Accounting Policies	9 Months Ended
Accounting Policies	Sep. 30, 2022
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. 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 2021 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 31, 2021 financial information has been derived from the Company’s audited consolidated financial statements. New Accounting Pronouncements Adopted In November 2021, the Financial Accounting Standards Board (FASB) issued new accounting guidance related to disclosures about certain types of government assistance. This new guidance requires business entities that account for transactions with a government by analogizing to a grant or contribution accounting model to make certain annual disclosures. It requires disclosure of the nature and significant terms and considerations of the transactions, the accounting policies used and the effects of those transactions on an entity’s financial statements. This new guidance is effective for the Company in fiscal year 2022, with early adoption permitted. The Company adopted this new accounting guidance effective January 1, 2022. The adoption of these changes does not have an impact on the Company’s consolidated financial statements other than the annual disclosure requirements. Pending Accounting Pronouncements In September 2022, the FASB issued new accounting guidance related to disclosures about supplier finance programs. Supplier finance programs allow a buyer to offer its suppliers the option for access to payment in advance of an invoice due date, which is paid by a third-party finance provider or intermediary on the basis of invoices that the buyer has confirmed as valid. This new guidance requires a buyer in a supplier finance program to disclose sufficient information about the program to allow a user of financial statements to understand the program’s nature, activity during the period, changes from period to period, and potential magnitude, using both qualitative and quantitative information about its supplier finance programs. This new guidance, with the exception of disclosures on rollforward information, will be effective for the Company in fiscal year 2023, with early adoption permitted. The rollforward information disclosures are effective for the Company in fiscal year 2024, with early adoption permitted. The Company does not plan to early adopt this guidance. The adoption of these changes is not expected to have an impact on the Company’s consolidated financial statements other than disclosure requirements.