v3.20.2

Debt 144A due 2025 (Details) - USD ($) $ in Millions				9 Months Ended
	May 13, 2020	Feb. 07, 2020	Feb. 05, 2020	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Debt instrument, face amount			$ 1,200
Proceeds from debt issuance			$ 1,168	$ 2,400	$ 0
Senior Secured Notes | First-Lien Notes due 2025, 6.000%
Debt Instrument [Line Items]
Debt instrument, face amount	$ 700
Fixed interest rate	6.00%
Proceeds from debt issuance	$ 691
Senior Secured Notes | First-Lien Notes due 2025, 6.000% | Minimum
Debt Instrument [Line Items]
Required prior notice for debt redemption, term	10 days
Senior Secured Notes | First-Lien Notes due 2025, 6.000% | Maximum
Debt Instrument [Line Items]
Required prior notice for debt redemption, term	60 days
Senior Secured Notes | First-Lien Notes due 2025, 6.000% | Redemption prior to May 15, 2022
Debt Instrument [Line Items]
Debt redemption percentage	100.00%
Debt redemption, additional make-whole charge	1.00%
Debt redemption, basis spread on discount rate		5000.00%
Senior Secured Notes | First-Lien Notes due 2025, 6.000% | Redemption prior to May 15, 2022; limited to 40% of aggregate principal
Debt Instrument [Line Items]
Debt redemption percentage	106.00%
Redemption limitation based on aggregate principal	40.00%
Minimum aggregate principal following redemption	60.00%
Maximum term allowed for redemption following offering	120 days
Senior Secured Notes | First-Lien Notes due 2025, 6.000% | Redemption after May 14, 2022
Debt Instrument [Line Items]
Debt redemption percentage	103.00%
Senior Secured Notes | First-Lien Notes due 2025, 6.000% | Redemption upon change in control
Debt Instrument [Line Items]
Debt redemption percentage	101.00%

X

- Definition

Debt Instrument, Notice for Debt Redemption, Term

+ References

No definition available.

+ Details

Name:	arnc_DebtInstrumentNoticeforDebtRedemptionTerm
Namespace Prefix:	arnc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Debt Instrument, Redemption, Additional Make-Whole Redemption Charge

+ References

No definition available.

+ Details

Name:	arnc_DebtInstrumentRedemptionAdditionalMakeWholeRedemptionCharge
Namespace Prefix:	arnc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Debt Instrument, Redemption, Basis Spread on Discount Rate

+ References

No definition available.

+ Details

Name:	arnc_DebtInstrumentRedemptionBasisSpreadonDiscountRate
Namespace Prefix:	arnc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Debt Instrument, Redemption, Limitation Based on Aggregate Principal

+ References

No definition available.

+ Details

Name:	arnc_DebtInstrumentRedemptionLimitationBasedonAggregatePrincipal
Namespace Prefix:	arnc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Debt Instrument, Redemption, Maximum Term Following Offering

+ References

No definition available.

+ Details

Name:	arnc_DebtInstrumentRedemptionMaximumTermFollowingOffering
Namespace Prefix:	arnc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Debt Instrument, Redemption, Minimum Aggregate Principal Following Redemption

+ References

No definition available.

+ Details

Name:	arnc_DebtInstrumentRedemptionMinimumAggregatePrincipalFollowingRedemption
Namespace Prefix:	arnc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

X

- Definition

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

+ References

Reference 1: 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 http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

X

- 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

X

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

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

X

- Details

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

X

- Details

Name:	us-gaap_DebtInstrumentAxis=arnc_FirstLienNotesdue20256.000Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=us-gaap_DebtInstrumentRedemptionPeriodOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=us-gaap_DebtInstrumentRedemptionPeriodTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=us-gaap_DebtInstrumentRedemptionPeriodThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=us-gaap_DebtInstrumentRedemptionPeriodFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: