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Fair Value Of Financial Instruments - Narrative (Details) | Dec. 29, 2024 | Dec. 31, 2023
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Allegheny Ludlum 6.95% Debentures due 2025 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt instrument interest rate stated percentage | 6.95% | 6.95%
Allegheny Ludlum 6.95% Debentures due 2025 [Member] | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt instrument interest rate stated percentage | 6.95% | 6.95%
ATI $350 million 5.875% Senior Notes due 2027 | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt instrument interest rate stated percentage | 5.875% | 5.875%
ATI 2029 Notes | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt instrument interest rate stated percentage | 4.875% | 4.875%
ATI 2031Notes | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt instrument interest rate stated percentage | 5.125% | 5.125%

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