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Joint Ventures (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 30, 2025 | Mar. 31, 2024 | Dec. 29, 2024
Schedule of Equity Method Investments [Line Items] |  |  |
Cash and cash equivalents | $ 475.8 |  | $ 721.2
Income (loss) from equity investments | 0.0 | $ (0.4) |
Next Gen Alloys LLC |  |  |
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% |  |
AA&S | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Cash and cash equivalents | $ 71.3 |  |
AA&S | Allegheny & Tsingshan Stainless |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Income (loss) from equity investments |  | $ (0.4) |
AA&S | Related Party | Allegheny & Tsingshan Stainless |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Other receivables | $ 0.5 |  | $ 0.6
China Baowu Steel Group Corporation Limited | AA&S | 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% |  |
ATI Inc. | Next Gen Alloys LLC |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Joint venture ownership percentage | 51.00% |  |
ATI Inc. | AA&S | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Joint venture ownership percentage | 60.00% |  |

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