Contingencies - Additional Information - 1 (Detail) $ in Millions				1 Months Ended			3 Months Ended			6 Months Ended
	Jul. 07, 2020 USD ($)	Jul. 07, 2020 AUD ($)	Jul. 06, 2018 USD ($)	Mar. 31, 2013 USD ($)	May 31, 2012 USD ($)	May 31, 2012 BRL (R$)	Sep. 30, 2020 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 EUR (€)	Jun. 30, 2020 USD ($)	Jun. 30, 2020 BRL (R$)	Sep. 30, 2020 AUD ($)	Jul. 07, 2020 AUD ($)	Jun. 30, 2020 AUD ($)	Dec. 31, 2019	Jul. 06, 2018 EUR (€)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 EUR (€)	Mar. 31, 2013 BRL (R$)
AWAC [Member] | Alumina Limited [Member]
Loss Contingencies [Line Items]
Non-controlling interest, ownership percentage										40.00%				40.00%	40.00%
Alcoa Corporation [Member] | AWAC [Member]
Loss Contingencies [Line Items]
Ownership interest percentage										60.00%				60.00%	60.00%
Tax Authority, Spain [Member]
Loss Contingencies [Line Items]
Charge recorded in provision for income taxes to establish liability for estimated loss								$ 30,000,000	€ 26,000,000
Percentage of share of the estimated loss								49.00%	49.00%
Tax Authority, Spain [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Total combined assessments			$ 25,000,000													€ 21,000,000
Tax Authority, Spain [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Total combined assessments			$ 61,000,000													€ 53,000,000
Tax Authority, Spain [Member] | Arconic Inc. [Member]
Loss Contingencies [Line Items]
Tax matters agreement, contribution percentage			51.00%
Tax Authority, Spain [Member] | Alcoa Corporation [Member]
Loss Contingencies [Line Items]
Tax matters agreement, contribution percentage			49.00%
Brazilian Federal Revenue Office [Member] | Alcoa World Alumina Brasil [Member]
Loss Contingencies [Line Items]
Disallowed tax credits				$ 110,000,000															R$ 220,000,000
Percentage of penalty of the gross disallowed amount				50.00%
Value added tax receivable					$ 41,000,000	R$ 82,000,000
Brazilian Federal Revenue Office [Member] | Alcoa World Alumina Brasil [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Charge recorded in provision for income taxes to establish liability for estimated loss										$ 0
Brazilian Federal Revenue Office [Member] | Alcoa World Alumina Brasil [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Charge recorded in provision for income taxes to establish liability for estimated loss										40,000,000	R$ 220,000,000
Australian Taxation Office [Member] | Foreign Jurisdiction [Member] | AofA [Member]
Loss Contingencies [Line Items]
Additional income tax payable, exclusive of interest and penalties										$ 144,000,000				$ 212
Australian Taxation Office [Member] | Foreign Jurisdiction [Member] | AofA [Member] | Subsequent Event [Member]
Loss Contingencies [Line Items]
Additional income tax payable, exclusive of interest and penalties	$ 147,000,000												$ 214
Increase in income tax payable, exclusive of interest and penalties	1,000,000	$ 2
Notices include claims for compounded interest on the tax amount	$ 488,000,000												$ 707
Australian Taxation Office [Member] | Foreign Jurisdiction [Member] | AofA [Member] | Scenario Forecast [Member]
Loss Contingencies [Line Items]
Payment of dispute resolution practices income tax percentage							50.00%
Assessed income tax amount exclusive of interest and penalties							$ 74,000,000					$ 107
Payment amount refund percentage							50.00%
Tax Year 2006 Through 2009 [Member] | Tax Authority, Spain [Member]
Loss Contingencies [Line Items]
Total combined assessments																	$ 152,000,000	€ 131,000,000