v3.26.1
Segment Information - Schedule of Operating Results, Capital Expenditures and Assets of Alcoa's Reportable Segments (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Segment Reporting Information [Line Items]          
Adjusted operating costs $ 2,531 $ 2,348 $ 4,698 $ 4,645  
Other segment items 917 895 1,739 1,880  
Segment Adjusted EBITDA 977 236 1,631 1,034  
Depreciation, depletion, and amortization 167 146 324 289  
Equity income (loss) 0 (6) 0 3  
Capital expenditures 179 127 289 217  
Equity investments 524   524   $ 473
Segment assets 12,757   12,757   11,822
Operating Segments [Member]          
Segment Reporting Information [Line Items]          
Total sales 4,425 3,479 8,068 7,559  
Intersegment Eliminations [Member]          
Segment Reporting Information [Line Items]          
Intersegment sales 458 472 908 1,188  
Third-Party Sales [Member] | Operating Segments [Member]          
Segment Reporting Information [Line Items]          
Third-party sales 3,967 3,007 7,160 6,371  
Alumina [Member]          
Segment Reporting Information [Line Items]          
Adjusted operating costs 843 770 1,580 1,493  
Other segment items 343 609 748 1,397  
Segment Adjusted EBITDA (96) 139 (136) 803  
Depreciation, depletion, and amortization 96 80 182 156  
Equity income (loss) 0 (9) 0 6  
Capital expenditures 106 73 170 126  
Equity investments 277   277   243
Segment assets 5,901   5,901   5,671
Alumina [Member] | Operating Segments [Member]          
Segment Reporting Information [Line Items]          
Total sales 1,090 1,518 2,192 3,693  
Alumina [Member] | Intersegment Eliminations [Member]          
Segment Reporting Information [Line Items]          
Intersegment sales 453 467 898 1,179  
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]          
Segment Reporting Information [Line Items]          
Third-party sales 637 1,051 1,294 2,514  
Aluminum [Member]          
Segment Reporting Information [Line Items]          
Adjusted operating costs 1,688 1,578 3,118 3,152  
Other segment items 574 286 991 483  
Segment Adjusted EBITDA 1,073 97 1,767 231  
Depreciation, depletion, and amortization 71 66 142 133  
Equity income (loss) 0 3 0 (3)  
Capital expenditures 73 54 119 91  
Equity investments 247   247   230
Segment assets 6,856   6,856   $ 6,151
Aluminum [Member] | Operating Segments [Member]          
Segment Reporting Information [Line Items]          
Total sales 3,335 1,961 5,876 3,866  
Aluminum [Member] | Intersegment Eliminations [Member]          
Segment Reporting Information [Line Items]          
Intersegment sales 5 5 10 9  
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]          
Segment Reporting Information [Line Items]          
Third-party sales $ 3,330 $ 1,956 $ 5,866 $ 3,857