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Business Segments - Additional Information (Details) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2024 USD ($) | Jul. 02, 2023 USD ($) | Apr. 02, 2023 USD ($) | Jun. 30, 2024 USD ($) segment | Jul. 02, 2023 USD ($) | Dec. 31, 2023 USD ($)
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Number of business segments | segment |  |  |  | 2 |  |
Employee retention credits | $ 8,600,000 |  |  | $ 8,600,000 |  |
Restructuring charges (credits) |  |  |  | 1,700,000 |  |
Restructuring and other charges | (5,400,000) | $ (9,200,000) |  | (8,500,000) | $ (10,400,000) |
Restructuring charges (credits) | (1,900,000) | 2,700,000 |  | (1,700,000) | 2,700,000 |
Start Up Costs |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring charges (credits) | 1,800,000 | 4,500,000 | $ 1,200,000 | 4,700,000 |  |
Asset Write-offs |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring charges (credits) |  | 2,000,000.0 |  |  |  |
Facility Closing |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Depreciation |  | $ 800,000 |  |  | $ 800,000 |
Inventory write-offs |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring charges (credits) | 5,500,000 |  |  | 5,500,000 |  |
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Debt, Gross | 291,400,000 |  |  | 291,400,000 |  | $ 291,400,000
High Performance Materials & Components |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Employee retention credits | 3,500,000 |  |  | 3,500,000 |  |
Advanced Alloys & Solutions |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Employee retention credits | $ 5,100,000 |  |  | $ 5,100,000 |  |

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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -SubTopic 10 -Topic 210 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-4 + Details
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- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 49 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482785/280-10-55-49 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-18 + Details
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