Contingencies and Commitments - Commitments (Details)	1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($)	Oct. 31, 2017 agreement	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) agreement	Sep. 30, 2018 agreement	Nov. 01, 2016 USD ($)
Loss Contingencies [Line Items]
Purchase obligations due in 2019	$ 770,000,000			$ 770,000,000
Purchase obligations due in 2020	169,000,000			169,000,000
Purchase obligations due in 2021	27,000,000			27,000,000
Purchase obligations due in 2022	24,000,000			24,000,000
Purchase obligations due in 2023	8,000,000			8,000,000
Purchase obligations due thereafter	5,000,000			5,000,000
Operating leases, expense				144,000,000	$ 113,000,000	$ 110,000,000
Long-term operating leases, minimum annual rentals due in 2019	94,000,000			94,000,000
Long-term operating leases, minimum annual rentals due in 2020	74,000,000			74,000,000
Long-term operating leases, minimum annual rentals due in 2021	54,000,000			54,000,000
Long-term operating leases, minimum annual rentals due in 2022	40,000,000			40,000,000
Long-term operating leases, minimum annual rentals due in 2023	30,000,000			30,000,000
Long-term operating leases, minimum annual rentals due thereafter	87,000,000			87,000,000
Guarantees of third party related to project financing	$ 37,000,000			37,000,000
Number of long term energy supply agreements | agreement							2
Number of agreements terminated | agreement		1
Other (expense) income, net				(79,000,000)	486,000,000	$ (41,000,000)
Insurance policy duration	1 year
Insurance policy limit	$ 80,000,000
Letters of credit	6,301,000,000		$ 6,807,000,000	6,301,000,000	6,807,000,000
Total amount committed under outstanding surety bonds	50,000,000			50,000,000
Amount of outstanding surety bonds relating to these liabilities	25,000,000			25,000,000
Purchase Agreement
Loss Contingencies [Line Items]
Number of long term energy supply agreements | agreement						2
Long-term purchase commitment, period						1 year
Guarantees of third party related to project financing						$ 245,000,000
Alcoa Corporation Workers Compensation Claims
Loss Contingencies [Line Items]
Letters of credit, total amount committed								$ 54,000,000
Bank Loan Obligations | Letter of Credit
Loss Contingencies [Line Items]
Letters of credit	136,000,000			136,000,000
Separation Agreement
Loss Contingencies [Line Items]
Combined fair value of guarantees	6,000,000		8,000,000	6,000,000	8,000,000
Other (expense) income, net			25,000,000
Total amount committed under outstanding surety bonds			25,000,000		25,000,000
Separation Agreement | Other Noncurrent Liabilities and Deferred Credits
Loss Contingencies [Line Items]
Guarantees of third party related to project financing	$ 1,087,000,000		$ 1,297,000,000	$ 1,087,000,000	$ 1,297,000,000