| Document and Entity Information - USD ($) $ in Millions | 12 Months Ended | ||
| Dec. 31, 2020 | Mar. 10, 2021 | Jun. 30, 2020 | |
| Cover [Abstract] | |||
| Document Type | 10-K/A | ||
| Amendment Flag | true | ||
| Amendment Description | The sole purpose of this Amendment No. 1 (“Amendment No. 1”) to the Annual Report on Form 10-K for the year ended December 31, 2020 of Syndax Pharmaceuticals, Inc. (the “Company”) filed with the Securities and Exchange Commission on March 12, 2021 (the “Form 10-K”) is to amend the certifications of the Company’s Principal Executive Officer and Principal Financial Officer pursuant to Rule 13a-14(a) or 15d-14(a) of the Exchange Act (the “Certifications”), to correct an inadvertent omission of a portion of paragraph 4 of the Certifications. The Certifications are included herewith as Exhibits 31.1 and 31.2. Except as described above, no other changes have been made to the Form 10-K. This Amendment No. 1 to the Form 10-K speaks as of the original filing date of the Form 10-K, does not reflect events that may have occurred subsequent to the original filing date, and does not modify or update in any way disclosures made in the original Form 10-K. Accordingly, this Amendment No. 1 should be read in conjunction with the original filing. | ||
| Document Period End Date | Dec. 31, 2020 | ||
| Document Fiscal Year Focus | 2020 | ||
| Document Fiscal Period Focus | FY | ||
| Trading Symbol | SNDX | ||
| Entity Registrant Name | Syndax Pharmaceuticals, Inc. | ||
| Entity Central Index Key | 0001395937 | ||
| Current Fiscal Year End Date | --12-31 | ||
| Entity Filer Category | Non-accelerated Filer | ||
| Entity Small Business | true | ||
| Entity Emerging Growth Company | true | ||
| Entity Ex Transition Period | true | ||
| Title of 12(b) Security | Common Stock | ||
| Entity Common Stock, Shares Outstanding | 48,235,759 | ||
| Entity Current Reporting Status | Yes | ||
| Entity Shell Company | false | ||
| Entity File Number | 001-37708 | ||
| 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 | ||
| Entity Interactive Data Current | Yes | ||
| Entity Incorporation, State or Country Code | DE | ||
| Document Annual Report | true | ||
| Document Transition Report | false | ||
| Security Exchange Name | NASDAQ | ||
| Entity Voluntary Filers | No | ||
| Entity Well-known Seasoned Issuer | Yes | ||
| ICFR Auditor Attestation Flag | false | ||
| Entity Public Float | $ 500.1 | ||
| Documents Incorporated by Reference | DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant’s definitive proxy statement for its 2021 Annual Meeting of Stockholders, which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant’s fiscal year ended December 31, 2020, are incorporated by reference into Part III of this Annual Report on Form 10-K. |
