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Debt - Additional Information (Detail) $ in Millions, kr in Billions		1 Months Ended	3 Months Ended	6 Months Ended
	Apr. 08, 2020 USD ($)	Jul. 31, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020	Oct. 02, 2019 USD ($)	Oct. 02, 2019 NOK (kr)
5.500% Senior Notes due 2027 [Member] \| Alcoa Nederland Holding BV [Member]
Debt Instrument [Line Items]
Debt instrument, frequency of periodic payment				semi-annually
5.500% Senior Notes due 2027 [Member] \| Alcoa Nederland Holding BV [Member] \| Subsequent Event [Member]
Debt Instrument [Line Items]
Principal amount of debt		$ 750
Senior notes, interest percentage		5.50%
Debt instrument maturity date		2027
Proceeds from issuance of debt		$ 736
Debt instrument, date of first required payment		Dec. 15, 2020
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Leverage ratio, indebtedness to EBITDA				3.00	2.50
Leverage ratio, indebtedness to EBITDA in next year				2.50
Principal amount of debt				$ 1,500
Revolving Credit Facility [Member] \| One-year, Multicurrency Agreement [Member] \| Alcoa Norway ANS [Member]
Debt Instrument [Line Items]
Principal amount of debt						$ 134	kr 1.3
Line of credit facility, current outstanding	$ 100
Line of credit facility, interest rate at period end	2.93%
Line of credit facility repayment due date	Jun. 29, 2020
Revolving Credit Facility [Member] \| Scenario Forecast [Member]
Debt Instrument [Line Items]
Line of credit facility, decrease in aggregate amount of commitments			$ 245