| Document and Entity Information | May 11, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | May 11, 2021 |
| Entity Registrant Name | SYNDAX PHARMACEUTICALS, INC. |
| Entity Central Index Key | 0001395937 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Entity File Number | 001-37708 |
| Entity Incorporation State Country Code | DE |
| Entity Tax Identification Number | 32-0162505 |
| Entity Address, Address Line One | 35 Gatehouse Drive |
| Entity Address, Address Line Two | Building D |
| Entity Address, Address Line Three | Floor 3 |
| Entity Address, City or Town | Waltham |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02451 |
| City Area Code | (781) |
| Local Phone Number | 419-1400 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock |
| Trading Symbol | SNDX |
| Security Exchange Name | NASDAQ |
