Restructuring and Other Charges, Net (2019 Actions) - Additional Information (Detail)	1 Months Ended					12 Months Ended
	Dec. 31, 2019 USD ($) Employee	Sep. 30, 2019 USD ($) Employee	Jun. 30, 2019 USD ($)	Feb. 28, 2019 USD ($) kt	Dec. 31, 2009	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) Employee	Jul. 05, 2019 kt	Dec. 31, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges						$ 1,128,000,000	$ 104,000,000	$ 1,031,000,000
Curtailment of certain pension and other postretirement employee benefits						(977,000,000)	(58,000,000)	(119,000,000)
Severance costs		$ 37,000,000				1,000,000	16,000,000	51,000,000
Asset impairments						75,000,000	2,000,000	225,000,000
Asset retirement obligations						101,000,000	107,000,000
Environmental remediation						15,000,000	1,000,000	69,000,000
Number of employees associated with employee termination and severance costs | Employee		260
Payments for restructuring						29,000,000	120,000,000	118,000,000
Restructuring reserve balance	$ 137,000,000					$ 93,000,000	$ 63,000,000	137,000,000		$ 47,000,000
Combined operating capacity | kt				124
Alcoa Joint Venture [Member]
Restructuring Cost and Reserve [Line Items]
Ownership interest in joint venture					25.10%
Equity interest			0.00%
Maaden Alcoa Joint Venture [Member]
Restructuring Cost and Reserve [Line Items]
Non-controlling interest, ownership percentage					25.10%	25.10%	25.10%
Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Ownership interest in joint venture						25.10%
Contributions to MRC			$ 100,000,000
Ma'aden [Member]
Restructuring Cost and Reserve [Line Items]
Contributions to MRC			100,000,000
Ma'aden Joint Venture [Member]
Restructuring Cost and Reserve [Line Items]
Contributions to MRC			100,000,000
Ma'aden Joint Venture [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Project financing Investment			296,000,000
Maaden Alcoa Joint Venture [Member]
Restructuring Cost and Reserve [Line Items]
Write off of MRC delinquent payables			$ 235,000,000
Cost of Goods Sold [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Other costs				$ 16,000,000				16,000,000
Saudi Arabia [Member] | Ma'aden Joint Venture [Member]
Restructuring Cost and Reserve [Line Items]
Write off of MRC delinquent payables			25.10%
Saudi Arabia [Member] | Ma'aden [Member] | Maaden Alcoa Joint Venture [Member]
Restructuring Cost and Reserve [Line Items]
Ownership interest in joint venture					74.90%	74.90%	74.90%
Divesture of MRC [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			$ 319,000,000					319,000,000
Divesture of MRC [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Write off of investment			161,000,000					161,000,000
Cash contribution to an equity method investment			100,000,000					100,000,000
Gain from write off of the fair value of debt guarantee			1,000,000					1,000,000
Write off delinduent payables due, forgiven			59,000,000					59,000,000
Divesture of MRC [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Write off of investment			161,000,000
Cash contribution to an equity method investment			100,000,000
Gain from write off of the fair value of debt guarantee			1,000,000
Write off delinduent payables due, forgiven			$ 59,000,000
Other Item Charges [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges						$ 3,000,000
Permanent Close [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges						(22,000,000)
Severance and Employee Termination Costs [Member]
Restructuring Cost and Reserve [Line Items]
Payments for restructuring						4,000,000	$ 41,000,000	19,000,000
Restructuring reserve balance	35,000,000					3,000,000	6,000,000	35,000,000		$ 5,000,000
2019 Restructuring Plan Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges								1,031,000,000
Costs related to smelter curtailment and subsequent divestiture								235,000,000
Curtailment of certain pension and other postretirement employee benefits								119,000,000
Severance costs								$ 37,000,000
Restructuring and related cost number of eliminated positions were not replaced | Employee								60
2019 Restructuring Plan Action [Member] | Divesture of MRC [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges								$ 319,000,000
2019 Restructuring Plan Action [Member] | Exit Cost [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges								274,000,000
2019 Restructuring Plan Action [Member] | Closure Cost [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges								9,000,000
2019 Restructuring Plan Action [Member] | Other Item Charges [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges								38,000,000
2019 Restructuring Plan Action [Member] | Severance and Employee Termination Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve balance	27,000,000							27,000,000
2019 Restructuring Plan Action [Member] | Elimination of Open Role Positions [Member]
Restructuring Cost and Reserve [Line Items]
Payments for restructuring							25,000,000
The Point Comfort alumina refinery {Member] | Permanent Close [Member] | Texas [Member]
Restructuring Cost and Reserve [Line Items]
Severance costs	4,000,000
Asset impairments	129,000,000
Asset retirement obligations	72,000,000
Environmental remediation	$ 69,000,000
Number of employees associated with employee termination and severance costs | Employee	40
Payments for restructuring							2,000,000
Restructuring and other charges (reversal)							(1,000,000)
The Point Comfort alumina refinery {Member] | Permanent Close [Member] | Texas [Member] | Cost of Goods Sold [Member]
Restructuring Cost and Reserve [Line Items]
Other costs	$ 2,000,000
Aviles and La Coruna Smelters [Member] | Spain [Member] | Avilés and La Coruña Aluminum Facilities [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges								127,000,000
Charge for working capital commitment and write-off remaining net book value of plants assets								32,000,000
Aviles and La Coruna Smelters [Member] | Spain [Member] | PARTER Capital Group AG [Member]
Restructuring Cost and Reserve [Line Items]
Number of plants to be purchased | kt									2
Aviles and La Coruna Smelters [Member] | Spain [Member] | PARTER Capital Group AG [Member] | Avilés and La Coruña Aluminum Facilities [Member]
Restructuring Cost and Reserve [Line Items]
Payments for restructuring								47,000,000
Restructuring and other charges (reversal)						(17,000,000)
Financial contributions						13,000,000	38,000,000
Remaining cash outflows for restructuring expected to be paid in quarterly installments through second quarter of 2021							$ 30,000,000
Aviles and La Coruna Smelters [Member] | Spain [Member] | PARTER Capital Group AG [Member] | Avilés and La Coruña Aluminum Facilities [Member] | Maximum [Member]
Restructuring Cost and Reserve [Line Items]
Financial contributions						$ 95,000,000		95,000,000
Restructuring And Other Charges [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Asset impairments				80,000,000				80,000,000
Severance and employee related costs								20,000,000
Contract termination costs				8,000,000				8,000,000
Restructuring And Other Charges [Member] | Selling, General Administrative, and Other Expenses [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				$ 2,000,000				$ 2,000,000