Long-Term Debt - Hercules Loan Agreements (Details)			1 Months Ended		3 Months Ended
	Mar. 18, 2019 $ / shares shares	Mar. 13, 2019 USD ($)	Dec. 31, 2018 USD ($)	Oct. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2019 USD ($)	Dec. 11, 2018 USD ($)
Line of Credit Facility [Line Items]
Fair value of warrants		$ 5,200,000			$ 288,000
Arsanis
Line of Credit Facility [Line Items]
Exchange ratio		0.5702
Fair value of warrants		$ 0
Hercules Loan Agreements
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 13,000,000.0
Proceeds from line of credit				$ 8,000,000.0
Fair value of warrants		$ 326,000	$ 282,000			$ 132,000
Hercules Loan Agreements | Arsanis
Line of Credit Facility [Line Items]
Convertible preferred stock exercise price (in dollars per share) | $ / shares	$ 19.80			$ 99
Warrants available to purchase (in shares) | shares	5,000
Exchange ratio	0.5702
Hercules Loan Agreements | Series B Redeemable Convertible Preferred Stock
Line of Credit Facility [Line Items]
Number of warrants issued (in shares) | shares				210,638
Convertible preferred stock exercise price (in dollars per share) | $ / shares				$ 1.88
Hercules Loan Agreements | Common Stock
Line of Credit Facility [Line Items]
Convertible preferred stock exercise price (in dollars per share) | $ / shares				$ 19.78
Warrants available to purchase (in shares) | shares				20,016
Hercules Loan Agreements | Additional Term Loan One
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 2,000,000.0	3,000,000.0
Proceeds from line of credit			$ 2,000,000.0		2,000,000.0
Remaining borrowing capacity					$ 3,000,000.0
Hercules Loan Agreements | Term Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 8,000,000.0