Restructuring and Other Charges, Net - Additional Information (Detail)		1 Months Ended			3 Months Ended				9 Months Ended		12 Months Ended
	Nov. 01, 2019 USD ($) Report Employee	Jul. 31, 2019 USD ($)	Feb. 28, 2019 kt	Dec. 31, 2009	Sep. 30, 2019 USD ($) Report	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Report	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jul. 05, 2019 Location	Jun. 27, 2019 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 185,000,000			$ 177,000,000	$ 668,000,000	$ 389,000,000
Employee termination and severance costs					37,000,000				37,000,000
Curtailments related to retirement benefits									38,000,000
Settlements of pension benefits					5,000,000				5,000,000
Combined operating capacity | kt			124
Payments for restructuring									93,000,000		$ 102,000,000
Curtailment of certain pension and other postretirement employee benefits								174,000,000		318,000,000
Noncurrent portion of the reserve					19,000,000				19,000,000
Alcoa Joint Venture [Member]
Restructuring Cost and Reserve [Line Items]
Equity interest						0.00%
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]
Equity investments					$ 615,000,000				$ 615,000,000		$ 874,000,000
Maaden Alcoa Joint Venture [Member]
Restructuring Cost and Reserve [Line Items]
Non-controlling interest, ownership percentage				25.10%	25.10%	25.10%			25.10%
Saudi Arabia [Member] | Ma'aden Joint Venture [Member]
Restructuring Cost and Reserve [Line Items]
Ownership interest percentage transferred						25.10%
PARTER Capital Group AG [Member]
Restructuring Cost and Reserve [Line Items]
Noncurrent portion of the reserve					$ 18,000,000				$ 18,000,000
Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Contributions to MRC						$ 100,000,000
Alcoa Corporation [Member] | Cost of Goods Sold [Member]
Restructuring Cost and Reserve [Line Items]
Write down of remaining inventories							$ 15,000,000
Ma’aden [Member] | Saudi Arabia [Member] | Maaden Alcoa Joint Venture [Member]
Restructuring Cost and Reserve [Line Items]
Ownership interest in joint venture				74.90%		74.90%			74.90%
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
Aviles and La Coruna Smelters [Member] | Spain [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					134,000,000
Charge for working capital commitment and write-off remaining net book value of plants assets					39,000,000
Aviles and La Coruna Smelters [Member] | PARTER Capital Group AG [Member] | Spain [Member]
Restructuring Cost and Reserve [Line Items]
Number of plants to be purchased | Location												2
Payments for restructuring		$ 37,000,000
Remaining cash outflows for restructuring expected to be paid in quarterly installments through second quarter of 2021					80,000,000				$ 80,000,000
Aviles and La Coruna Smelters [Member] | PARTER Capital Group AG [Member] | Spain [Member] | Maximum [Member]
Restructuring Cost and Reserve [Line Items]
Financial contributions					$ 95,000,000
Restructuring And Other Charges [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Asset impairment							80,000,000
Contract termination costs							8,000,000
Severance costs excluding employee related cost						3,000,000
Employee related expenses excluding severance costs						2,000,000	15,000,000
Restructuring And Other Charges [Member] | Alcoa Corporation [Member] | Selling, General Administrative, and Other Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges							$ 2,000,000
Scenario, Forecast [Member]
Restructuring Cost and Reserve [Line Items]
Employee termination and severance costs	$ 37,000,000
Number of employees associated with employee termination and severance costs | Employee	260
Chief Executive Officer [Member]
Restructuring Cost and Reserve [Line Items]
Number Of direct reports | Report					12				12
Chief Executive Officer [Member] | Scenario, Forecast [Member]
Restructuring Cost and Reserve [Line Items]
Number Of direct reports | Report	7
Exit Cost [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 134,000,000				$ 242,000,000
Closure Cost [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					1,000,000				9,000,000
Divesture of MRC [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges									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
Miscellaneous Items Charges [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 8,000,000			1,000,000	$ 18,000,000
Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges								$ 2,000,000		86,000,000
Net benefit										15,000,000
Contract Termination [Member] | Energy supply Agreement [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges										$ 73,000,000