Segment and Related Information - Schedule of Sales by Product Division (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Disaggregation Of Revenue [Line Items]
Sales	$ 2,719	$ 3,090
Primary Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales	1,394	1,647
Alumina [Member]
Disaggregation Of Revenue [Line Items]
Sales	897	913
Flat-Rolled Aluminum [Member]
Disaggregation Of Revenue [Line Items]
Sales	312	429
Energy [Member]
Disaggregation Of Revenue [Line Items]
Sales	69	73
Bauxite [Member]
Disaggregation Of Revenue [Line Items]
Sales	58	45
Other Products [Member]
Disaggregation Of Revenue [Line Items]
Sales	$ (11)	$ (17)

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration