CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK, CONTINGENTLY REDEEMABLE RESTRICTED COMMON STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Redeemable convertible preferred stock (Series A and B)	Contingently Redeemable Restricted Common Stock
Balance at the beginning of the period (in shares) at Dec. 31, 2017						93,222,418
Balance at the beginning of the period at Dec. 31, 2017						$ 174,895	$ 355
Changes in temporary equity
Stock-based compensation expense							210
Accretion of redeemable convertible preferred stock to redemption value						$ 6,371
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock (in shares)						(93,222,418)
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock						$ (181,266)	(565)
Balance at the end of the period (in shares) at Dec. 31, 2018						0
Balance at the end of the period at Dec. 31, 2018						$ 0	0
Balance at the beginning of the period (in shares) at Dec. 31, 2017		3,302,927
Balance at the beginning of the period at Dec. 31, 2017	$ (93,572)	$ 3	$ 0	$ (13)	$ (93,562)
Changes in equity
Stock-based compensation expense	6,019		6,019
Accretion of redeemable convertible preferred stock to redemption value	(6,371)		(3,105)		(3,266)
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock (in shares)		18,644,462
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock	181,831	$ 19	181,812
Issuance of common stock, net of issuance costs (in shares)		6,250,000
Issuance of common stock, net of issuance costs	89,912	$ 6	89,906
Exercise of stock options (in shares)		129,510
Exercise of stock options	404		404
Cancellation of restricted common stock (in shares)		(12,625)
Cancellation of restricted common stock	0
Vesting of restricted common stock	22		22
Unrealized gains on marketable securities	(62)			(62)
Net loss	$ (76,934)				(76,934)
Balance at the end of the period (in shares) at Dec. 31, 2018	28,314,274	28,314,274
Balance at the end of the period at Dec. 31, 2018	$ 101,249	$ 28	275,058	(75)	(173,762)
Balance at the end of the period (in shares) at Dec. 31, 2019						0
Balance at the end of the period at Dec. 31, 2019						$ 0	$ 0
Changes in equity
Stock-based compensation expense	7,340		7,340
Conversion of redeemable convertible preferred stock and contingently redeemable restricted common stock to common stock	0
Issuance of common stock, net of issuance costs (in shares)		327,602
Issuance of common stock, net of issuance costs	334	$ 1	333
Exercise of stock options (in shares)		44,132
Exercise of stock options	115		115
Issuance of common stock; ESPP purchase (in shares)		62,927
Issuance of common stock; ESPP purchase	63		63
Issuance of restricted common stock (in shares)		25,000
Issuance of restricted common stock	0
Cancellation of restricted common stock (in shares)		(44,210)
Cancellation of restricted common stock	0
Vesting of restricted common stock	17		17
Unrealized gains on marketable securities	75			75
Net loss	$ (79,776)				(79,776)
Balance at the end of the period (in shares) at Dec. 31, 2019	28,729,725	28,729,725
Balance at the end of the period at Dec. 31, 2019	$ 29,417	$ 29	$ 282,926	$ 0	$ (253,538)