Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Common Stock	Additional Pain In Capital	Accumulated Deficit	Comprehensive Income (Loss)
Balance at Jul. 04, 2016	$ 0			$ 0	$ 0	$ 0	$ 0
Balance, shares at Jul. 04, 2016				0
Vesting of restricted shares, value	1			$ 1
Vesting of restricted shares				2,082,860
Net loss	(1)					(1)
Balance at Dec. 31, 2016	0			$ 1		(1)
Balance, shares at Dec. 31, 2016				2,082,860
Issuance of common shares to PureTech, shares				1,886,363
Issuance of redeemable convertible preferred stock		$ 41,674	$ 39,946
Issuance of redeemable convertible preferred stock, shares		15,527,951	4,792,716
Issuance of redeemable convertible preferred stock, adjustments to additional paid in capital	1,379				1,379
Vesting of restricted shares				593,417
Stock-based compensation expense	470				470
Net loss	(33,778)					(33,778)
Balance at Dec. 31, 2017	(31,929)			$ 1	1,849	(33,779)
Balance, temporary equity, shares at Dec. 31, 2017		15,527,951	4,792,716
Balance, temporary equity at Dec. 31, 2017		$ 41,674	$ 39,946
Balance, shares at Dec. 31, 2017				4,562,640
Conversion of convertible preferred stock into common stock upon the closing of initial public offering	81,620	$ (41,674)	$ (39,946)	$ 1	81,619
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, Shares		(15,527,951)	(4,792,716)	15,870,559
Issuance of common stock	89,370			$ 1	89,369
Issuance of common stock, Shares				6,516,667
Vesting of restricted shares, value	865				865
Vesting of restricted shares				1,097,449
Exercise of stock options	5				5
Exercise of stock options, Shares				7,029
Stock-based compensation expense	1,928				1,928
Net loss	(37,614)					(37,614)
Unrealized losses on marketable securities	(41)						(41)
Balance at Dec. 31, 2018	$ 104,204			$ 3	$ 175,635	$ (71,393)	$ (41)
Balance, temporary equity, shares at Dec. 31, 2018		0	0
Balance, temporary equity at Dec. 31, 2018		$ 0	$ 0
Balance, shares at Dec. 31, 2018				28,054,344