Segment and Related Information - Schedule of Segment Adjusted EBITDA to Consolidated Net (Loss) Income Attributable to Alcoa Corporation (Parenthetical) (Detail) - Change in Accounting Principle from LIFO to Average Cost [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 47	$ 14
Increase (decrease) in intersegment eliminations	$ (19)	$ 27