v3.26.1
Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive Loss by Component (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Mar. 31, 2026
Jun. 30, 2025
Mar. 31, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Pension and other postretirement benefits (L)              
Total Other comprehensive income $ (14)   $ (11)   $ (25) $ (16)  
Foreign currency translation              
Other comprehensive income (loss) 7   217   145 402  
Joint venture formation (C)       $ (76)      
Cash flow hedges (M)              
Net change from periodic revaluations   $ (16)   (213) (58) (128)  
Net amount reclassified to earnings   331   175 190 80  
Total Accumulated other comprehensive (loss) income (4,735)       (4,735)   $ (5,189)
Alcoa Corporation [Member]              
Pension and other postretirement benefits (L)              
Balance at beginning of period (63) (74) (6) (11) (74) (11)  
Unrecognized net actuarial gain/loss and prior service cost/benefit 6   1   7    
Tax (expense) benefit (1)   3   (1) 2  
Total Other comprehensive income before reclassifications, net of tax 5   4   6 2  
Amortization of net actuarial gain/loss and prior service cost/benefit 9   7   19 14  
Total amount reclassified from Accumulated other comprehensive loss, net of tax 9   7   19 14  
Total Other comprehensive income 14   11   25 16  
Balance at end of period (49) (63) 5 (6) (49) 5  
Foreign currency translation              
Balance at beginning of period (3,639) (3,780) (4,040) (4,194) (3,780) (4,194)  
Other comprehensive income (loss) 7   207   148 392  
Joint venture formation (C)           (31)  
Balance at end of period (3,632) (3,639) (3,833) (4,040) (3,632) (3,833)  
Cash flow hedges (M)              
Balance at beginning of period (1,179) (1,335) (889) (905) (1,335) (905)  
Net change from periodic revaluations (58)   (128)   (16) (213)  
Tax benefit 22   18   20 41  
Total Other comprehensive income (loss) before reclassifications, net of tax (36)   (110)   4 (172)  
Net amount reclassified to earnings 190   80   331 175  
Tax expense (29)   (17)   (54) (34)  
Total amount reclassified from Accumulated other comprehensive loss, net of tax 161   63   277 141  
Total Other comprehensive income (loss) 125   (47)   281 (31)  
Balance at end of period (1,054) (1,179) (936) $ (889) (1,054) (936)  
Total Accumulated other comprehensive (loss) income (4,735)   (4,764)   (4,735) (4,764)  
Alcoa Corporation [Member] | Aluminum Contracts [Member]              
Cash flow hedges (M)              
Net amount reclassified to earnings 187   78   318 171  
Alcoa Corporation [Member] | Financial Contracts [Member]              
Cash flow hedges (M)              
Net amount reclassified to earnings 6   2   17 2  
Alcoa Corporation [Member] | Foreign Exchange Contracts [Member]              
Cash flow hedges (M)              
Net amount reclassified to earnings (3)       (4) 2  
Non-controlling Interest [Member]              
Foreign currency translation              
Balance at beginning of period 8 11     11    
Other comprehensive income (loss)     10   (3) 10  
Balance at end of period 8 $ 8 10   8 10  
Cash flow hedges (M)              
Total Accumulated other comprehensive (loss) income $ 8   $ 10   $ 8 $ 10