Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Mar. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers between fair value levels	$ 0	$ 0
Genzyme Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative liability		$ 183,000
Measurement Input, Expected Dividend Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expected dividend yield	0.00%
Arsanis | Genzyme Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative liability			$ 0
Series A Convertible Preferred Stock | Arsanis And Merger Sub
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred stock Fair value per share (in usd per share)		$ 1.70
Series B Convertible Preferred Stock | Arsanis And Merger Sub
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred stock Fair value per share (in usd per share)		$ 1.86
Convertible Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants for purchase of convertible preferred shares (in shares)	0