Nature of Business	12 Months Ended
Dec. 31, 2017
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 a portfolio of potent and selective inhibitors targeting the binding interaction of Menin with MLLr. 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, which included 409,475 shares issued pursuant to the underwriters’ partial exercise of their over-allotment option to purchase additional shares of common stock. The aggregate net proceeds received by the Company from the offering were $50.5 million, net of underwriting discounts and commissions of $4.0 million and offering expenses of $3.1 million. Upon the closing of the IPO, all outstanding shares of the Company’s outstanding convertible preferred stock converted into 12,872,551 shares of common stock; and the Company’s outstanding warrant liability to purchase 357,840 shares of the Company’s common stock valued at $4.6 million was reclassified to additional paid-in capital. The shares trade on the Nasdaq Global Select Market under the symbol “SNDX.” In connection with the closing of the Company’s IPO, the Company filed an amended and restated certificate of incorporation and adopted amended and restated bylaws, both of which were approved by the Company’s board of directors and stockholders on September 28, 2015 and February 24, 2016, respectively. Pursuant to the amended and restated certificate of incorporation, the Company is now authorized to issue 100,000,000 shares of common stock and 10,000,000 shares of preferred stock. 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”) 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. In 2017, the Company sold 148,421 shares of Common Stock pursuant to the ATM program, at an average price of $12.45 per share for gross proceeds of $1.8 million, resulting in net proceeds of $1.7 million after deducting sales commissions and offering expenses. As of December 31, 2017, $48.2 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, which included 200,190 shares issued pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock. 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 Mixed Lineage Leukemia (“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 MLL-r driven malignancies and believes that the Menin Assets have the potential to be used to treat a genetically-defined subset of acute leukemias with chromosomal rearrangements in the MLL gene (“MLL-r”). The Company is in the process of performing a number of pre-clinical studies related to the Menin Assets. 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, short-term and long-term investments balances as of December 31, 2017 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.

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 a portfolio of potent and selective inhibitors targeting the binding interaction of Menin with MLLr. 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, which included 409,475 shares issued pursuant to the underwriters’ partial exercise of their over-allotment option to purchase additional shares of common stock. The aggregate net proceeds received by the Company from the offering were $50.5 million, net of underwriting discounts and commissions of $4.0 million and offering expenses of $3.1 million. Upon the closing of the IPO, all outstanding shares of the Company’s outstanding convertible preferred stock converted into 12,872,551 shares of common stock; and the Company’s outstanding warrant liability to purchase 357,840 shares of the Company’s common stock valued at $4.6 million was reclassified to additional paid-in capital. The shares trade on the Nasdaq Global Select Market under the symbol “SNDX.” In connection with the closing of the Company’s IPO, the Company filed an amended and restated certificate of incorporation and adopted amended and restated bylaws, both of which were approved by the Company’s board of directors and stockholders on September 28, 2015 and February 24, 2016, respectively. Pursuant to the amended and restated certificate of incorporation, the Company is now authorized to issue 100,000,000 shares of common stock and 10,000,000 shares of preferred stock.

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”) 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. In 2017, the Company sold 148,421 shares of Common Stock pursuant to the ATM program, at an average price of $12.45 per share for gross proceeds of $1.8 million, resulting in net proceeds of $1.7 million after deducting sales commissions and offering expenses. As of December 31, 2017, $48.2 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, which included 200,190 shares issued pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock. 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 Mixed Lineage Leukemia (“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 MLL-r driven malignancies and believes that the Menin Assets have the potential to be used to treat a genetically-defined subset of acute leukemias with chromosomal rearrangements in the MLL gene (“MLL-r”). The Company is in the process of performing a number of pre-clinical studies related to the Menin Assets.

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, short-term and long-term investments balances as of December 31, 2017 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.