Preferred Stock and Redeemable Convertible Preferred Stock - Additional Information (Details) - USD ($)				1 Months Ended	12 Months Ended
	Nov. 29, 2017	Oct. 12, 2017	Mar. 23, 2017	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Jan. 31, 2018	Jan. 30, 2018	Oct. 27, 2017	Aug. 29, 2017
Temporary Equity [Line Items]
Preferred stock, shares authorized					10,000,000	0		10,000,000
Preferred stock, shares authorized						20,320,667
Preferred stock, shares issued					0	0
Preferred stock, shares outstanding					0	0
Accretion of redeemable convertible preferred shares to redemption value						$ 0
Issuance of common stock					$ 89,370,000
Fair value of tranche rights liabilities/assets issuance date					Mar. 23, 2017
Fair value of tranche rights liabilities/assets settlement date					Nov. 29, 2017
Other Income, net					$ 2,117,000	$ (14,896,000)
Series B Tranche Right
Temporary Equity [Line Items]
Issuance price per shares		$ 8.346
Side Letter | PureTech Health LLC
Temporary Equity [Line Items]
Shares issued			5,175,984
Side Letter | PureTech Health LLC | Fourth Closing
Temporary Equity [Line Items]
Issuance price per shares			$ 1.932
Common Stock
Temporary Equity [Line Items]
Issuance of common stock					$ 1,000
Initial Public Offering
Temporary Equity [Line Items]
Convertible preferred stock converted into common stock								15,870,559
Issuance price per shares								$ 15.00
Initial Public Offering | Common Stock
Temporary Equity [Line Items]
Convertible preferred stock converted into common stock							15,870,559
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Preferred stock, shares authorized					0	15,527,951
Preferred stock, shares issued					0	15,527,951
Preferred stock, shares outstanding					0	15,527,951
Preferred stock, par value					$ 0.0001	$ 0.0001
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Preferred stock, shares authorized					0	4,792,716
Preferred stock, shares issued					0	4,792,716
Preferred stock, shares outstanding					0	4,792,716
Preferred stock, par value					$ 0.0001	$ 0.0001
Series A Preferred Stock
Temporary Equity [Line Items]
Issuance price per shares				$ 1.34
Issuance of common stock		$ 6,800,000
Series A Preferred Stock | Series A Tranche Rights
Temporary Equity [Line Items]
Shares issued				2,329,193
Issuance price per shares		$ 4.11
Issuance of common stock			$ 2,000,000	$ 4,500,000
Estimated fair value		$ 10,100,000		1,400,000
Other income		$ 8,800,000		$ 600,000
Series A Preferred Stock | Series A Tranche Rights | Second Closing
Temporary Equity [Line Items]
Issuance price per shares										$ 1.932
Series A Preferred Stock | Purchase Agreement
Temporary Equity [Line Items]
Shares issued		7,763,975
Issuance price per shares		$ 1.932	$ 1.22
Issuance of common stock		$ 15,000,000
Series A Preferred Stock | Purchase Agreement | New Investor
Temporary Equity [Line Items]
Issuance of common stock		6,000,000
Series A Preferred Stock | Purchase Agreement | Tranche Rights
Temporary Equity [Line Items]
Shares issued			4,917,185
Issuance of common stock			$ 9,500,000
Series A Preferred Stock | Purchase Agreement | Tranche Rights | Third Closing
Temporary Equity [Line Items]
Issuance price per shares			$ 1.932
Series A Preferred Stock | Purchase Agreement | PureTech Health LLC
Temporary Equity [Line Items]
Shares issued			2,846,791
Issuance price per shares			$ 1.932
Issuance of common stock		9,000,000	$ 5,500,000
Series A Preferred Stock | Purchase Agreement | Novartis International Pharmaceutical Ltd.
Temporary Equity [Line Items]
Shares issued			2,587,992
Series B Preferred Stock
Temporary Equity [Line Items]
Shares issued	4,792,716								4,792,716
Issuance price per shares	$ 8.346								$ 8.346
Issuance of common stock		$ 20,000,000
Series B Preferred Stock | Series B Tranche Right
Temporary Equity [Line Items]
Issuance of common stock	$ 20,000,000
Series B Preferred Stock | Series B Financing
Temporary Equity [Line Items]
Issuance of common stock	$ 40,000,000