Long Term Debt - Hercules Loan Agreement - Additional Information (Detail)			1 Months Ended			3 Months Ended		9 Months Ended
	Mar. 18, 2019 $ / shares shares	Mar. 13, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Oct. 31, 2018 USD ($) $ / shares shares	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 27, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 11, 2018 USD ($)
Schedule Of Long Term Debt [Line Items]
Fair value of derivative liability		$ 5,200,000
Minimum liquidity as a percentage of debt						125.00%		125.00%
Maximum liquidity other than cash and cash equivalents						$ 2,500,000		$ 2,500,000
Unamortized debt discount				$ 226,000		317,000		317,000
Arsanis
Schedule Of Long Term Debt [Line Items]
Business acquisition share exchange ratio		0.5702
Fair value of derivative liability		$ 0
Hercules Loan Agreement
Schedule Of Long Term Debt [Line Items]
Line of credit facility, maximum borrowing capacity					$ 13,000,000.0
Proceeds from lines of credit					$ 8,000,000.0
Fair value of derivative liability		$ 326,000		282,000								$ 132,000
Interest expense, debt						382,000	$ 0	739,000	$ 0
Amount of interest expense including accretion of debt discount.						101,000		215,000
Unamortized debt discount						$ 104,000		104,000
Effective interest rate of loan			11.00%
Principal payments								$ 0
Hercules Loan Agreement | Arsanis
Schedule Of Long Term Debt [Line Items]
Warrant exercise price (usd per share) | $ / shares	$ 19.80				$ 99
Number of warrants for purchase of convertible preferred shares (in shares) | shares	5,000
Business acquisition share exchange ratio	0.5702
Hercules Loan Agreement | Series B Redeemable Convertible Preferred Stock
Schedule Of Long Term Debt [Line Items]
Warrant issued to purchase stock (in shares) | shares					210,638
Warrant exercise price (usd per share) | $ / shares					$ 1.88
Hercules Loan Agreement | Common Stock
Schedule Of Long Term Debt [Line Items]
Warrant exercise price (usd per share) | $ / shares					$ 19.78
Number of warrants for purchase of convertible preferred shares (in shares) | shares					20,016
Hercules Loan Agreement | Additional Term Loan One
Schedule Of Long Term Debt [Line Items]
Line of credit facility, maximum borrowing capacity					$ 2,000,000.0
Proceeds from lines of credit				$ 2,000,000.0	2,000,000.0
Expired borrowing capacity											$ 3,000,000.0
Hercules Loan Agreement | Additional Term Loan Two
Schedule Of Long Term Debt [Line Items]
Line of credit facility, maximum borrowing capacity					3,000,000.0
Hercules Loan Agreement | Term Loan
Schedule Of Long Term Debt [Line Items]
Line of credit facility, maximum borrowing capacity					$ 8,000,000.0
June 2019 Loan Agreement
Schedule Of Long Term Debt [Line Items]
Line of credit facility, maximum borrowing capacity			$ 35,000,000.0
Line of credit facility, outstanding			$ 10,000,000.0
Line of credit facility, current borrowing capacity										$ 20,000,000.0
Debt instrument variable percentage			8.75%
Interest rate increase percentage			4.00%
Line of credit facility periodic payment			$ 800,000
Line of credit principal amount drawn percentage			4.00%
Minimum liquidity as a percentage of debt			125.00%
Minimum liquidity as a percentage of debt			100.00%
Maximum liquidity other than cash and cash equivalents			$ 2,500,000
Fair value of derivative liability			18,000
Upfront payments			180,000
Amortization of debt discount			$ 180,000
June 2019 Loan Agreement | Prime Rate
Schedule Of Long Term Debt [Line Items]
Line of credit facility interest rate			6.00%
June 2019 Loan Agreement | Minimum
Schedule Of Long Term Debt [Line Items]
Debt instrument variable percentage			8.75%
June 2019 Loan Agreement | Maximum
Schedule Of Long Term Debt [Line Items]
Debt instrument pre payment premium percentage			2.00%
June 2019 Loan Agreement | Additional Term Loan One
Schedule Of Long Term Debt [Line Items]
Line of credit facility, additional borrowing capacity			$ 5,000,000.0
June 2019 Loan Agreement | Additional Term Loan Two
Schedule Of Long Term Debt [Line Items]
Line of credit facility, additional borrowing capacity			$ 10,000,000.0
June 2019 Loan Agreement | New Borrowings
Schedule Of Long Term Debt [Line Items]
Line of credit facility, current borrowing capacity										10,000,000.0
June 2019 Loan Agreement | Previous Agreement
Schedule Of Long Term Debt [Line Items]
Line of credit facility, current borrowing capacity										$ 10,000,000.0