Contingencies and Commitments - Narrative (Detail) € in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) location	Mar. 31, 2018 EUR (€)	Dec. 31, 2017 USD ($)	Nov. 01, 2016 USD ($)
Loss Contingencies [Line Items]
Number of cleanup locations | location	100
Remediation reserve balance	$ 292		$ 294
Remediation reserve balance, classified as a current liability	40		41
Payments related to remediation expenses applied against the reserve	$ 3
Actual remediation fieldwork period	4 years
Assessment for 2006 through 2009 tax years	$ 161	€ 130
Guarantees of third party related to project financing	27
Long-term debt	6,312		6,807
Total amount committed under outstanding surety bonds	55
Massena West, NY
Loss Contingencies [Line Items]
Remediation reserve balance	$ 213		215
Recurring Costs of Managing Hazardous Substances and Environmental Programs | Maximum
Loss Contingencies [Line Items]
Percentage of cost of goods sold	1.00%
Supply Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing	$ 1,188		1,297
Alcoa Corporation Workers Compensation Claims
Loss Contingencies [Line Items]
Letters of credit, total amount committed				$ 53
Other Noncurrent Liabilities and Deferred Credits
Loss Contingencies [Line Items]
Combined fair value of guarantees	7		8
Separation Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing	0		$ 0
Total amount committed under outstanding surety bonds	25
Bank Loan Obligations [Member] | Letter of Credit
Loss Contingencies [Line Items]
Long-term debt	$ 126