Other Expense (Income), Net - Additional Information (Details) € in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2020 EUR (€)
Other Non operating Income Expense [Line Items]
Income tax benefit	$ 40	$ (84)	$ (119)
Alcoa Corporation
Other Non operating Income Expense [Line Items]
Tax agreement, indemnification of ultimate liability, percent				49.00%
Spain
Other Non operating Income Expense [Line Items]
Income taxes receivable | €				€ 53
Tax agreement, indemnification of ultimate liability, percent			49.00%	33.66%
Spain | Other Nonoperating Income (Expense) | Alcoa Corporation
Other Non operating Income Expense [Line Items]
Income tax benefit			$ 29