Acquisitions and Divestitures - Additional Information (Detail) $ in Millions					3 Months Ended		9 Months Ended
	Sep. 15, 2024 USD ($) shares	Sep. 12, 2024 USD ($)	Aug. 01, 2024 USD ($) shares	Jul. 31, 2024 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jul. 31, 2024 $ / shares
Business Disposition [Line Items]
Acquisition of cash			$ 9
Net income (loss), Noncontrolling interest					$ 8	$ (25)	$ (36)	$ (45)
Warrick Rolling Mill [Member] | Disposal Group, Held-for-Sale, Not Discontinued Operations [Member]
Business Disposition [Line Items]
Estimated liabilities for future site separation commitment charge							15	17
Payments against the reserve					7	$ 19	19	$ 44
Remaining reserve					7		7
Alumina Limited [Member]
Business Disposition [Line Items]
Business acquisition, shares outstanding | shares			2,760,056,014
Exchange ratio value per share | $ / shares									$ 1.45
Aggregate purchase consideration				$ 2,700
Other current liabilities			$ 1
Noncontrolling interest			1,472
Net assets acquired			1,216
Acquisition of noncontrolling interest			$ 1,099
Saudi Arabia Joint Venture [Member]
Business Disposition [Line Items]
Ownership interest in joint venture	25.10%
Sale of ownership interest under agreement exchange for issuance shares | shares	85,977,547
Sale of ownership interest under agreement exchange for issuance shares value		$ 950
Minimum holding period	3 years
Percentage of shares transferable	33.33%
Sale of ownership interest under agreement exchange for cash	$ 150
Alcoa Corporation [Member]
Business Disposition [Line Items]
Carrying value of investment					$ 538		$ 538
Nonvoting Convertible Preferred Stock [Member] | Alumina Limited [Member]
Business Disposition [Line Items]
Business acquisition, shares issued | shares			4,041,989
Business acquisition, shares outstanding | shares			141,625,403
AWAC [Member] | Alumina Limited [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage			40.00%
Common Stock [Member] | Alumina Limited [Member]
Business Disposition [Line Items]
Business acquisition, shares issued | shares			78,772,422
Additional Paid-in Capital [Member] | Alumina Limited [Member]
Business Disposition [Line Items]
Transaction costs			$ 32
Scheme Implementation Deed [Member] | Alumina Limited [Member]
Business Disposition [Line Items]
Business acquision, number of shares | shares			4
Assumption of indebtedness			$ 385
Deferred tax assets			$ 121
Scheme Implementation Deed [Member] | New Alcoa Preferred Stock [Member] | Alumina Limited [Member]
Business Disposition [Line Items]
Business acquisition, shares issued | shares			0.02854
Scheme Implementation Deed [Member] | Alcoa Common Stock [Member] | Alumina Limited [Member]
Business Disposition [Line Items]
Business acquisition, shares issued | shares			0.02854
Scheme Implementation Deed [Member] | CHESS Depositary Interests [Member] | Alumina Limited [Member]
Business Disposition [Line Items]
Business acquisition, shares issued | shares			0.02854