Stockholders' equity	3 Months Ended
Mar. 31, 2019
Equity [Abstract]
Stockholders' equity	Stockholders' equity Agreement with Lincoln Park Capital In October 2019, the Company entered into a purchase agreement with Lincoln Park Capital (“LPC”) pursuant to which LPC purchased $2.5 million of shares of the Company's common stock at a purchase price of $2.587 per share. In addition, for a period of thirty months, the Company has the right, at its sole discretion, to sell up to an additional $27.5 million of the Company's common stock based on prevailing market prices of its common stock at the time of each sale. In consideration for entering into the purchase agreement, the Company issued 289,966 shares of its common stock to LPC as a commitment fee. The purchase agreement limits the Company's sales of shares of common stock to LPC to 5,227,323 shares of common stock, representing 19.99% of the shares of common stock outstanding on the date of the purchase agreement. The purchase agreement also prohibits the Company from directing LPC to purchase any shares of common stock if those shares, when aggregated with all other shares of the Company's common stock then beneficially owned by LPC and its affiliates, would result in LPC and its affiliates having beneficial ownership, at any single point in time, of more than 9.99% of the then total outstanding shares of the Company's common stock. In January 2020, the Company sold 186,986 shares of common stock to LPC, and received net proceeds of approximately $0.4 million. 2019 Public Offering In June 2019, the Company entered into an underwriting agreement relating to the public offering of 10,500,000 shares of the Company’s common stock, at a price of $3.50 per share, for gross proceeds of approximately $36.8 million (the “2019 Public Offering”). The Company also granted the underwriters an option to purchase up to an additional 1,575,000 shares of common stock (“Overallotment Option”). On June 26, 2019, the underwriters exercised this option in full. The Company received approximately $5.5 million in gross proceeds from the underwriter’s exercise of the Overallotment Option. In connection with the 2019 Public Offering, inclusive of the Overallotment Option, the Company received net proceeds of $38.4 million. Private Placement In February 2019, the Company completed a private placement financing transaction (the “Private Placement”). The Company issued 3,199,998 shares of common stock, prefunded warrants (the “Pre-Funded Warrants”) to purchase 531,250 shares of common stock (the “Pre-Funded Warrant Shares”), and warrants (the “Private Placement Warrants”) to purchase up to 932,812 shares of common stock (the “Warrant Shares”). The shares, Pre-Funded Warrants and Private Placement Warrants (collectively, the “Units”) were sold at a purchase price of $4.02 per Unit. The Company received net cash proceeds of approximately $13.8 million for the purchase of the shares, Pre-Funded Warrant Shares and Warrant Shares. See Note 8. Warrants. The Company had the option to issue additional shares of common stock in a second closing (the “Second Closing”) for gross proceeds of up to $24.2 million. The occurrence of the Second Closing was conditioned on top-line results from Part A of the Company's Phase 1/2a clinical trial for GEN-009 and a decision by the Company's board of directors to proceed with the Second Closing. In June 2019, the Company announced top-line results from this trial but elected not to proceed with the Second Closing. In lieu of the Second Closing, the Company proceeded with the 2019 Public Offering. At-the-market equity offering program In 2015, the Company entered into an agreement, as amended, with Cowen and Company, LLC to establish an at-the-market equity offering program (“ATM”) pursuant to which it was able to offer and sell shares of its common stock at prevailing market prices from time to time. Through March 31, 2020, the Company has sold an aggregate of 463,887 shares under the ATM and received approximately $4.0 million in net proceeds.

Stockholders' equity

Agreement with Lincoln Park Capital

In October 2019, the Company entered into a purchase agreement with Lincoln Park Capital (“LPC”) pursuant to which LPC purchased $2.5 million of shares of the Company's common stock at a purchase price of $2.587 per share. In addition, for a period of thirty months, the Company has the right, at its sole discretion, to sell up to an additional $27.5 million of the Company's common stock based on prevailing market prices of its common stock at the time of each sale. In consideration for entering into the purchase agreement, the Company issued 289,966 shares of its common stock to LPC as a commitment fee. The purchase agreement limits the Company's sales of shares of common stock to LPC to 5,227,323 shares of common stock, representing 19.99% of the shares of common stock outstanding on the date of the purchase agreement. The purchase agreement also prohibits the Company from directing LPC to purchase any shares of common stock if those shares, when aggregated with all other shares of the Company's common stock then beneficially owned by LPC and its affiliates, would result in LPC and its affiliates having beneficial ownership, at any single point in time, of more than 9.99% of the then total outstanding shares of the Company's common stock.

In January 2020, the Company sold 186,986 shares of common stock to LPC, and received net proceeds of approximately $0.4 million.

2019 Public Offering

In June 2019, the Company entered into an underwriting agreement relating to the public offering of 10,500,000 shares of the Company’s common stock, at a price of $3.50 per share, for gross proceeds of approximately $36.8 million (the “2019 Public Offering”). The Company also granted the underwriters an option to purchase up to an additional 1,575,000 shares of common stock (“Overallotment Option”). On June 26, 2019, the underwriters exercised this option in full. The Company received approximately $5.5 million in gross proceeds from the underwriter’s exercise of the Overallotment Option. In connection with the 2019 Public Offering, inclusive of the Overallotment Option, the Company received net proceeds of $38.4 million.

Private Placement

In February 2019, the Company completed a private placement financing transaction (the “Private Placement”). The Company issued 3,199,998 shares of common stock, prefunded warrants (the “Pre-Funded Warrants”) to purchase 531,250 shares of common stock (the “Pre-Funded Warrant Shares”), and warrants (the “Private Placement Warrants”) to purchase up to 932,812 shares of common stock (the “Warrant Shares”). The shares, Pre-Funded Warrants and Private Placement Warrants (collectively, the “Units”) were sold at a purchase price of $4.02 per Unit. The Company received net cash proceeds of approximately $13.8 million for the purchase of the shares, Pre-Funded Warrant Shares and Warrant Shares. See Note 8. Warrants.

The Company had the option to issue additional shares of common stock in a second closing (the “Second Closing”) for gross proceeds of up to $24.2 million. The occurrence of the Second Closing was conditioned on top-line results from Part A of the Company's Phase 1/2a clinical trial for GEN-009 and a decision by the Company's board of directors to proceed with the Second Closing. In June 2019, the Company announced top-line results from this trial but elected not to proceed with the Second Closing. In lieu of the Second Closing, the Company proceeded with the 2019 Public Offering.

At-the-market equity offering program

In 2015, the Company entered into an agreement, as amended, with Cowen and Company, LLC to establish an at-the-market equity offering program (“ATM”) pursuant to which it was able to offer and sell shares of its common stock at prevailing market prices from time to time. Through March 31, 2020, the Company has sold an aggregate of 463,887 shares under the ATM and received approximately $4.0 million in net proceeds.