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Long Term Debt - Hercules Loan Agreement - Additional Information (Detail) $ / shares in Units, $ in Thousands |  |  |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |
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Mar. 31, 2019 USD ($) shares | Mar. 18, 2019 $ / shares shares | Dec. 31, 2018 USD ($) $ / shares shares | Oct. 31, 2018 USD ($) $ / shares shares | Oct. 31, 2018 USD ($) $ / shares shares | Mar. 31, 2019 USD ($) shares | Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) $ / shares shares | Mar. 13, 2019 USD ($) | Dec. 11, 2018 USD ($) | Oct. 19, 2018 USD ($)
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Fair value of warrants | $ 0 |  |  |  |  | $ 0 |  |  | $ 5,235 |  |
Upfront payments |  |  |  | $ 92 |  |  |  |  |  |  |
Non-cash interest expense |  |  |  |  |  | 153 | $ 27 |  |  |  |
Unamortized debt discount | $ 1,019 |  | $ 226 |  |  | $ 1,019 |  | $ 226 |  |  |
Arsanis [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Warrant issued to purchase stock | shares | 271,230,000 |  |  |  |  | 271,230,000 |  |  |  |  |
Business acquisition share exchange ratio |  |  |  |  |  | 0.5702 |  |  |  |  |
Hercules Loan Agreement [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  | 13,000 | $ 13,000 |  |  |  |  |  |
Proceeds from lines of credit |  |  |  | 8,000 |  |  |  |  |  |  |
Interest rate increase percentage |  |  |  |  |  | 4.00% |  |  |  |  |
Debt instrument interest rate | 9.80% |  | 9.75% |  |  | 9.80% |  | 9.75% |  |  |
Line of credit interest rate, Description |  |  |  |  |  | Borrowings under the Hercules Loan Agreement bear interest at variable rates, with the first tranche bearing interest at a variable rate equal to the greater of (i) 9.5% or (ii) 9.5% plus The Wall Street Journal prime rate minus 5.25%, the second tranche bearing interest at a variable rate, subject to completion of specified financing conditions, equal to either (A) the greater of (i) 9.5% or (ii) 9.5% plus The Wall Street Journal prime rate minus 5.25% or (B) the greater of (i) 8.75% or (ii) 8.75% plus The Wall Street Journal prime rate minus 5.25%, and the third tranche bearing interest at a variable rate equal to the greater of (i) 8.75% or (ii) 8.75% plus The Wall Street Journal prime rate minus 5.25%. In an event of default, as defined, and until such event is no longer continuing, the interest rate applicable to borrowings under the Hercules Loan Agreement would be increased by 4.0%. As of March 31, 2019 and December 31, 2018, the interest rate applicable to borrowings under the Hercules Loan Agreement was 9.80% and 9.75%. |  |  |  |  |
Repayments of lines of credit |  |  |  | 953 |  |  |  |  |  |  |
Line of credit facility, periodic payment, interest |  |  |  | 794 |  |  |  |  |  |  |
Fair value of derivative liability |  |  |  | $ 18 | $ 18 |  |  |  |  |  |
Minimum liquidity as a percentage of debt |  |  |  | 125.00% | 125.00% |  |  |  |  |  |
Minimum liquidity as a percentage of debt |  |  |  | 100.00% | 100.00% |  |  |  |  |  |
Fair value of warrants |  |  | $ 282 |  |  |  |  | $ 282 | $ 326 | $ 132 | $ 132
Interest expense recognized |  |  |  |  |  | $ 357 |  | 0 |  |  |
Non-cash interest expense |  |  |  |  |  | 113 |  |  |  |  |
Unamortized debt discount | $ 182 |  | $ 226 |  |  | $ 182 |  | $ 226 |  |  |
Effective interest rate of loan | 14.20% |  | 14.20% |  |  | 14.20% |  | 14.20% |  |  |
Principal payments |  |  |  |  |  | $ 0 |  |  |  |  |
Hercules Loan Agreement [Member] | Series B Redeemable Convertible Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Warrant issued to purchase stock | shares |  |  |  | 210,638 | 210,638 |  |  |  |  |  |
Warrant exercise price | $ / shares |  |  |  | $ 1.88 | $ 1.88 |  |  |  |  |  |
Class of warrant expiration period |  |  |  |  | 2028-10 |  |  |  |  |  |
Hercules Loan Agreement [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument pre payment premium percentage |  |  |  | 2.00% | 2.00% |  |  |  |  |  |
Hercules Loan Agreement [Member] | Arsanis [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Warrant exercise price | $ / shares |  | $ 19.80 | $ 99 |  |  |  |  | $ 99 |  |  |
Number of warrant | shares |  | 5,000 | 52,659 |  |  |  |  | 52,659 |  |  |
Business acquisition share exchange ratio |  | 0.5702 |  |  |  |  |  |  |  |  |
Reverse stock split, conversion ratio |  | 0.166 |  |  |  |  |  |  |  |  |
Debt instrument redemption description |  | warrants was determined to be 52,659 shares, calculated by dividing $99 by the $1.88 price per share |  |  |  |  |  |  |  |  |
Hercules Loan Agreement [Member] | First Tranche [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument variable percentage |  |  |  |  |  | 9.50% |  |  |  |  |
Hercules Loan Agreement [Member] | First Tranche [Member] | Prime Rate [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit facility interest rate |  |  |  |  |  | 5.25% |  |  |  |  |
Hercules Loan Agreement [Member] | First Tranche [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument variable percentage |  |  |  |  |  | 9.50% |  |  |  |  |
Hercules Loan Agreement [Member] | Second Tranche A [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument variable percentage |  |  |  |  |  | 9.50% |  |  |  |  |
Hercules Loan Agreement [Member] | Second Tranche A [Member] | Prime Rate [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit facility interest rate |  |  |  |  |  | 5.25% |  |  |  |  |
Hercules Loan Agreement [Member] | Second Tranche A [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument variable percentage |  |  |  |  |  | 9.50% |  |  |  |  |
Hercules Loan Agreement [Member] | Second Tranche B [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument variable percentage |  |  |  |  |  | 8.75% |  |  |  |  |
Hercules Loan Agreement [Member] | Second Tranche B [Member] | Prime Rate [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit facility interest rate |  |  |  |  |  | 5.25% |  |  |  |  |
Hercules Loan Agreement [Member] | Second Tranche B [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument variable percentage |  |  |  |  |  | 8.75% |  |  |  |  |
Hercules Loan Agreement [Member] | Third Tranche [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument variable percentage |  |  |  |  |  | 8.75% |  |  |  |  |
Hercules Loan Agreement [Member] | Third Tranche [Member] | Prime Rate [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit facility interest rate |  |  |  |  |  | 5.25% |  |  |  |  |
Hercules Loan Agreement [Member] | Third Tranche [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument variable percentage |  |  |  |  |  | 8.75% |  |  |  |  |
Hercules Loan Agreement [Member] | Term Loan [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  | $ 8,000 | $ 8,000 |  |  |  |  |  |
Hercules Loan Agreement [Member] | Additional Term Loan One [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  | 2,000 | 2,000 |  |  |  |  |  |
Proceeds from lines of credit |  |  | $ 2,000 |  |  |  |  |  |  |  |
Line of credit facility, expiration date |  |  | Nov. 01, 2021 |  |  |  |  |  |  |  |
Hercules Loan Agreement [Member] | Additional Term Loan Two [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Long Term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  | $ 3,000 | $ 3,000 |  |  |  |  |  |
Line of credit facility, expiration date | Mar. 01, 2019 |  |  |  |  |  |  |  |  |  |

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