Percentage Of Forecasted Natural Gas Usage Hedged for Year 1

No definition available.

Name:	ati_PercentageOfForecastedNaturalGasUsageHedgedforYear1
Namespace Prefix:	ati_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate notional amount of price risk derivatives for instruments with notional amounts expressed in percentage of estimated annual domestic (U.S.) usage

+ References

No definition available.

+ Details

Name:	ati_PercentageOfForecastedNaturalGasUsageHedgedforYear2
Namespace Prefix:	ati_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate notional amount of price risk derivatives for instruments with notional amounts expressed in percentage of annual estimated usage in production

+ References

No definition available.

+ Details

Name:	ati_PercentageOfRawMaterialUsageHedged
Namespace Prefix:	ati_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=121549185&loc=d3e80748-113994

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1B
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract
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

No definition available.

+ Details

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

- Definition

Nominal mass used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeNonmonetaryNotionalAmountMass
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:massItemType
Balance Type:	na
Period Type:	duration

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ati_NickelMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ati_NaturalGasContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ati_DerivativeMaturityRangeAxis=ati_MaturityDatesThrough2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ati_A2027TermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Financial Instruments and Hedging - Narrative (Details) - 12 months ended Dec. 31, 2022 € in Millions, lb in Millions, $ in Millions	USD ($) lb	EUR (€)	USD ($)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Percentage of estimated annual nickel requirements	8.00%
Derivative [Line Items]
Pre-tax cash flow hedge gain (loss) to be reclassified within twelve months	$ 12.2
Nickel [Member]
Derivative [Line Items]
Derivative notional amount (in pounds of nickel) \| lb	6
Cash Flow Hedging [Member] \| Natural gas contracts [Member] \| Designated as Hedging Instrument [Member]
Derivative [Line Items]
Percentage Of Forecasted Natural Gas Usage Hedged for 2023	70.00%
Percentage of forecasted natural gas usage hedged for 2024	25.00%
Cash Flow Hedging [Member] \| Foreign Exchange Forward [Member] \| Designated as Hedging Instrument [Member] \| Maturity Dates Through 2022
Derivative [Line Items]
Derivative notional amount \| €		€ 0
2027 Term Loan
Derivative [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.21%	4.21%
2027 Term Loan \| Interest Rate Swap [Member]
Derivative [Line Items]
Derivative notional amount			$ 50.0