v3.19.1
Document and Entity Information - shares
3 Months Ended
Mar. 31, 2019
May 02, 2019
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Mar. 31, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q1  
Trading Symbol FPRX  
Entity Registrant Name FIVE PRIME THERAPEUTICS INC  
Entity Central Index Key 0001175505  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company false  
Entity Common Stock, Shares Outstanding   36,062,578