Contingencies and Commitments (Details) € in Millions, $ in Millions		3 Months Ended		9 Months Ended
	Jun. 06, 2019 plaintiff	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($) location	Sep. 30, 2019 EUR (€) location	Sep. 30, 2019 EUR (€)	Dec. 31, 2018 USD ($)	Nov. 01, 2016 USD ($)
Loss Contingencies [Line Items]
Number of cleanup locations | location				100	100
Remediation reserve balance		$ 253		$ 253			$ 266
Remediation reserve balance, classified as a current liability		88		88			81
Payments related to remediation expenses applied against the reserve		22		38
umber of plaintiffs | plaintiff	247
Income tax examination assessment outstanding		166		166		€ 152
Guarantees of third party related to project financing		31		31
Total amount committed under outstanding surety bonds		49		49
Bank Loan Obligations
Loss Contingencies [Line Items]
Letters of credit, total amount		146		146
Separation Agreement
Loss Contingencies [Line Items]
Total amount committed under outstanding surety bonds		24		24
Other Noncurrent Liabilities and Deferred Credits
Loss Contingencies [Line Items]
Combined fair value of guarantees		8		8			6
Other Noncurrent Liabilities and Deferred Credits | Separation Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing		1,265		1,265			1,087
Massena West, NY
Loss Contingencies [Line Items]
Remediation reserve balance		191		191			$ 198
Increase in accrual for environmental loss contingency			$ 25
Spain
Loss Contingencies [Line Items]
Established tax reserve				58	€ 53
Tax indemnification receivable		$ 28		$ 28		€ 26
Alcoa Corporation
Loss Contingencies [Line Items]
Tax agreement, indemnification of ultimate liability, percent		49.00%		49.00%		49.00%
Recurring Costs of Managing Hazardous Substances and Environmental Programs | Maximum
Loss Contingencies [Line Items]
Percentage of cost of goods sold				1.00%	1.00%
Alcoa Corporation Workers Compensation Claims
Loss Contingencies [Line Items]
Letters of credit, total amount outstanding								$ 52