Nature of Business	12 Months Ended
Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business	1. Nature of Business Syndax Pharmaceuticals, Inc. (the Company) is a clinical stage biopharmaceutical company developing an innovative pipeline of cancer therapies. The Company is developing its lead product candidate, entinostat, a once-weekly, oral, small molecule, Class I HDAC inhibitor, in combination with exemestane and several approved PD-1/PD-L1 antagonists. The Company’s pipeline also includes SNDX-6352, a monoclonal antibody that blocks the colony stimulating factor 1 (CSF-1) receptor, as well as SNDX-5613, a selective inhibitor targeting the binding interaction of Menin with the mixed lineage leukemia (“MLL”) protein. The Company plans to continue to leverage the technical and business expertise of our management team and scientific collaborators to license, acquire and develop additional cancer therapies to expand our pipeline. In March 2016, the Company completed its initial public offering (“IPO”) whereby it sold 4,809,475 shares of common stock at the initial public offering price of $12.00 per share. The aggregate net proceeds received by the Company from the offering were $50.5 million, net of underwriting discounts and commissions and offering expenses. In April 2017, the Company entered into a sales agreement with Cowen and Company, LLC (“Cowen”) under which the Company may issue and sell shares of our common stock having aggregate sales proceeds of up to $50.0 million from time to time through Cowen, acting as agent, in a series of one or more at-the-market (“ATM program”) equity offerings. Cowen is not required to sell any specific amount but acts as our sales agent using commercially reasonable efforts consistent with its normal trading and sales practices. Shares sold pursuant to the sales agreement will be sold pursuant to a shelf registration statement, which became effective on April 20, 2017. Our common stock will be sold at prevailing market prices at the time of the sale; and as a result, prices may vary. We will pay Cowen up to 3% of the gross proceeds from any common stock sold through the sales agreement. Since inception of the ATM program in 2017, though March 6, 2019, the Company has sold 2,403,409 shares of common stock pursuant to the ATM program, at an average price of $7.82 per share for gross proceeds of $18.8 million, resulting in net proceeds of $18.2 million after deducting sales commissions and offering expenses. As of December 31, 2018, $32.1 million of common stock remained available for sale under the ATM program. In 2019, the Company sold 140,819 shares of common stock pursuant to the ATM program, with net proceeds of $0.9 million. As of March 6, 2019, $31.2 million of common stock remained available for sale under the ATM program. In May 2017, the Company completed a follow-on public offering whereby the Company sold 3,950,190 shares of common stock at a price of $13.25 per share. The aggregate net proceeds received by the Company from the offering were approximately $48.7 million, net of underwriting discounts and commissions and estimated offering expenses payable by the Company. On October 13, 2017, the Company entered into a license agreement with Vitae Pharmaceuticals, Inc., a subsidiary of Allergan plc (“Allergan”), under which Allergan granted to the Company a worldwide, sublicenseable, exclusive license to a portfolio of preclinical, orally-available, small molecule inhibitors of the interaction of Menin with the MLL protein (the “Menin Assets”). The Company made a nonrefundable upfront payment of $5.0 million to Allergan in the fourth quarter of 2017. The Company is developing the Menin Assets to potentially treat two genetically defined acute leukemias: (i) a genetically-defined subset of acute leukemias with chromosomal rearrangements in the MLL gene (“MLL-r”) and (ii) acute myeloid leukemia (“AML”) with a mutated nucleophosmin 1 (“NPM1”). On October 17, 2017, the Company entered into a purchase agreement with Biotech Value Fund, L.P. (“BVF”) and certain entities affiliated with BVF (the “Purchase Agreement”). Pursuant to the Purchase Agreement, the Company issued directly to BVF in a registered direct offering (the “Offering”), 2,021,018 shares of the Company’s common stock at a price of $12.37 per share, representing the closing price of the Company’s shares on the Nasdaq Global Select Market on Friday, October 13, 2017. The net proceeds from the Offering, after deducting estimated expenses, were $24.9 million. Since its inception, the Company has devoted its efforts principally to research and development and raising capital. The Company is subject to risks common to companies in the development stage, including, but not limited to, successful development of therapeutics, obtaining additional funding, protection of proprietary therapeutics, compliance with government regulations, fluctuations in operating results, dependence on key personnel and collaborative partners, and risks associated with industry changes. The Company’s long-term success is dependent upon its ability to successfully develop and market its product candidates, expand its oncology drug pipeline, earn revenue, obtain additional capital when needed, and ultimately, achieve profitable operations. The Company anticipates that it will be several years before any of its product candidates is approved, if ever, and the Company begins to generate revenue from sales of such product candidates. Accordingly, management expects to incur substantial losses on the ongoing development of its product candidates and does not expect to achieve positive cash flow from operations for the foreseeable future, if ever. As a result, the Company will continue to require additional capital to move forward with its business plan. While certain amounts of this additional capital were raised in the past, there can be no assurance that funds necessary beyond these amounts will be available in amounts or on terms sufficient to ensure ongoing operations. The Company’s management believes that the cash, cash equivalents and short-term investments balances as of December 31, 2018 should enable the Company to maintain its planned operations for at least twelve months from the date these financial statements were issued. The Company’s ability to fund all of its planned operations internally beyond that date, including the completion of its ongoing and planned clinical trial activities, may be substantially dependent upon whether the Company can obtain sufficient funding on terms acceptable to the Company. Proceeds from additional capital transactions would allow the Company to accelerate and/or expand its planned research and development activities. In the event that sufficient funds were not available, the Company may be required to delay or reduce expenditures to conserve cash, which could involve scaling back or curtailing development and general and administrative activities.

Nature of Business

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

1. Nature of Business

Syndax Pharmaceuticals, Inc. (the Company) is a clinical stage biopharmaceutical company developing an innovative pipeline of cancer therapies. The Company is developing its lead product candidate, entinostat, a once-weekly, oral, small molecule, Class I HDAC inhibitor, in combination with exemestane and several approved PD-1/PD-L1 antagonists. The Company’s pipeline also includes SNDX-6352, a monoclonal antibody that blocks the colony stimulating factor 1 (CSF-1) receptor, as well as SNDX-5613, a selective inhibitor targeting the binding interaction of Menin with the mixed lineage leukemia (“MLL”) protein. The Company plans to continue to leverage the technical and business expertise of our management team and scientific collaborators to license, acquire and develop additional cancer therapies to expand our pipeline.

In March 2016, the Company completed its initial public offering (“IPO”) whereby it sold 4,809,475 shares of common stock at the initial public offering price of $12.00 per share. The aggregate net proceeds received by the Company from the offering were $50.5 million, net of underwriting discounts and commissions and offering expenses.

In April 2017, the Company entered into a sales agreement with Cowen and Company, LLC (“Cowen”) under which the Company may issue and sell shares of our common stock having aggregate sales proceeds of up to $50.0 million from time to time through Cowen, acting as agent, in a series of one or more at-the-market (“ATM program”) equity offerings. Cowen is not required to sell any specific amount but acts as our sales agent using commercially reasonable efforts consistent with its normal trading and sales practices. Shares sold pursuant to the sales agreement will be sold pursuant to a shelf registration statement, which became effective on April 20, 2017. Our common stock will be sold at prevailing market prices at the time of the sale; and as a result, prices may vary. We will pay Cowen up to 3% of the gross proceeds from any common stock sold through the sales agreement. Since inception of the ATM program in 2017, though March 6, 2019, the Company has sold 2,403,409 shares of common stock pursuant to the ATM program, at an average price of $7.82 per share for gross proceeds of $18.8 million, resulting in net proceeds of $18.2 million after deducting sales commissions and offering expenses. As of December 31, 2018, $32.1 million of common stock remained available for sale under the ATM program. In 2019, the Company sold 140,819 shares of common stock pursuant to the ATM program, with net proceeds of $0.9 million. As of March 6, 2019, $31.2 million of common stock remained available for sale under the ATM program.

In May 2017, the Company completed a follow-on public offering whereby the Company sold 3,950,190 shares of common stock at a price of $13.25 per share. The aggregate net proceeds received by the Company from the offering were approximately $48.7 million, net of underwriting discounts and commissions and estimated offering expenses payable by the Company.

On October 13, 2017, the Company entered into a license agreement with Vitae Pharmaceuticals, Inc., a subsidiary of Allergan plc (“Allergan”), under which Allergan granted to the Company a worldwide, sublicenseable, exclusive license to a portfolio of preclinical, orally-available, small molecule inhibitors of the interaction of Menin with the MLL protein (the “Menin Assets”). The Company made a nonrefundable upfront payment of $5.0 million to Allergan in the fourth quarter of 2017. The Company is developing the Menin Assets to potentially treat two genetically defined acute leukemias: (i) a genetically-defined subset of acute leukemias with chromosomal rearrangements in the MLL gene (“MLL-r”) and (ii) acute myeloid leukemia (“AML”) with a mutated nucleophosmin 1 (“NPM1”).

On October 17, 2017, the Company entered into a purchase agreement with Biotech Value Fund, L.P. (“BVF”) and certain entities affiliated with BVF (the “Purchase Agreement”). Pursuant to the Purchase Agreement, the Company issued directly to BVF in a registered direct offering (the “Offering”), 2,021,018 shares of the Company’s common stock at a price of $12.37 per share, representing the closing price of the Company’s shares on the Nasdaq Global Select Market on Friday, October 13, 2017. The net proceeds from the Offering, after deducting estimated expenses, were $24.9 million.

Since its inception, the Company has devoted its efforts principally to research and development and raising capital. The Company is subject to risks common to companies in the development stage, including, but not limited to, successful development of therapeutics, obtaining additional funding, protection of proprietary therapeutics, compliance with government regulations, fluctuations in operating results, dependence on key personnel and collaborative partners, and risks associated with industry changes. The Company’s long-term success is dependent upon its ability to successfully develop and market its product candidates, expand its oncology drug pipeline, earn revenue, obtain additional capital when needed, and ultimately, achieve profitable operations. The Company anticipates that it will be several years before any of its product candidates is approved, if ever, and the Company begins to generate revenue from sales of such product candidates. Accordingly, management expects to incur substantial losses on the ongoing development of its product candidates and does not expect to achieve positive cash flow from operations for the foreseeable future, if ever. As a result, the Company will continue to require additional capital to move forward with its business plan. While certain amounts of this additional capital were raised in the past, there can be no assurance that funds necessary beyond these amounts will be available in amounts or on terms sufficient to ensure ongoing operations.

The Company’s management believes that the cash, cash equivalents and short-term investments balances as of December 31, 2018 should enable the Company to maintain its planned operations for at least twelve months from the date these financial statements were issued. The Company’s ability to fund all of its planned operations internally beyond that date, including the completion of its ongoing and planned clinical trial activities, may be substantially dependent upon whether the Company can obtain sufficient funding on terms acceptable to the Company. Proceeds from additional capital transactions would allow the Company to accelerate and/or expand its planned research and development activities. In the event that sufficient funds were not available, the Company may be required to delay or reduce expenditures to conserve cash, which could involve scaling back or curtailing development and general and administrative activities.