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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
	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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