Business Segments - Narrative (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 29, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 29, 2025 USD ($) segment	Jun. 30, 2024 USD ($)
Segment Reporting Information [Line Items]
Number of business segments \| segment			2
Revenue from international sales	$ 490.0	$ 456.8	$ 990.6	$ 928.1
Sales by U.S. operations to foreign countries	393.4	355.3	808.3	716.6
Restructuring and other charges	(7.4)	(5.4)	(13.0)	(8.5)
Restructuring charges (credits)			1.3
Restructuring charges (credits)	(1.3)	(1.9)	(1.3)	(1.7)
Customer programs
Segment Reporting Information [Line Items]
Gain (loss) on sale of accounts receivable	1.3	1.3	2.7	2.9
Customer programs & Accounts receivable purchase agreement
Segment Reporting Information [Line Items]
Gain (loss) on sale of accounts receivable	1.6		3.2
Start Up Costs
Segment Reporting Information [Line Items]
Restructuring charges (credits)	$ 7.1	1.8	$ 11.1	4.7
Inventory write-offs
Segment Reporting Information [Line Items]
Restructuring charges (credits)		$ 5.5		$ 5.5

total foreign country sales from domicile operations

No definition available.

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

- Definition

Restructuring and other credits (charges)

+ References

No definition available.

+ Details

Name:	ati_RestructuringAndOtherCreditsCharges
Namespace Prefix:	ati_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Restructuring, Settlement And Impairment Provisions, Excluding Goodwill Impairment Loss

+ References

No definition available.

+ Details

Name:	ati_RestructuringSettlementAndImpairmentProvisionsExcludingGoodwillImpairmentLoss
Namespace Prefix:	ati_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Revenue From International Sales

+ References

No definition available.

+ Details

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

- Definition

Amount of gain (loss) on sale of accounts receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481326/860-20-50-5

+ Details

Name:	us-gaap_GainLossOnSaleOfAccountsReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 49
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482785/280-10-55-49

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-18

+ Details

Name:	us-gaap_NumberOfOperatingSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 18
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476153/220-40-55-18

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 420
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 5.P.4.b.1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479823/420-10-S99-2

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 420
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482047/420-10-45-3

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 420
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 5.P.3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479823/420-10-S99-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476148/220-40-50-21

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476148/220-40-50-6

+ Details

Name:	us-gaap_RestructuringCharges
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

No definition available.

+ Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=ati_CustomerProgramsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=ati_CustomerProgramsAccountsReceivablePurchaseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=ati_StartUpCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=ati_InventoryWriteOffsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: