v3.10.0.1
Document and Entity Information - shares
9 Months Ended
Sep. 30, 2018
Nov. 02, 2018
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
Trading Symbol CTMX  
Entity Registrant Name CytomX Therapeutics, Inc.  
Entity Central Index Key 0001501989  
Current Fiscal Year End Date --12-31  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Common Stock, Shares Outstanding   45,007,104