Subsequent Events	12 Months Ended
Dec. 31, 2021
Subsequent Events [Abstract]
Subsequent Events	18. Subsequent Events Second Amendment to Loan and Security Agreement with Hercules Effective as of February 24, 2022 (the “Effective Date”), the Company entered into a Second Amendment to Loan and Security Agreement (the “Second Amendment”), with the lenders party thereto (the “Lenders”), and Hercules Capital, Inc., in its capacity as the administrative agent and the collateral agent for the Lenders, which amended the Original Credit Facility. Under the Original Credit Facility, term loans in an aggregate principal amount of up to $50,000 were available to the Company, of which $25,000 (the “first tranche”) has been advanced to the Company and approximately $22,100 were outstanding immediately prior to the Effective Date. Pursuant to the Second Amendment, term loans in an aggregate principal amount of up to $100,000 (the “New Credit Facility”) have become available to the Company in five tranches including the first tranche, subject to certain terms and conditions. The first tranche in an aggregate principal amount of $25,000 is outstanding as of the Effective Date, after taking into account reborrowing by the Company on the Effective Date of a previously-repaid principal amount of approximately $2,900. The second tranche in an aggregate principal amount of $12,500 and the third tranche in an aggregate principal amount of $12,500 have been advanced to the Company and are outstanding as of the Effective Date. The fourth tranche in an aggregate principal amount of $25,000 is available upon satisfaction of certain conditions, including the approval by the U.S. Food and Drug Administration of a biologics license application in respect of SER-109 (the "Regulatory Approval Milestone") by no later than December 15, 2023. The fifth tranche in an aggregate principal amount of up to $25,000 is available through the Amortization Date (as defined below) upon satisfaction of certain conditions, including the Lenders’ investment committee approval. All advances outstanding under the New Credit Facility will bear interest at a rate equal to the greater of either (i) the Prime Rate (as reported in The Wall Street Journal) plus 6.40%, and (ii) 9.65%. For all advances outstanding under the New Credit Facility, the Company will make interest only payments through December 31, 2023, extendable to December 31, 2024 upon satisfaction of certain conditions (such applicable date, the “Amortization Date”). The principal balance and interest of the advances will be repaid in equal monthly installments after the Amortization Date and continuing through October 1, 2024, extendable to October 1, 2025, upon satisfaction of certain conditions (such applicable date, the “Maturity Date”). The Company may prepay advances under the New Credit Facility, in whole or in part, at any time subject to a prepayment charge equal to: (a) 2.0% of amounts so prepaid, if such prepayment occurs during the first year following the Effective Date; (b) 1.5% of the amount so prepaid, if such prepayment occurs during the second year following the Effective Date, and (c) 1.0% of the amount so prepaid, if such prepayment occurs during the third year following the Effective Date. The Company will pay an end of term charge of 4.85% of the aggregate amount of the advances made under the Old Credit Facility on the earliest date of (i) November 1, 2023; (ii) the date that the Company prepays all of the outstanding principal in full, or (iii) the date the loan payments are accelerated due to an event of default. The Company will pay an additional end of term charge of 1.75% of the aggregate amount of the advances under the New Credit Facility (including the first tranche of $25,000) on the earliest date of (i) the Maturity Date; (ii) the date that the Company prepays all of the outstanding principal in full, or (iii) the date the loan payments are accelerated due to an event of default. Other terms of the New Credit Facility remain generally identical to those under the Old Credit Facility, with certain covenants amended by the Second Amendment to provide the Company with additional operational flexibility, including the ability for the Company to issue up to $350.0 million in convertible notes. The New Credit Facility includes a conditional liquidity covenant commencing on June 15, 2023, which ceases to apply if certain conditions are satisfied.

Subsequent Events

12 Months Ended

Dec. 31, 2021

18.

Subsequent Events

Second Amendment to Loan and Security Agreement with Hercules

Effective as of February 24, 2022 (the “Effective Date”), the Company entered into a Second Amendment to Loan and Security Agreement (the “Second Amendment”), with the lenders party thereto (the “Lenders”), and Hercules Capital, Inc., in its capacity as the administrative agent and the collateral agent for the Lenders, which amended the Original Credit Facility. Under the Original Credit Facility, term loans in an aggregate principal amount of up to $50,000 were available to the Company, of which $25,000 (the “first tranche”) has been advanced to the Company and approximately $22,100 were outstanding immediately prior to the Effective Date. Pursuant to the Second Amendment, term loans in an aggregate principal amount of up to $100,000 (the “New Credit Facility”) have become available to the Company in five tranches including the first tranche, subject to certain terms and conditions.

The first tranche in an aggregate principal amount of $25,000 is outstanding as of the Effective Date, after taking into account reborrowing by the Company on the Effective Date of a previously-repaid principal amount of approximately $2,900. The second tranche in an aggregate principal amount of $12,500 and the third tranche in an aggregate principal amount of $12,500 have been advanced to the Company and are outstanding as of the Effective Date. The fourth tranche in an aggregate principal amount of $25,000 is available upon satisfaction of certain conditions, including the approval by the U.S. Food and Drug Administration of a biologics license application in respect of SER-109 (the "Regulatory Approval Milestone") by no later than December 15, 2023. The fifth tranche in an aggregate principal amount of up to $25,000 is available through the Amortization Date (as defined below) upon satisfaction of certain conditions, including the Lenders’ investment committee approval.

All advances outstanding under the New Credit Facility will bear interest at a rate equal to the greater of either (i) the Prime Rate (as reported in The Wall Street Journal) plus 6.40%, and (ii) 9.65%. For all advances outstanding under the New Credit Facility, the Company will make interest only payments through December 31, 2023, extendable to December 31, 2024 upon satisfaction of certain conditions (such applicable date, the “Amortization Date”). The principal balance and interest of the advances will be repaid in equal monthly installments after the Amortization Date and continuing through October 1, 2024, extendable to October 1, 2025, upon satisfaction of certain conditions (such applicable date, the “Maturity Date”).

The Company may prepay advances under the New Credit Facility, in whole or in part, at any time subject to a prepayment charge equal to: (a) 2.0% of amounts so prepaid, if such prepayment occurs during the first year following the Effective Date; (b) 1.5% of the amount so prepaid, if such prepayment occurs during the second year following the Effective Date, and (c) 1.0% of the amount so prepaid, if such prepayment occurs during the third year following the Effective Date.

The Company will pay an end of term charge of 4.85% of the aggregate amount of the advances made under the Old Credit Facility on the earliest date of (i) November 1, 2023; (ii) the date that the Company prepays all of the outstanding principal in full, or (iii) the date the loan payments are accelerated due to an event of default. The Company will pay an additional end of term charge of 1.75% of the aggregate amount of the advances under the New Credit Facility (including the first tranche of $25,000) on the earliest date of (i) the Maturity Date; (ii) the date that the Company prepays all of the outstanding principal in full, or (iii) the date the loan payments are accelerated due to an event of default.

Other terms of the New Credit Facility remain generally identical to those under the Old Credit Facility, with certain covenants amended by the Second Amendment to provide the Company with additional operational flexibility, including the ability for the Company to issue up to $350.0 million in convertible notes. The New Credit Facility includes a conditional liquidity covenant commencing on June 15, 2023, which ceases to apply if certain conditions are satisfied.