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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

+ Details

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- 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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- Definition

This element represents a description of the nature of common ownership or management control relationships with other entities, regardless of there being transactions between the entities, when the existence of that control could result in operating results or financial position of the reporting entity significantly different from that which would have been obtained if the entities' were autonomous.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39691-107864

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- 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

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Investments - Schedule of Equity Investment (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2014
Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest		60.00%
Halco Mining Inc [Member]
Schedule of Equity Method Investments [Line Items]
Nature of ownership	Halco Mining, Inc. owns 100% of Boké Investment Company, which owns 51% of Compagnie des Bauxites de Guinée.
Pechiney Reynolds Quebec Inc [Member]
Schedule of Equity Method Investments [Line Items]
Nature of ownership	Pechiney Reynolds Quebec, Inc. owns a 50.1% interest in the Bécancour smelter in Quebec, Canada thereby entitling Alcoa Corporation to a 25.05% interest in the smelter. Through two wholly-owned Canadian subsidiaries, Alcoa Corporation also owns 49.9% of the Bécancour smelter.
Pechiney Reynolds Quebec Inc [Member] \| Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	49.90%
Boke Investment Company [Member] \| Halco Mining Inc [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	100.00%
Compagnie Des Bauxites De Guinee [Member] \| Halco Mining Inc [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	51.00%
Becancour Smelter [Member] \| Pechiney Reynolds Quebec Inc [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	50.10%
Becancour Smelter [Member] \| Pechiney Reynolds Quebec Inc [Member] \| Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	25.05%