| Document and Entity Information | Sep. 03, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 3, 2021 |
| Entity Registrant Name | KAISER ALUMINUM CORP |
| Entity Central Index Key | 0000811596 |
| Entity Emerging Growth Company | false |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 1-09447 |
| Entity Tax Identification Number | 94-3030279 |
| Entity Address, Address Line One | 27422 Portola Parkway |
| Entity Address, Address Line Two | Suite 200 |
| Entity Address, City or Town | Foothill Ranch |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 92610-2831 |
| City Area Code | (949) |
| Local Phone Number | 614-1740 |
| 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 | KALU |
| Security Exchange Name | NASDAQ |
