v3.26.1
Debt - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended
May 15, 2026
Apr. 24, 2026
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Debt Instrument [Line Items]              
Short-term borrowings     $ 0   $ 0   $ 9,000,000
Borrowings of inventory related to agreement       $ 7,000,000 104,000,000 $ 51,000,000  
Repurchase of inventory related to agreement     109,000,000 44,000,000 113,000,000 93,000,000  
Alcoa Nederland Holding BV [Member] | 6.125% Notes, due 2028 [Member]              
Debt Instrument [Line Items]              
Repayment of senior debt $ 219,000,000            
Senior notes, interest percentage 6.125%            
Debt instrument maturity date 2028            
Debt instrument redemption price percentage of principal amount redeemed 100.00%            
Revolving Credit Facility [Member]              
Debt Instrument [Line Items]              
Principal amount of debt     0   0   0
Amounts borrowed under the credit facility     0 0 0 0  
Unsecured revolving credit facility     1,250,000,000   1,250,000,000    
Japanese Yen Revolving Credit Facility [Member] | Alcoa Nederland Holding BV [Member]              
Debt Instrument [Line Items]              
Amounts borrowed under the credit facility     0 $ 0 0 $ 0  
Line of credit facility, amounts outstanding   $ 0         $ 0
Unsecured revolving credit facility     200,000,000   200,000,000    
Unsecured revolving credit facility maturity date   Apr. 24, 2026          
Bridge Term Loan Credit Facility [Member]              
Debt Instrument [Line Items]              
Bridge financing facility maximum commitment amount     $ 3,100,000,000   $ 3,100,000,000