Accounting Policies	9 Months Ended
Accounting Policies	Sep. 28, 2025
Accounting Policies [Abstract]
Accounting Policies	Accounting Policies The interim consolidated financial statements include the accounts of ATI Inc. and its subsidiaries. Unless the context requires otherwise, “ATI” and “the Company” refer to ATI Inc. and its subsidiaries. The Company follows a 4-4-5 or 5-4-4 fiscal calendar, whereby each fiscal quarter consists of thirteen weeks grouped into two four-week months and one five-week month, and its fiscal year ends on the Sunday closest to December 31. Unless otherwise stated, references to years and quarters in this Quarterly Report on Form 10-Q relate to fiscal years and quarters, rather than calendar years and quarters. These unaudited consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and note disclosures required by U.S. generally accepted accounting principles for complete financial statements. In management’s opinion, all adjustments (which include only normal recurring adjustments) considered necessary for a fair presentation have been included. These unaudited consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Company’s 2024 Annual Report on Form 10-K. The results of operations for these interim periods are not necessarily indicative of the operating results for any future period. The December 29, 2024 financial information has been derived from the Company’s audited consolidated financial statements. New Accounting Pronouncements Adopted In November 2023, the Financial Accounting Standards Board (FASB) issued new accounting guidance related to segment reporting disclosures. This guidance requires additional disclosures on an annual and interim basis of segment information, including significant segment expenses that are regularly provided to the chief operating decision maker (CODM) and the presentation and composition of other segment items, which is the difference between segment revenue less segment expenses and the measure of segment profit or loss. The guidance also requires that all current segment disclosures required on an annual basis be provided on an interim basis and requires disclosure of the title and position of the CODM and how the CODM uses the reported measure of segment profit or loss in assessing performance and allocating resources. This guidance does not change how an entity identifies its reportable segments. This new guidance, with the exception of interim disclosures, was effective for the Company in fiscal year 2024, and the Company adopted this new accounting guidance for annual disclosures effective January 1, 2024. The interim disclosures are effective for the Company in fiscal year 2025. The adoption of these changes did not have an impact on the Company’s consolidated financial statements other than disclosure requirements which are included in Note 11. Pending Accounting Pronouncements In December 2023, the FASB issued new accounting guidance related to income tax disclosures. This guidance requires an entity to disclose specific categories in its annual rate reconciliation and provide additional information for reconciling items that meet a quantitative threshold. This guidance also requires additional annual disclosures for income taxes paid and requires disaggregation of income before tax, between domestic and foreign, and income tax expense, between federal, state and foreign. This guidance also eliminates several current disclosure requirements related to: (1) the nature and estimate of the range of the reasonably possible change in the unrecognized tax benefits balance in the next 12 months, (2) making a statement that an estimate of the range cannot be made, and (3) disclosing the cumulative amount of each type of temporary difference when a deferred tax liability is not recognized because of the exceptions to comprehensive recognition of deferred taxes related to subsidiaries and corporate joint ventures. This new guidance includes annual disclosure requirements that will be effective for the Company for fiscal year 2025 and must be applied on a prospective basis with retrospective application permitted. The Company does not expect these changes to have an impact on the Company’s consolidated financial statements other than disclosure requirements. In November 2024, the FASB issued new accounting guidance related to expense disaggregation disclosures. This guidance requires entities to disclose specified information about certain costs and expenses including (1) the amounts of purchases of inventory, employee compensation, depreciation, and intangible asset amortization, (2) include certain amounts that are already required to be disclosed under current generally accepted accounting principles in the same disclosure as the other disaggregation requirements, (3) a qualitative description of the amounts remaining in relevant expense captions that are not separately disaggregated quantitatively, and (4) the total amount of selling expenses and, in annual reporting periods, an entity’s definition of selling expenses. This new guidance for annual disclosures will be effective for the Company for fiscal year 2027 and for interim disclosures will be effective for the Company for fiscal year 2028. The guidance can be applied prospectively or retrospectively and early adoption is permitted. The Company does not expect to early adopt this guidance and does not expect these changes to have an impact on the Company’s consolidated financial statements other than disclosure requirements.