Contingencies and Commitments (Details) € in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2019 EUR (€)	Dec. 31, 2018 USD ($) location	Mar. 31, 2019 EUR (€)	Nov. 01, 2016 USD ($)
Loss Contingencies [Line Items]
Number of cleanup locations | location			100
Remediation reserve balance	$ 261		$ 266
Remediation reserve balance, classified as a current liability	79		81
Payments related to remediation expenses applied against the reserve	3
Income tax examination assessment outstanding	171			€ 152
Guarantees of third party related to project financing	29
Total amount committed under outstanding surety bonds	50
Bank Loan Obligations
Loss Contingencies [Line Items]
Letters of credit, total amount	136
Separation Agreement
Loss Contingencies [Line Items]
Total amount committed under outstanding surety bonds	25
Other Noncurrent Liabilities and Deferred Credits
Loss Contingencies [Line Items]
Combined fair value of guarantees	7		6
Other Noncurrent Liabilities and Deferred Credits | Separation Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing	1,143		1,087
Massena West, NY
Loss Contingencies [Line Items]
Remediation reserve balance	195		$ 198
Spain
Loss Contingencies [Line Items]
Established tax reserve	59	€ 52
Tax indemnification receivable	$ 28			€ 25
Alcoa Corporation
Loss Contingencies [Line Items]
Tax agreement, indemnification of ultimate liability, percent	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