% |  | 51.00% |  |  |
ATI Inc. | Advanced Alloys & Solutions | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Joint venture ownership percentage | 60.00% |  | 60.00% |  |  |

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