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v3.23.3

Debt - Convertible Notes Narrative (Details) | 3 Months Ended | 9 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2023 USD ($) $ / shares | Sep. 30, 2022 USD ($) | Sep. 30, 2023 USD ($) tradingDays $ / shares | Sep. 30, 2022 USD ($) | Jun. 30, 2023 USD ($) | Dec. 31, 2022 USD ($) | Jun. 30, 2022 USD ($) | Dec. 31, 2021 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Equity adjustment for adoption | $ 1,355,900,000 | $ 938,400,000 | $ 1,355,900,000 | $ 938,400,000 | $ 1,309,200,000 | $ 1,157,200,000 | $ 893,900,000 | $ 832,700,000
Debt issuance costs | 20,700,000 |  | 20,700,000 |  |  | 17,200,000 |  |
Interest expense, net | 23,800,000 | $ 20,800,000 | 65,000,000.0 | $ 67,800,000 |  |  |  |
Estimate Of Fair Value Fair Value Disclosure |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt | 2,359,300,000 |  | 2,359,300,000 |  |  | 1,964,500,000 |  |
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt | 2,044,800,000 |  | 2,044,800,000 |  |  | 1,665,700,000 |  |
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 | $ 3,400,000 |  | $ 3,400,000 |  |  | $ 4,800,000 |  |
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 | $ 2,500,000 | $ 2,500,000 | $ 7,600,000 | $ 7,600,000 |  |  |  |
Amortization of Debt Issuance Costs | 500,000 | 400,000 | 1,400,000 | 1,300,000 |  |  |  |
Interest expense, net | 3,000,000.0 | $ 2,900,000 | $ 9,000,000.0 | $ 8,900,000 |  |  |  |
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 | $ 786,000,000 |  | $ 786,000,000 |  |  | $ 590,000,000 |  |

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