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v3.24.1.u1

Debt - Convertible Notes Narrative (Details) | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2024 USD ($) tradingDays $ / shares | Apr. 02, 2023 USD ($) | Dec. 31, 2023 USD ($) | Jan. 01, 2023 USD ($)
Debt Instrument [Line Items] |  |  |  |
Equity adjustment for adoption | $ 1,373,200,000 | $ 1,214,300,000 | $ 1,480,500,000 | $ 1,157,200,000
Debt issuance costs | 18,500,000 |  | 19,600,000 |
Interest expense, net | 26,600,000 | $ 19,900,000 |  |
Estimate Of Fair Value Fair Value Disclosure |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt | 2,836,800,000 |  | 2,746,700,000 |
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt | 2,536,200,000 |  | 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 |
Interest rate | 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 |  |  |
Debt issuance costs | $ 2,400,000 |  | $ 2,900,000 |
Debt instrument, interest rate, effective percentage | 4.20% | 4.20% |  |
Cap price (in dollars per share) | $ / shares | $ 19.76 |  |  |
Interest Expense, Debt, Excluding Amortization | $ 2,500,000 | $ 2,500,000 |  |
Amortization of Debt Issuance Costs | 500,000 | 500,000 |  |
Interest expense, net | $ 3,000,000.0 | $ 3,000,000.0 |  |
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 | $ 967,000,000 |  | $ 864,000,000 |

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