Segment Information - Schedule of Segment Adjusted EBITDA to Consolidated Net (Loss) Income Attributable to Alcoa Corporation (Parenthetical) (Detail) - Effects of Change in Accounting Principle from LIFO to Average Cost [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 11	$ 44
Increase (decrease) in intersegment eliminations	$ 38	$ (37)