Basis of Presentation [Line Items]

No definition available.

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- Definition

Number of operating locations.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Ownership interest in joint venture.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Payment for separation transaction, including payments made subsequent to the effective date of the separation transaction.

+ References

No definition available.

+ Details

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Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Face amount or stated value per share of common stock.

+ 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(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of countries in which the entity operates as of balance sheet date.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash inflow from the sale of property, plant and equipment (capital expenditures), software, and other intangible assets.

+ 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 http://asc.fasb.org/extlink&oid=118932676&loc=d3e3179-108585

+ Details

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

- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Millions		12 Months Ended
	Nov. 01, 2016 USD ($) $ / shares	Dec. 31, 2019 Country Location $ / shares	Dec. 31, 2017 USD ($) $ / shares
Basis Of Presentation [Line Items]
Number of countries in which entity operates \| Country		9
Cash payment related to separation transaction \| $	$ 1,072		$ 247
Proceeds from sale of assets \| $			$ 243
Common stock par value \| $ / shares		$ 0.01
Aluminum Segment [Member]
Basis Of Presentation [Line Items]
Ownership interest in joint venture		55.00%
AWAC [Member] \| Alumina Limited [Member]
Basis Of Presentation [Line Items]
Non-controlling interest, ownership percentage		40.00%
Parent Co
Basis Of Presentation [Line Items]
Common stock par value \| $ / shares	$ 0.01		$ 0.01
Alcoa Corporation [Member]
Basis Of Presentation [Line Items]
Ownership interest in joint venture		25.10%
Alcoa Corporation [Member] \| AWAC [Member]
Basis Of Presentation [Line Items]
Ownership interest percentage		60.00%
Minimum [Member]
Basis Of Presentation [Line Items]
Number of operating locations \| Location		30
Maximum [Member]
Basis Of Presentation [Line Items]
Percent of equity investments in other entity		50.00%