Fair Value of Financial Assets and Liabilities - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Genzyme Agreement
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability		$ 183,000
Measurement Input, Expected Dividend Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Preferred stock, dividend rate	0.00%
Convertible Preferred Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants available to purchase (in shares)	0
Arsanis and Merger Sub | Class A Convertible Preferred Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Preferred stock, fair value (in dollars per share)		$ 1.70
Arsanis and Merger Sub | Class B Convertible Preferred Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Preferred stock, fair value (in dollars per share)	$ 1.86
Arsanis | Genzyme Agreement
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of derivative liability		$ 0