Restructuring and Other Charges, Net (2017 Actions) - Additional Information (Detail) $ in Millions	1 Months Ended				3 Months Ended								9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) Employee	Dec. 31, 2017 a kt	Oct. 31, 2017 USD ($) a	Jul. 31, 2017 USD ($) kt Potline	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) Employee	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employee	260
Cash payment														$ 118	$ 102	$ 73
Sales					$ 2,436	$ 2,567	$ 2,711	$ 2,719	$ 3,344	$ 3,390	$ 3,579	$ 3,090		10,433	13,403	11,652
Asset impairments														225	18	40
Other costs														52	48	49
Asset retirement obligations														90	80
Environmental remediation														69	2	8
Severance costs	$ 37													51	2	15
Carrying value of the smelter and related assets					$ 14,631				$ 16,132					$ 14,631	16,132
Smelting and Related Assets [Member]
Restructuring Cost and Reserve [Line Items]
Carrying value of the smelter and related assets				$ 0
2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Other costs																16
Other costs																6
Number of potlines restarted | Potline				3
Capacity of restarted potlines | kt				161
Total number of potlines | Potline				5
Total capacity of potlines | kt				269
Liabilities related to original closure decision reversed				$ 33
Asset retirement obligations				20
Environmental remediation				4
Severance costs				$ 9
2017 Restructuring Plans Action [Member] | Smelting and Related Assets [Member]
Restructuring Cost and Reserve [Line Items]
Carrying value of the smelter and related assets																	$ 0	$ 0
Alcoa Corporation [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges																$ 309
Number of employees associated with layoff costs | Employee																130
Alcoa Corporation [Member] | 2017 Restructuring Plans Action [Member] | Aluminum Segment [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employee																115
Rarly Termination of Power Contract [Member] | Alcoa Corporation [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges																$ 244
Exit Cost [Member] | Alcoa Corporation [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges																49
Contract Termination [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Cash payment			$ 238
Area of land transferred | a			2,200
Net asset carrying value			$ 6
Sales													$ 105
Cost Of Goods And Services Sold													$ 148
Capacity closure | kt		191
Area of land owned | a		30,000
Asset impairments																32
Cash payments made against the layoff reserves																1
Other costs																$ 16
Contract Termination [Member] | 2017 Restructuring Plans Action [Member] | Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employee																10
Contract Termination [Member] | Alcoa Corporation [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges																$ 41
Restructuring Programs Layoffs 2017 [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Cash payments made against the layoff reserves															$ 3	9
Restructuring Programs Layoffs 2017 [Member] | Alcoa Corporation [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges																22
Reserve Previously Established [Member] | Alcoa Corporation [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges																22
Other Item Charges [Member] | Alcoa Corporation [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges																18
Other Adjustments [Member] | Alcoa Corporation [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges																43
Asset Retirement Obligations [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Other costs																8
Environmental Remediation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges																$ 8