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] $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 34
Increase in intersegment eliminations	$ 45