Summary of Significant Accounting Policies - Government Assistance (Details) - US States' Economic Development Incentive Program - USD ($) $ in Millions	12 Months Ended
Dec. 28, 2025	Dec. 29, 2024	Dec. 31, 2023
Government Assistance [Line Items]
Reduction of expenses from grant benefits	$ 2.4	$ 1.7	$ 1.4
Cash receipts from government assistance	1.8	2.5	$ 3.4
Grants receivable	$ 1.2	$ 1.2

Government Assistance, Grant Benefit, Cash Receipts

No definition available.

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- Definition

Government Assistance, Grant Benefit, Reduction In Operating Expenses

+ References

No definition available.

+ Details

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Namespace Prefix:	ati_
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 832
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483507/832-10-50-3

+ Details

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- Definition

Carrying amount as of the balance sheet date of amounts due under the terms of governmental, corporate, or foundation grants.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(a)(4))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

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Balance Type:	debit
Period Type:	instant

- Details

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