Percentage Of Forecasted Natural Gas Usage Hedged for Year 3

No definition available.

Name:	ati_PercentageOfForecastedNaturalGasUsageHedgedForYear3
Namespace Prefix:	ati_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage Of Forecasted Natural Gas Usage Hedged for Year 1

+ References

No definition available.

+ Details

Name:	ati_PercentageOfForecastedNaturalGasUsageHedgedforYear1
Namespace Prefix:	ati_
Data Type:	dtr-types1: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-types1: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-types1: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
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480870/815-30-50-1

+ 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
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

+ Details

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

- 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 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4E
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-4E

+ Details

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

- Definition

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 4D
-Subparagraph (b)
-SubTopic 10
-Topic 815
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-4D

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-4C

+ Details

Name:	us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-1B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-1A

+ Details

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

- Definition

Amount, after tax and before reclassification, of gain (loss) from derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482790/220-10-45-11

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 10A
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482790/220-10-45-10A

+ Details

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

- References

No definition available.

+ Details

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

- Details

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

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_OtherComprehensiveIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ati_NickelMember
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:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ati_NickelAndOtherRawMaterialContractsMember
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_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
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:	us-gaap_DebtInstrumentAxis=ati_A2027TermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Financial Instruments and Hedging (Details) € in Millions, lb in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($) lb	Jun. 30, 2022 USD ($)	Jun. 30, 2023 EUR (€)
Derivative Instruments Gain Loss [Line Items]
Percentage of estimated annual nickel requirements			10.00%
Percentage Of Forecasted Natural Gas Usage Hedged for 2023			80.00%
Percentage Of Forecasted Natural Gas Usage Hedged for 2024			50.00%
Percentage Of Forecasted Natural Gas Usage Hedged for 2025			15.00%
Reclassification of Cash Flow Hedge Gain (Loss) [Abstract]
Cash Flow Hedge Gain (Loss) To Be Reclassified Within Twelve Months			$ (2.1)
Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	$ (2.9)	$ 15.5	1.3	$ 19.3
Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	(1.9)	12.3	$ (13.3)	31.0
Nickel
Derivative Instruments Gain Loss [Line Items]
Notional amount of nickel hedge (in pounds of nickel) \| lb			8
Interest Rate Swap \| Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	0.3	(0.1)	$ 0.5	(0.3)
Interest Rate Swap \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	0.3	0.2	0.2	1.5
Nickel and other raw material contracts \| Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	(1.4)	12.1	3.9	14.5
Nickel and other raw material contracts \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	(1.6)	9.3	(6.9)	17.3
Natural gas contracts \| Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	(1.8)	3.4	(3.2)	5.0
Natural gas contracts \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	(0.7)	2.5	(6.8)	11.8
Foreign exchange contracts \| Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments Gain Loss Reclassified From Accumulated OCI Into Income Effective Portion Net	0.0	0.1	0.1	0.1
Foreign exchange contracts \| Cash Flow Hedging \| Other Comprehensive Income (Loss) [Member]
Derivative Instruments Gain Loss [Line Items]
Net derivative gain (loss) on hedge transactions	$ 0.1	$ 0.3	$ 0.2	$ 0.4
Designated as Hedging Instrument \| Foreign exchange forward \| Cash Flow Hedging
Derivative Instruments Gain Loss [Line Items]
Notional amount of derivative \| €					€ 0
2027 Term Loan
Derivative Instruments Gain Loss [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.21%		4.21%		4.21%
2027 Term Loan \| Interest Rate Swap
Derivative Instruments Gain Loss [Line Items]
Notional amount of derivative	$ 50.0		$ 50.0