Other Expense (Income), Net - Summary of Other Non-Operating Income and Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other Income and Expenses [Abstract]
Non-service related net periodic benefit cost	$ 28	$ 38
Interest income	(6)	(4)
Foreign currency gains, net	(3)	(5)
Other, net	(2)	4
Total	(20)	316
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposition of assets	$ (3)	349
Alcoa Corporation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposition of assets		351
Cash received on sale of operations		$ 888