v3.19.2
Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Aug. 01, 2019
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
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 Current Reporting Status Yes    
Entity Shell Company false  
Entity File Number 1-36070  
Entity Tax Identification Number 260038620  
Entity Address, Address Line One 111 Oyster Point Boulevard  
Entity Address, City or Town South San Francisco  
Entity Address, State or Province California  
Entity Address, Postal Zip Code 94080  
City Area Code 415  
Local Phone Number 365-5600  
Entity Common Stock, Shares Outstanding   36,781,385