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Leases - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 29, 2024 | Dec. 31, 2023 | Jan. 01, 2023 | Dec. 30, 2024
Leases [Abstract] |  |  |  |
Progress payments made on behalf of company, paid | $ 54.3 |  |  |
Progress payments made on behalf of company, scheduled to be paid |  |  |  | $ 41.1
Lessee, finance lease, lease not yet commenced, right-of-use asset to be recognized |  |  |  | 95.4
Lessee, finance lease, lease not yet commenced, liability to be recognized |  |  |  | $ 95.4
Proceeds from sale of ongoing CIP projects converted to leases | $ 9.7 | $ 2.8 | $ 1.8 |

X
- Definition Lessee, Finance Lease, Lease Not Yet Commenced, Liability To Be Recognized + References No definition available. + Details
Name: | ati_LesseeFinanceLeaseLeaseNotYetCommencedLiabilityToBeRecognized
Namespace Prefix: | ati_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Lessee, Finance Lease, Lease Not Yet Commenced, Right-Of-Use Asset To Be Recognized + References No definition available. + Details
Name: | ati_LesseeFinanceLeaseLeaseNotYetCommencedRightOfUseAssetToBeRecognized
Namespace Prefix: | ati_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Progress Payments Made On Behalf Of Company, Paid + References No definition available. + Details
Name: | ati_ProgressPaymentsMadeOnBehalfOfCompanyPaid
Namespace Prefix: | ati_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Progress Payments Made On Behalf Of Company, Scheduled To Be Paid + References No definition available. + Details
Name: | ati_ProgressPaymentsMadeOnBehalfOfCompanyScheduledToBePaid
Namespace Prefix: | ati_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_LeasesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of cash inflow for the sale of long-lived, physical assets used to produce goods and services and not intended for resale, classified as other. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 12 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-12 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration