Quarterly report pursuant to Section 13 or 15(d)

Long-Term Debt - Silicon Valley Bank Loan Agreement - Additional Information (Details)

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Long-Term Debt - Silicon Valley Bank Loan Agreement - Additional Information (Details) - USD ($)
 3 Months Ended
May 07, 2018
Sep. 30, 2018
Dec. 31, 2017
 [1]
Debt Instrument [Line Items]      
Long-term debt   $ 22,034,000 $ 14,572,000
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 6,332    
Exercise price of per share $ 23.69    
Exercisable period of warrants 10 years    
Fair value of warrant $ 100,000    
Debt issuance costs 200,000    
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    
Earlier effecitve date of term loan Mar. 31, 2019    
Effective date of term loan extended Mar. 31, 2020    
Gross cash proceeds received from milestone and licensing payments $ 20,000,000    
Basis spread on variable rate 4.75%    
Variable rate basis The interest-only period will be followed by equal monthly payments of principal and interest over 24 months. 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%    
Percentage of unused portion of term loan 1.00%    
Loan agreement description (i) the termination of the Loan Agreement, or (ii) the Draw Period if the aggregate original principal amount of the Term Loan Advances is less than $5.0 million.    
Unamortized amount of deferred charges   300,000  
Non-cash interest expense resulting from amortization of discount and accretion of final payment   11,000  
Loan and Security Agreement [Member] | Term Loan [Member] | Silicon Valley Bank [Member] | Agenus [Member]      
Debt Instrument [Line Items]      
Loan borrowed   7,500,000  
Long-term debt   7,200,000  
Discount included in long-term debt   $ 300,000  
Loan and Security Agreement [Member] | Term Loan [Member] | Silicon Valley Bank [Member] | Maximum [Member]      
Debt Instrument [Line Items]      
Aggregate original principal amount $ 5,000,000    
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] | 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    
[1] The condensed consolidated balance sheet as of December 31, 2017 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, 2017.