| Document and Entity Information | Mar. 31, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001675149 |
| Document Type | 8-K |
| Document Period End Date | Mar. 31, 2021 |
| Entity Registrant Name | ALCOA CORP |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 1-37816 |
| Entity Tax Identification Number | 81-1789115 |
| Entity Address, Address Line One | 201 Isabella Street |
| Entity Address, Address Line Two | Suite 500 |
| Entity Address, City or Town | Pittsburgh |
| Entity Address, State or Province | PA |
| Entity Address, Postal Zip Code | 15212-5858 |
| City Area Code | 412 |
| Local Phone Number | 315-2900 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.01 per share |
| Trading Symbol | AA |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
