Other Expense, Net - Narrative (Details) - Alcoa Corporation - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Tax agreement, indemnification of ultimate liability, percent	49.00%
Spain | Other, net
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Tax agreement, indemnification, receivable	$ 29	$ 29
Tax agreement, indemnification of ultimate liability, percent	49.00%