| Cover Page | Feb. 10, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Feb. 10, 2021 |
| Entity Registrant Name | EXELIXIS, INC. |
| Entity Central Index Key | 0000939767 |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common Stock $.001 Par Value per Share |
| Trading Symbol | EXEL |
| Security Exchange Name | NASDAQ |
| Entity File Number | 000-30235 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 04-3257395 |
| Entity Address, Address Line One | 1851 Harbor Bay Parkway |
| Entity Address, City or Town | Alameda |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94502 |
| City Area Code | 650 |
| Local Phone Number | 837-7000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
