Segment and Geographic Area Information - Additional Information (Detail)	12 Months Ended
Dec. 31, 2017	Sep. 28, 2015
Alcoa Corporation [Member]
Segment Reporting Information [Line Items]
Minority interest percentage	25.10%	25.10%
Alcoa Joint Venture [Member]
Segment Reporting Information [Line Items]
Ownership interest in joint venture	25.10%

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.

No definition available.

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