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Joint Ventures (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 29, 2025 | Jun. 30, 2024 | Jun. 29, 2025 | Jun. 30, 2024 | Dec. 29, 2024
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Cash and cash equivalents | $ 319.6 |  | $ 319.6 |  | $ 721.2
Other | 0.0 | $ (0.4) | 0.0 | $ (0.8) |
Next Gen Alloys LLC |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Cash and cash equivalents | 1.0 |  | 1.0 |  |
AA&S | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Cash and cash equivalents | $ 86.7 |  | $ 86.7 |  |
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% |  | 40.00% |  |
ATI Inc. | Next Gen Alloys LLC |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Joint venture ownership percentage | 51.00% |  | 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% |  | 60.00% |  |

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