| Document and Entity Information | Oct. 10, 2019 |
| Cover [Abstract] | |
| Entity Registrant Name | FIVE PRIME THERAPEUTICS, INC. |
| Security Exchange Name | NASDAQ |
| Amendment Flag | true |
| Entity Central Index Key | 0001175505 |
| Document Type | 8-K/A |
| Document Period End Date | Oct. 10, 2019 |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-36070 |
| Entity Tax Identification Number | 26-0038620 |
| Entity Address, Address Line One | 111 Oyster Point Boulevard |
| Entity Address, City or Town | South San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94080 |
| City Area Code | 415 |
| Local Phone Number | 365-5600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.001 per share |
| Trading Symbol | FPRX |
| Entity Emerging Growth Company | false |
| Amendment Description | This Current Report on Form 8-K/A (this “Current Report”) amends the Current Report on Form 8-K filed by Five Prime Therapeutics, Inc., a Delaware corporation (“FivePrime”), with the Securities and Exchange Commission (“SEC”) on October 10, 2019 (the “Original Report”) and is being filed in order to amend and supplement FivePrime’s disclosure under Item 2.05 of the Original Report. The Original Report otherwise remains unchanged. |
