Line of Credit Facility, Additional Borrowing Capacity

No definition available.

Name:	arnc_LineofCreditFacilityAdditionalBorrowingCapacity
Namespace Prefix:	arnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Incremental percentage increase (decrease) in the stated rate on a debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentInterestRateIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

+ Details

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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

No definition available.

+ Details

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

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt premium.

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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow from a contractual arrangement with the lender, including letter of credit, standby letter of credit and revolving credit arrangements, under which borrowings can be made up to a specific amount at any point in time with maturities due beyond one year or the operating cycle, if longer.

+ References

+ Details

Name:	us-gaap_ProceedsFromLongTermLinesOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

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

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Write-off of amounts previously capitalized as debt issuance cost in an extinguishment of debt.

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Balance Type:	debit
Period Type:	duration

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LoansPayableMember
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Namespace Prefix:
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Period Type:

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Period Type:

- Details

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Namespace Prefix:
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- Details

Name:	us-gaap_DebtInstrumentAxis=arnc_ABLCreditAgreementMember
Namespace Prefix:
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- Details

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Debt (Details) - USD ($)							2 Months Ended	3 Months Ended	9 Months Ended
Debt (Details) - USD ($)	Mar. 03, 2021	May 13, 2020	Apr. 02, 2020	Mar. 25, 2020	Feb. 07, 2020	Feb. 05, 2020	Mar. 25, 2020	May 13, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Apr. 01, 2020
Debt Instrument [Line Items]
Debt instrument, face amount						$ 1,200,000,000
Proceeds from debt issuance						$ 1,168,000,000			$ 319,000,000	$ 2,400,000,000
Cash										60,000,000		$ 500,000,000
Repayments of Long-term Debt									0	$ 1,100,000,000
Senior Secured Notes
Debt Instrument [Line Items]
Fixed interest rate					6.125%
Senior Secured Loans
Debt Instrument [Line Items]
Debt Instrument, Unamortized Premium	$ 19,000,000
Unamortized Debt Issuance Expense	5,000,000
Second-Lien Notes due 2028, 6.125% \| Senior Secured Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 300,000,000				$ 600,000,000
Debt Instrument, Interest Rate, Increase (Decrease)	106.25%
Proceeds from debt issuance					$ 593,000,000
Debt, carrying value									1,593,000,000		$ 1,278,000,000
Debt, fair value									1,691,000,000		1,399,000,000
Second-Lien Notes due 2028, 6.125% \| Senior Secured Loans
Debt Instrument [Line Items]
Proceeds from debt issuance	$ 315,000,000
First-Lien Term B Loan Facility
Debt Instrument [Line Items]
Debt Issuance Costs, Net				$ 30,000,000			$ 30,000,000		14,000,000
Write off of Deferred Debt Issuance Cost							16,000,000	$ 19,000,000
First-Lien Term B Loan Facility \| Senior Secured Loans
Debt Instrument [Line Items]
Debt instrument, face amount				600,000,000			600,000,000
Proceeds from debt issuance				$ 575,000,000
Debt instrument, term				7 years
Repayments of Long-term Debt		$ 600,000,000
First-Lien Revolving Credit Facility
Debt Instrument [Line Items]
Debt Issuance Costs, Net				$ 30,000,000			30,000,000		14,000,000
Write off of Deferred Debt Issuance Cost							16,000,000	19,000,000
First-Lien Revolving Credit Facility \| Senior Secured Credit Facility \| Revolving Credit Facility
Debt Instrument [Line Items]
Debt instrument, face amount				$ 1,000			1,000
Debt instrument, term				5 years
Proceeds from revolving credit facility			$ 500,000,000
Repayments of Long-term Debt		500,000,000
First-Lien Revolving Credit Facility \| Senior Secured Credit Facility \| Revolving Credit Facility \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate			2.00%
First-Lien Notes due 2025, 6.000%
Debt Instrument [Line Items]
Debt Issuance Costs, Net		15,000,000						15,000,000	12,000,000
Write off of Deferred Debt Issuance Cost		3,000,000						19,000,000
First-Lien Notes due 2025, 6.000% \| Senior Secured Notes
Debt Instrument [Line Items]
Debt instrument, face amount		$ 700,000,000						$ 700,000,000
Fixed interest rate		6.00%						6.00%
Proceeds from debt issuance		$ 691,000,000
Debt, carrying value									1,593,000,000		1,278,000,000
Debt, fair value									1,691,000,000		$ 1,399,000,000
ABL Credit Agreement
Debt Instrument [Line Items]
Debt Issuance Costs, Net		15,000,000						$ 15,000,000	12,000,000
Write off of Deferred Debt Issuance Cost		3,000,000						19,000,000
ABL Credit Agreement \| Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility, maximum borrowing capacity		800,000,000						800,000,000
Revolving credit facility, additional borrowing capacity, accordion feature		$ 350,000,000						$ 350,000,000
Line of Credit Facility, Remaining Borrowing Capacity									$ 785,000,000
First-Lien Revolving Credit Facility and Second-Lien Notes due 2028, 6.125% [Member]
Debt Instrument [Line Items]
Debt Issuance Costs, Net				$ 42,000,000			$ 42,000,000