Consolidated Statements of Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]	Convertible Preferred Stock [Member]
Beginning balance at Dec. 31, 2018	$ (110,927)	$ 3	$ 3,247		$ (114,177)	$ 532,120
Beginning balance, shares at Dec. 31, 2018		2,998,017				210,863,764
Issuance of Series C preferred stock, net of issuance costs						$ 5,686
Issuance of Series C preferred stock, net of issuance costs, shares						1,779,093
Issuance of common stock upon exercise of stock options	29		29
Issuance of common stock upon exercise of stock options, shares		7,040
Vesting of restricted common stock	101		101
Vesting of restricted common stock ,shares		216,777
Stock-based compensation expense	820		820
Net loss	(14,182)				(14,182)
Ending balance at Mar. 31, 2019	(124,159)	$ 3	4,197		(128,359)	$ 537,806
Ending balance, shares at Mar. 31, 2019		3,221,834				212,642,857
Beginning balance at Dec. 31, 2018	(110,927)	$ 3	3,247		(114,177)	$ 532,120
Beginning balance, shares at Dec. 31, 2018		2,998,017				210,863,764
Net loss	(51,392)
Ending balance at Sep. 30, 2019	(158,291)	$ 4	6,867	$ 407	(165,569)	$ 537,781
Ending balance, shares at Sep. 30, 2019		3,701,910				212,642,857
Beginning balance at Mar. 31, 2019	(124,159)	$ 3	4,197		(128,359)	$ 537,806
Beginning balance, shares at Mar. 31, 2019		3,221,834				212,642,857
Series C preferred stock issuance costs						$ (21)
Issuance of common stock upon exercise of stock options	23		23
Issuance of common stock upon exercise of stock options, shares		16,897
Vesting of restricted common stock	100		100
Vesting of restricted common stock ,shares		216,777
Stock-based compensation expense	1,082		1,082
Unrealized gain(Loss) on investments	475			475
Net loss	(17,537)				(17,537)
Ending balance at Jun. 30, 2019	(140,016)	$ 3	5,402	475	(145,896)	$ 537,785
Ending balance, shares at Jun. 30, 2019		3,455,508				212,642,857
Series C preferred stock issuance costs						$ (4)
Issuance of common stock upon exercise of stock options	120		120
Issuance of common stock upon exercise of stock options, shares		31,581
Vesting of restricted common stock	98	$ 1	97
Vesting of restricted common stock ,shares		214,821
Stock-based compensation expense	1,248		1,248
Unrealized gain(Loss) on investments	(68)			(68)
Net loss	(19,673)				(19,673)
Ending balance at Sep. 30, 2019	(158,291)	$ 4	6,867	407	(165,569)	$ 537,781
Ending balance, shares at Sep. 30, 2019		3,701,910				212,642,857
Beginning balance at Dec. 31, 2019	(180,438)	$ 4	8,715	325	(189,482)	$ 537,781
Beginning balance, shares at Dec. 31, 2019		4,037,476				212,642,857
Issuance of common stock upon exercise of stock options	351		351
Issuance of common stock upon exercise of stock options, shares		85,845
Vesting of restricted common stock	98		98
Vesting of restricted common stock ,shares		210,516
Stock-based compensation expense	1,455		1,455
Unrealized gain(Loss) on investments	1,069			1,069
Net loss	(24,886)				(24,886)
Ending balance at Mar. 31, 2020	(202,351)	$ 4	10,619	1,394	(214,368)	$ 537,781
Ending balance, shares at Mar. 31, 2020		4,333,837				212,642,857
Beginning balance at Dec. 31, 2019	$ (180,438)	$ 4	8,715	325	(189,482)	$ 537,781
Beginning balance, shares at Dec. 31, 2019		4,037,476				212,642,857
Issuance of common stock upon exercise of stock options, shares	283,275
Net loss	$ (87,688)
Ending balance at Sep. 30, 2020	713,525	$ 90	1,152,763	176	(439,504)
Ending balance, shares at Sep. 30, 2020		89,803,785
Beginning balance at Mar. 31, 2020	(202,351)	$ 4	10,619	1,394	(214,368)	$ 537,781
Beginning balance, shares at Mar. 31, 2020		4,333,837				212,642,857
Issuance of common stock upon exercise of stock options	367		367
Issuance of common stock upon exercise of stock options, shares		95,573
Vesting of restricted common stock	47	$ 1	46
Vesting of restricted common stock ,shares		163,321
Stock-based compensation expense	4,032		4,032
Unrealized gain(Loss) on investments	(763)			(763)
Net loss	(26,724)				(26,724)
Ending balance at Jun. 30, 2020	(225,392)	$ 5	15,064	631	(241,092)	$ 537,781
Ending balance, shares at Jun. 30, 2020		4,592,731				212,642,857
Extinguishment upon modification of Series C preferred stock	(177,789)		(15,455)		(162,334)	$ 177,789
Conversion of convertible preferred stock into common stock upon initial public offering	715,570	$ 62	715,508			$ (715,570)
Conversion of convertible preferred stock into common stock upon initial public offering, shares		61,992,534				(212,642,857)
Issuance of common stock in public offering, net of discounts and issuance costs of $34,707	425,293	$ 23	425,270
Issuance of common stock in public offering, net of discounts and issuance costs of $34,707, shares		23,000,000
Issuance of common stock upon exercise of stock options	442		442
Issuance of common stock upon exercise of stock options, shares		101,861
Vesting of restricted common stock	5		5
Vesting of restricted common stock ,shares		116,659
Stock-based compensation expense	11,929		11,929
Unrealized gain(Loss) on investments	(455)			(455)
Net loss	(36,078)				(36,078)
Ending balance at Sep. 30, 2020	$ 713,525	$ 90	$ 1,152,763	$ 176	$ (439,504)
Ending balance, shares at Sep. 30, 2020		89,803,785