Contingencies and Commitments (Details) € in Millions, $ in Millions		3 Months Ended	6 Months Ended
	Jun. 06, 2019 plaintiff	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($) location	Jun. 30, 2019 EUR (€) location	Jun. 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		$ 275	$ 275			$ 266
Remediation reserve balance, classified as a current liability		95	95			81
Payments related to remediation expenses applied against the reserve		13	16
umber of plaintiffs | plaintiff	247
Income tax examination assessment outstanding		173	173		€ 152
Guarantees of third party related to project financing		31	31
Total amount committed under outstanding surety bonds		50	50
Bank Loan Obligations
Loss Contingencies [Line Items]
Letters of credit, total amount		137	137
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		1,215	1,215			1,087
Massena West, NY
Loss Contingencies [Line Items]
Remediation reserve balance		210	210			$ 198
Increase in accrual for environmental loss contingency		25
Spain
Loss Contingencies [Line Items]
Established tax reserve			60	€ 52
Tax indemnification receivable		$ 29	$ 29		€ 25
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							$ 54