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Leases - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended |
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Dec. 31, 2022 | Dec. 31, 2021 | Jan. 01, 2023
Leases [Abstract] |  |  |
Progress payments made on behalf of company, paid | $ 27.9 |  |
Progress payments made on behalf of company, scheduled to be paid |  |  | $ 13.7
Lessee, finance lease, lease not yet commenced, right-of-use asset to be recognized |  |  | 41.6
Lessee, finance lease, lease not yet commenced, liability to be recognized |  |  | $ 41.6
Proceeds from sale of ongoing CIP projects converted to leases | $ 1.8 | $ 16.2 |

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- 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

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- 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

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- 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 -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3179-108585 + Details
Name: | us-gaap_ProceedsFromSaleOfOtherPropertyPlantAndEquipment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration