Government Assistance, Grant Benefit, Cash Receipts

No definition available.

Name:	ati_GovernmentAssistanceGrantBenefitCashReceipts
Namespace Prefix:	ati_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Government Assistance, Grant Benefit, Reduction In Operating Expenses

+ References

No definition available.

+ Details

Name:	ati_GovernmentAssistanceGrantBenefitReductionInOperatingExpenses
Namespace Prefix:	ati_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_AccountingPoliciesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	us-gaap_GovernmentAssistanceLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Duration of government assistance transaction, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 832
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483507/832-10-50-4

+ Details

Name:	us-gaap_GovernmentAssistanceTransactionDuration
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	us-gaap_GrantsReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_GovernmentAssistanceTypeAxis=ati_AviationManufacturingJobsProtectionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GovernmentAssistanceTypeAxis=ati_USStatesEconomicDevelopmentIncentiveProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Summary of Significant Accounting Policies - Government Receivable (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 29, 2024	Dec. 31, 2023	Jan. 01, 2023	Nov. 30, 2021
Aviation Manufacturing Jobs Protection
Accounting Policies [Abstract]
Grants receivable				$ 22.2
Government Assistance [Line Items]
Grants receivable				$ 22.2
Government assistance, transaction duration			6 months
Reduction of expenses from grant benefits			$ 16.6
Cash receipts from government assistance			11.0
US States' Economic Development Incentive Program
Accounting Policies [Abstract]
Grants receivable	$ 1.2	$ 1.2
Government Assistance [Line Items]
Grants receivable	1.2	1.2
Reduction of expenses from grant benefits	1.7	1.4	1.6
Cash receipts from government assistance	$ 2.5	$ 3.4	$ 2.8