v3.8.0.1
Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Feb. 20, 2018
Jun. 30, 2017
Document And Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2017    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Trading Symbol FPRX    
Entity Registrant Name FIVE PRIME THERAPEUTICS INC    
Entity Central Index Key 0001175505    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer Yes    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Large Accelerated Filer    
Entity Common Stock, Shares Outstanding   34,860,499  
Entity Public Float     $ 619