Subsequent Event	12 Months Ended
Dec. 28, 2025
Subsequent Events [Abstract]
Subsequent Events	Subsequent EventOn February 18, 2026, ATI's Board of Directors of ATI authorized additional share repurchase of up to $500 million, which is in addition to the existing share repurchase authorization of approximately $120.0 million. Repurchases under the program may be made in the open market or in privately negotiated transactions, with the amount and timing of repurchases depending on market conditions and corporate needs. Open market repurchases will be structured to occur within the pricing and volume requirements of SEC Rule 10b-18. The stock repurchase program does not obligate the Company to repurchase any specific number of shares, and it may be modified, suspended, or terminated at any time by the Board of Directors without prior notice.

The entire disclosure for significant events or transactions that occurred after the balance sheet date through the date the financial statements were issued or the date the financial statements were available to be issued. Examples include: the sale of a capital stock issue, purchase of a business, settlement of litigation, catastrophic loss, significant foreign exchange rate changes, loans to insiders or affiliates, and transactions not in the ordinary course of business.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 855
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org/855/tableOfContent

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 855
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483399/855-10-50-2

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