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Debt Schedule of Debt (Details) - USD ($) | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 29, 2024 | Dec. 31, 2023 | Mar. 31, 2024
Debt Instrument [Line Items] |  |  |
Debt Issuance Costs, Net | $ (14,800,000) | $ (19,600,000) |
Debt | 1,883,400,000 | 2,179,600,000 |
Short-term debt and current portion of long-term debt | 27,900,000 | 31,900,000 |
Long-term debt | 1,855,500,000 | 2,147,700,000 |
ATI Inc. 5.875% Notes due 2027 |  |  |
Debt Instrument [Line Items] |  |  |
Debt, Gross | $ 350,000,000.0 | $ 350,000,000.0 |
Debt Instrument, Issuer | ATI Inc. | ATI Inc. |
Interest rate | 5.875% | 5.875% |
Debt Instrument, Maturity Date | Dec. 01, 2027 | Dec. 01, 2027 |
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt, Gross | $ 0 | $ 291,400,000 |
Debt Issuance Costs, Net |  | $ (2,900,000) |
Debt Instrument, Issuer |  | ATI Inc. |
Interest rate |  | 3.50% |
Debt Instrument, Maturity Date |  | Jun. 15, 2025 |
Allegheny Ludlum 6.95% Debentures due 2025 (a) |  |  |
Debt Instrument [Line Items] |  |  |
Debt, Gross | $ 150,000,000.0 | $ 150,000,000.0 |
Debt Instrument, Issuer | Allegheny Ludlum | Allegheny Ludlum |
Interest rate | 6.95% | 6.95% |
Debt Instrument, Maturity Date | Dec. 15, 2025 | Dec. 15, 2025 |
Foreign credit facilities |  |  |
Debt Instrument [Line Items] |  |  |
Short-term Debt | $ 0 | $ 5,000,000.0 |
Finance leases and other |  |  |
Debt Instrument [Line Items] |  |  |
Debt, Gross | 98,200,000 | 102,800,000 |
ATI 2031 Notes |  |  |
Debt Instrument [Line Items] |  |  |
Debt, Gross | $ 350,000,000.0 | $ 350,000,000.0 |
Debt Instrument, Issuer | ATI Inc. | ATI Inc. |
Interest rate | 5.125% | 5.125% |
Debt Instrument, Maturity Date | Oct. 01, 2031 | Oct. 01, 2031 |
ATI 2029 Notes |  |  |
Debt Instrument [Line Items] |  |  |
Debt, Gross | $ 325,000,000.0 | $ 325,000,000.0 |
Debt Instrument, Issuer | ATI Inc. | ATI Inc. |
Interest rate | 4.875% | 4.875% |
Debt Instrument, Maturity Date | Oct. 01, 2029 | Oct. 01, 2029 |
Domestic Bank Group $600 million asset-based credit facility |  |  |
Debt Instrument [Line Items] |  |  |
Short-term Debt | $ 0 | $ 0 |
2027 Term Loan |  |  |
Debt Instrument [Line Items] |  |  |
Debt, Gross | $ 200,000,000.0 | $ 200,000,000.0 |
Debt Instrument, Issuer | ATI Inc. | ATI Inc. |
Interest rate |  |  | 4.21%
Debt Instrument, Maturity Date | Sep. 09, 2027 | Sep. 09, 2027 |
ATI 2030 Notes |  |  |
Debt Instrument [Line Items] |  |  |
Debt, Gross | $ 425,000,000.0 | $ 425,000,000.0 |
Debt Instrument, Issuer | ATI Inc. | ATI Inc. |
Interest rate | 7.25% | 7.25% |
Debt Instrument, Maturity Date | Aug. 15, 2030 | Aug. 15, 2030 |

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