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Debt - Revolving Credit Facility Narrative (Details) - USD ($) | 9 Months Ended |  |  |
--- | --- | --- | --- | ---
Sep. 29, 2024 | Oct. 01, 2023 | Sep. 09, 2027 | Mar. 31, 2024 | Dec. 31, 2023
Debt Instrument [Line Items] |  |  |  |  |
Debt issuance costs | $ 14,800,000 |  |  |  | $ 19,600,000
Foreign credit facilities |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Maximum borrowing capacity | 59,000,000 |  |  |  |
Short-term Debt | 0 |  |  |  | 5,000,000.0
Domestic Bank Group $600 million asset-based credit facility |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Bridge loan | 60,000,000 |  |  |  |
Amount utilized to support the issuance of letters of credit | 31,700,000 |  |  |  |
Average borrowings during period | 0 | $ 17,000,000 |  |  |
Short-term Debt | 0 |  |  |  | 0
Interest rate during period |  | 6.50% |  |  |
Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Maximum borrowing capacity | $ 600,000,000 |  |  |  |
Minimum fixed charge coverage ratio allowed in event of default | 1.00 |  |  |  |
Percentage of borrowing capacity unavailable due to fixed charge coverage ratio covenant | 10.00% |  |  |  |
Borrowing capacity unavailable due to fixed charge coverage ratio covenant, minimum | $ 60,000,000.0 |  |  |  |
Minimum required liquidity number of days prior to maturity of senior notes | 90 days |  |  |  |
Debt covenant, undrawn availability requirement, amount | $ 120,000,000 |  |  |  |
Debt covenant, undrawn availability requirement, percent of total available liquidity | 20.00% |  |  |  |
Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility | Minimum | Base rate borrowings |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Variable interest rate (percentage) | 0.25% |  |  |  |
Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility | Minimum | Secured Overnight Financing Rate (SOFR) |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Variable interest rate (percentage) | 1.25% |  |  |  |
Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility | Maximum | Base rate borrowings |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Variable interest rate (percentage) | 0.75% |  |  |  |
Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility | Maximum | Secured Overnight Financing Rate (SOFR) |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Variable interest rate (percentage) | 1.75% |  |  |  |
Domestic Bank Group $600 million asset-based credit facility | Letter of credit sub-facility |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Maximum borrowing capacity | $ 200,000,000 |  |  |  |
2027 Term Loan |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Outstanding borrowings | 200,000,000.0 |  |  |  | 200,000,000.0
Minimum prepayment increments allowed | $ 25,000,000 |  |  |  |
Interest rate |  |  |  | 4.21% |
2027 Term Loan | Interest Rate Swap |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Notional amount of derivative |  |  |  | $ 50,000,000 |
2027 Term Loan | Secured Overnight Financing Rate (SOFR) |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Variable interest rate (percentage) | 2.00% |  |  |  |
Allegheny Ludlum 6.95% Debentures due 2025 (a) |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Outstanding borrowings | $ 150,000,000.0 |  |  |  | $ 150,000,000.0
Interest rate | 6.95% |  |  |  | 6.95%
ATI 2030 Notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Outstanding borrowings | $ 425,000,000.0 |  |  |  | $ 425,000,000.0
Interest rate | 7.25% |  |  |  | 7.25%
Forecast | Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Line of credit facility, increase to maximum borrowing capacity |  |  | $ 300,000,000 |  |

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