Restructuring and Other Charges, Net - Additional Information (Detail)		1 Months Ended	3 Months Ended					9 Months Ended		12 Months Ended
	Apr. 22, 2020 kt	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) Employee	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			$ 5,000,000		$ 185,000,000			$ 44,000,000	$ 668,000,000
Settlements of pension benefits			(5,000,000)		(5,000,000)			(5,000,000)	(5,000,000)
Net curtailment gain								(2,000,000)	(38,000,000)
Payments for restructuring								101,000,000		$ 118,000,000
Employee termination and severance costs					37,000,000				37,000,000
Restructuring reserve balance			75,000,000					75,000,000		137,000,000	$ 47,000,000
Noncurrent portion of the reserve			1,000,000					1,000,000		13,000,000
Aviles and La Coruna Smelters [Member]
Restructuring Cost and Reserve [Line Items]
Payments for restructuring			6,000,000					30,000,000
Restructuring reserve balance			38,000,000					38,000,000		68,000,000
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
2019 Restructuring Plan Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					37,000,000			2,000,000
Payments for restructuring			2,000,000					$ 24,000,000
Number of affected employees associated with employee termination and severance costs | Employee								260
Number of separated employees associated with employee termination and severance costs | Employee								235
Increase (reduction) in foreign currency								$ (1,000,000)
Restructuring and related cost number of eliminated positions were not replaced | Employee								60
The Point Comfort alumina refinery {Member] | Texas [Member]
Restructuring Cost and Reserve [Line Items]
Payments for restructuring								$ 2,000,000
Number of affected employees associated with employee termination and severance costs | Employee								40
Restructuring and related cost number of positions expected to be eliminated | Employee								35
Alcoa Corporation [Member] | Cost of Goods Sold [Member]
Restructuring Cost and Reserve [Line Items]
Write down of remaining inventories							$ 15,000,000
Alcoa Corporation [Member] | Restructuring And Other Charges [Member]
Restructuring Cost and Reserve [Line Items]
Contract termination costs							8,000,000
Asset impairment							80,000,000
Severance costs excluding employee related cost						$ 3,000,000
Employee related expenses excluding severance costs						2,000,000	15,000,000
Alcoa Corporation [Member] | Restructuring And Other Charges [Member] | Selling, General Administrative, and Other Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Miscellaneous charges							$ 2,000,000
PARTER Capital Group AG [Member]
Restructuring Cost and Reserve [Line Items]
Noncurrent portion of the reserve										12,000,000
PARTER Capital Group AG [Member] | Aviles and La Coruna Smelters [Member] | Spain [Member] | Maximum [Member]
Restructuring Cost and Reserve [Line Items]
Financial contributions					95,000,000
Intalco [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				$ 27,000,000
Restructuring and other charges (reversal)			(4,000,000)					$ (4,000,000)
Curtailment of smelting capacity | kt	230
Curtailed smelting capacity | kt	49
Contract termination costs				11,000,000
Net curtailment gain				1,000,000
Payments for restructuring			8,000,000					$ 10,000,000
Number of affected employees associated with employee termination and severance costs | Employee								685
Number of separated employees associated with employee termination and severance costs | Employee								590
Payment of contract termination cost			1,000,000					$ 1,000,000
Severance and Exit Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (reversal)			(4,000,000)					23,000,000
Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			4,000,000					17,000,000
Severance and Other Employee Termination Costs [Member]
Restructuring Cost and Reserve [Line Items]
Payments for restructuring								38,000,000		19,000,000
Restructuring reserve balance			$ 11,000,000					$ 11,000,000		35,000,000	$ 5,000,000
Severance and Other Employee Termination Costs [Member] | 2019 Restructuring Plan Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve balance										27,000,000
Severance and Other Employee Termination Costs [Member] | The Point Comfort alumina refinery {Member] | Texas [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve balance										$ 4,000,000
Severance and Other Employee Termination Costs [Member] | Intalco [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				$ 17,000,000
Exit Cost [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					134,000,000				242,000,000
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
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 delinquent payables due, forgiven		$ 59,000,000
Closure Cost [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					1,000,000				9,000,000
Other Items Charges [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 8,000,000				$ 18,000,000