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Debt Schedule of Debt (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2024 | Dec. 31, 2023
Debt Instrument [Line Items] |  |
Debt Issuance Costs, Net | $ (18,500,000) | $ (19,600,000)
Debt | 2,173,500,000 | 2,179,600,000
Short-term debt and current portion of long-term debt | 27,100,000 | 31,900,000
Long-term debt | 2,146,400,000 | 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 | $ 291,400,000 | $ 291,400,000
Debt Issuance Costs, Net | $ (2,400,000) | $ (2,900,000)
Debt Instrument, Issuer | ATI Inc. | ATI Inc.
Interest rate | 3.50% | 3.50%
Debt Instrument, Maturity Date | Jun. 15, 2025 | 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 | 100,600,000 | 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 | $ 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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