| Document And Entity Information | Nov. 04, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 4, 2021 |
| Entity Registrant Name | CytomX Therapeutics, Inc. |
| Entity Central Index Key | 0001501989 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-37587 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 27-3521219 |
| Entity Address, Address Line One | 151 Oyster Point Blvd |
| Entity Address, Address Line Two | Suite 400 |
| Entity Address, City or Town | South San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94080 |
| City Area Code | 650 |
| Local Phone Number | 515-3185 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.00001 par value per share |
| Trading Symbol | CTMX |
| Security Exchange Name | NASDAQ |
