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Debt - Convertible Notes Narrative (Details) | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2024 USD ($) $ / shares | Jul. 02, 2023 USD ($) | Jun. 30, 2024 USD ($) tradingDays $ / shares | Jul. 02, 2023 USD ($) | Mar. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) | Apr. 02, 2023 USD ($) | Jan. 01, 2023 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Equity adjustment for adoption | $ 1,467,900,000 | $ 1,309,200,000 | $ 1,467,900,000 | $ 1,309,200,000 | $ 1,373,200,000 | $ 1,480,500,000 | $ 1,214,300,000 | $ 1,157,200,000
Debt issuance costs | 17,400,000 |  | 17,400,000 |  |  | 19,600,000 |  |
Interest expense, net | 28,400,000 | $ 21,300,000 | 55,000,000.0 | $ 41,200,000 |  |  |  |
Estimate Of Fair Value Fair Value Disclosure |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt | 2,905,700,000 |  | 2,905,700,000 |  |  | 2,746,700,000 |  |
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt | 2,608,900,000 |  | 2,608,900,000 |  |  | 2,438,900,000 |  |
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt, Gross | $ 291,400,000 |  | $ 291,400,000 |  |  | $ 291,400,000 |  |
Interest rate | 3.50% |  | 3.50% |  |  | 3.50% |  |
Debt instrument, convertible, redemption price as a percent of principal amount |  |  | 100.00% |  |  |  |  |
Debt instrument, convertible, threshold percentage of stock price trigger |  |  | 130.00% |  |  |  |  |
Debt instrument, convertible, threshold trading days | tradingDays |  |  | 20 |  |  |  |  |
Debt instrument, convertible, threshold consecutive trading days | tradingDays |  |  | 30 |  |  |  |  |
Debt instrument, convertible, conversion ratio |  |  | 0.0645745 |  |  |  |  |
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares | $ 15.49 |  | $ 15.49 |  |  |  |  |
Debt issuance costs | $ 2,000,000.0 |  | $ 2,000,000.0 |  |  | $ 2,900,000 |  |
Debt instrument, interest rate, effective percentage | 4.20% | 4.20% | 4.20% | 4.20% |  |  |  |
Cap price (in dollars per share) | $ / shares | $ 19.76 |  | $ 19.76 |  |  |  |  |
Interest Expense, Debt, Excluding Amortization | $ 2,600,000 | $ 2,600,000 | $ 5,100,000 | $ 5,100,000 |  |  |  |
Amortization of Debt Issuance Costs | 500,000 | 400,000 | 1,000,000.0 | 900,000 |  |  |  |
Interest expense, net | 3,100,000 | $ 3,000,000.0 | $ 6,100,000 | $ 6,000,000.0 |  |  |  |
Debt instrument, convertible, number of equity instruments (in shares) |  |  | 18,800,000 |  |  |  |  |
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt | $ 1,040,000,000.00 |  | $ 1,040,000,000.00 |  |  | $ 864,000,000 |  |

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