Contingencies and Commitments - Commitments (Details) $ in Millions	1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($) agreement	Oct. 31, 2017 agreement	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) agreement	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2018 agreement	Nov. 01, 2016 USD ($)
Loss Contingencies [Line Items]
Purchase obligations due in 2020	$ 604			$ 604
Purchase obligations due in 2021	93			93
Purchase obligations due in 2022	45			45
Purchase obligations due in 2023	12			12
Purchase obligations due in 2024	4			4
Purchase obligations due thereafter	2			2
Guarantees of third party related to project financing	$ 31			$ 31
Number of long term energy supply agreements | agreement	2			2			2
Number of agreements terminated | agreement		1
Other (expense) income, net				$ (122)	$ (79)	$ 486
Insurance policy coverage limit	$ 80
Insurance policy term	1 year
Total amount committed under outstanding surety bonds	$ 50			50
Amount of outstanding surety bonds relating to these liabilities	24			24
Alcoa Corporation Workers Compensation Claims
Loss Contingencies [Line Items]
Letters of credit, total amount committed								$ 52
Separation Agreement
Loss Contingencies [Line Items]
Combined fair value of guarantees	9			9	6
Other (expense) income, net			$ 25
Total amount committed under outstanding surety bonds			$ 25			$ 25
Separation Agreement | Other Noncurrent Liabilities and Deferred Credits
Loss Contingencies [Line Items]
Guarantees of third party related to project financing	1,353			1,353	$ 1,087
Bank Loan Obligations
Loss Contingencies [Line Items]
Letters of credit	$ 142			$ 142