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Long-Term Debt - Silicon Valley Bank Loan Agreement - Additional Information (Details) - USD ($) |  |  | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | ---
Mar. 04, 2019 | May 07, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Current portion of long-term debt |  |  | $ 3,981,000 |  | $ 3,981,000 |  | $ 789,000 | [1]
Long-term debt |  |  | 28,698,000 |  | $ 28,698,000 |  | 21,690,000 | [1]
Loan and Security Agreement [Member] | Silicon Valley Bank [Member] | Warrants for Common Stock [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Aggregate number of shares of common stock called by warrants | 4,845 | 6,332 |  |  |  |  |  |
Exercise price of per share | $ 14.71 | $ 23.69 |  |  |  |  |  |
Fair value of warrant | $ 100,000 | $ 100,000 |  |  |  |  |  |
Exercisable period of warrants |  |  |  |  | 10 years |  |  |
Loan and Security Agreement [Member] | Term Loan [Member] | Silicon Valley Bank [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Maximum borrowing capacity under loan agreement |  | 20,000,000 |  |  |  |  |  |
Increase borrowing capacity under loan agreement |  | $ 40,000,000 |  |  |  |  |  |
End of the draw period of the term loan |  | Mar. 31, 2019 |  |  |  |  |  |
Variable rate basis |  | The interest rate will be calculated at a rate equal to the greater of (i) 4.75%, and (ii) 0.25% plus the prime rate as reported from time to time in The Wall Street Journal. |  |  |  |  |  |
Debt instrument, frequency of periodic payment |  | The interest-only period will be followed by equal monthly payments of principal and interest over 24 months |  |  |  |  |  |
Debt instrument, maturity date |  | Each Term Loan Advance will mature at the earlier of (i) the 23 months following the applicable term loan amortization date for each such Term Loan Advance (ii) March 1, 2023, or (iii) 30 days prior to the earliest maturity of any portion of the Company’s loan with Novartis (the “Loan Maturity Date”). After repayment, no Term Loan Advance (or any portion thereof) may be reborrowed. |  |  |  |  |  |
Period of monthly payments of principal and interest |  | 24 months |  |  |  |  |  |
Repayment of term loan |  | $ 0 |  |  |  |  |  |
Loan borrowed |  | $ 0 |  |  |  |  |  |
Percentage of final payment fee of Loan |  | 8.50% |  |  |  |  |  |
Percentage of prepayment fee |  | 3.00% |  |  |  |  |  |
Additional interest rate of outstanding payments due |  | 4.00% |  |  |  |  |  |
Debt issuance costs |  | $ 200,000 |  |  |  |  |  |
Extended end of the draw period of the term loan | Mar. 31, 2020 |  |  |  |  |  |  |
Loan and Security Agreement [Member] | Term Loan [Member] | Silicon Valley Bank [Member] | Agenus [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Carrying value of debt |  |  | 17,100,000 |  | $ 17,100,000 |  | 7,300,000 |
Current portion of long-term debt |  |  | 4,000,000 |  | 4,000,000 |  | 800,000 |
Long-term debt |  |  | 13,100,000 |  | 13,100,000 |  | $ 6,500,000 |
Loan and Security Agreement [Member] | Term Loan [Member] | Silicon Valley Bank [Member] | First Anniversary [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Percentage of prepayment fee |  | 2.00% |  |  |  |  |  |
Loan and Security Agreement [Member] | Term Loan [Member] | Silicon Valley Bank [Member] | Second Anniversary [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Percentage of prepayment fee |  | 1.00% |  |  |  |  |  |
Loan and Security Agreement [Member] | Term Loan [Member] | Silicon Valley Bank [Member] | Minimum [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Minimum effective interest rate on debt |  | 4.75% |  |  |  |  |  |
Loan and Security Agreement [Member] | Term Loan [Member] | Prime Rate [Member] | Silicon Valley Bank [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate |  | 0.25% |  |  |  |  |  |
Loan and Security Agreement [Member] | Novartis [Member] | Silicon Valley Bank [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Maturity date |  | Mar. 01, 2023 |  |  |  |  |  |
Days of interest, period |  | 30 days |  |  |  |  |  |
Earliest maturity of loan |  | 30 days |  |  |  |  |  |
Loan and Security Agreement [Member] | First Term Loan [Member] | Silicon Valley Bank [Member] | Agenus [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Loan borrowed |  |  |  | $ 7,500,000 |  | $ 7,500,000 |  |
Discount against debt |  |  |  | 300,000 |  | 300,000 |  |
Non-cash interest expense resulting from amortization of discount and accretion of final payment |  |  | 200,000 | $ 11,000 | 400,000 | $ 11,000 |  |
Loan and Security Agreement [Member] | Second Term Loan [Member] | Silicon Valley Bank [Member] | Aronora [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Loan borrowed |  |  | 9,500,000 |  | 9,500,000 |  |  |
Discount against debt |  |  | $ 45,000 |  | $ 45,000 |  |  |
[1] | The consolidated balance sheet as of December 31, 2018 has been derived from the audited consolidated financial statements as of that date included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018.

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