Contingencies and Commitments - Commitments (Detail)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) agreement guarantee	Dec. 31, 2015 USD ($)	Nov. 01, 2016 USD ($)
Loss Contingencies [Line Items]
Purchase obligations due in 2018	$ 861,000,000	$ 861,000,000
Purchase obligations due in 2019	176,000,000	176,000,000
Purchase obligations due in 2020	74,000,000	74,000,000
Purchase obligations due in 2021	13,000,000	13,000,000
Purchase obligations due in 2022	6,000,000	6,000,000
Purchase obligations due thereafter	5,000,000	5,000,000
Operating leases, expense		113,000,000	$ 110,000,000	$ 112,000,000
Long-term operating leases, minimum annual rentals due in 2018	108,000,000	108,000,000
Long-term operating leases, minimum annual rentals due in 2019	88,000,000	88,000,000
Long-term operating leases, minimum annual rentals due in 2020	62,000,000	62,000,000
Long-term operating leases, minimum annual rentals due in 2021	46,000,000	46,000,000
Long-term operating leases, minimum annual rentals due in 2022	38,000,000	38,000,000
Long-term operating leases, minimum annual rentals due thereafter	100,000,000	100,000,000
Guarantees of third party related to project financing	29,000,000	29,000,000
Other income, net		640,000,000	94,000,000	$ 28,000,000
Letters of credit, total amount committed	120,000,000	120,000,000
Total amount committed under outstanding surety bonds	54,000,000	54,000,000
Other Noncurrent Liabilities and Deferred Credits
Loss Contingencies [Line Items]
Combined fair value of guarantees	8,000,000	8,000,000	35,000,000
Separation Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing	197,000,000	197,000,000	354,000,000
Other income, net	25,000,000
Combined fair value of guarantees	1,297,000,000	1,297,000,000
Total amount committed under outstanding surety bonds	$ 25,000,000	$ 25,000,000			$ 25,000,000
Alcoa Corporation Energy Supply Contracts Guarantee
Loss Contingencies [Line Items]
Guarantees of third party related to project financing			$ 50,000,000
Guarantor obligations, number of contracts | guarantee			2
Alcoa Corporation Environmental Liabilities Guarantee
Loss Contingencies [Line Items]
Guarantees of third party related to project financing			$ 53,000,000
Purchase Agreement
Loss Contingencies [Line Items]
Long-term purchase commitment, period			1 year
Guarantees of third party related to project financing			$ 245,000,000
Number of long term energy supply agreements | agreement			2
Alcoa Corporation Workers Compensation Claims
Loss Contingencies [Line Items]
Letters of credit, total amount committed					62,000,000
Alcoa Corporation Equipment Leases And Energy Contracts
Loss Contingencies [Line Items]
Letters of credit, total amount committed					$ 103,000,000