Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income Loss [Line Items]
Settlements and/or curtailments related to retirement benefits	$ 38	$ (23)
Alcoa Corporation [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Settlements and/or curtailments related to retirement benefits	116	330
Non-controlling Interest [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Settlements and/or curtailments related to retirement benefits	$ 3	$ 1

Amount of gain (loss) recognized in net periodic benefit (cost) credit from event reducing expected years of future service of present employees or eliminating accrual of defined benefits for some or all future services of present employees.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(7)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)(7)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920

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