CONDENSED CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' DEFICIT - USD ($) $ in Thousands	Total	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Series C Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Additional Paid-in Capital Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment
Beginning balance at Dec. 31, 2018	$ (108,991)					$ 2		$ (9)	$ (108,984)
Temporary equity, Beginning balance, shares at Dec. 31, 2018		33,200,000	20,479,162	20,204,079
Temporary equity, Beginning balance at Dec. 31, 2018		$ 41,738	$ 75,900	$ 83,385
Beginning balance, shares at Dec. 31, 2018					2,921,268
Issuance of Series B redeemable convertible preferred stock in conjunction with sponsored research agreement			$ 253
Issuance of Series B redeemable convertible preferred stock in conjunction with sponsored research agreement, shares			41,666
Exercise of options to purchase common stock	254					254
Exercise of options to purchase common stock, shares					74,649
Stock-based compensation expense	4,883					4,883
Accretion of redeemable convertible preferred stock to redemption value	(9,984)					(5,120)			(4,864)
Temporary equity, Accretion of redeemable convertible preferred stock to redemption value		$ 1,761	$ 3,644	$ 4,579
Other comprehensive income, net	72							72
Net loss	(56,322)								(56,322)
Ending balance at Sep. 30, 2019	$ (170,088)					2	$ (17)	63	(170,153)	$ 17
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201807Member
Temporary equity, Ending balance, shares at Sep. 30, 2019		33,200,000	20,520,828	20,204,079
Temporary equity, Ending balance at Sep. 30, 2019		$ 43,499	$ 79,797	$ 87,964
Ending balance, shares at Sep. 30, 2019					2,995,917
Beginning balance at Jun. 30, 2019	$ (148,314)					2		71	(148,387)
Temporary equity, Beginning balance, shares at Jun. 30, 2019		33,200,000	20,520,828	20,204,079
Temporary equity, Beginning balance at Jun. 30, 2019		$ 42,830	$ 78,487	$ 86,421
Beginning balance, shares at Jun. 30, 2019					2,995,917
Issuance of Series B redeemable convertible preferred stock in conjunction with sponsored research agreement			70
Stock-based compensation expense	2,349					2,349
Accretion of redeemable convertible preferred stock to redemption value	(3,452)					(2,349)			(1,103)
Temporary equity, Accretion of redeemable convertible preferred stock to redemption value		$ 669	$ 1,240	$ 1,543
Other comprehensive income, net	(8)							(8)
Net loss	(20,663)								(20,663)
Ending balance at Sep. 30, 2019	$ (170,088)					2	$ (17)	63	(170,153)	$ 17
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201807Member
Temporary equity, Ending balance, shares at Sep. 30, 2019		33,200,000	20,520,828	20,204,079
Temporary equity, Ending balance at Sep. 30, 2019		$ 43,499	$ 79,797	$ 87,964
Ending balance, shares at Sep. 30, 2019					2,995,917
Beginning balance at Dec. 31, 2019	$ (192,833)					2		43	(192,878)
Temporary equity, Beginning balance, shares at Dec. 31, 2019	73,924,907	33,200,000	20,520,828	20,204,079
Temporary equity, Beginning balance at Dec. 31, 2019	$ 214,784	$ 44,169	$ 81,108	$ 89,507
Beginning balance, shares at Dec. 31, 2019					2,997,040
Issuance of Series B redeemable convertible preferred stock in conjunction with sponsored research agreement, shares			62,500
Issuance of common stock in connection with license agreement	2,660					2,660
Issuance of common stock in connection with license agreement, shares					177,318
Exercise of options to purchase common stock	$ 142					142
Exercise of options to purchase common stock, shares	20,998				20,997
Stock-based compensation expense	$ 4,961					4,961
Accretion of redeemable convertible preferred stock to redemption value	(10,249)					(6,702)			(3,547)
Temporary equity, Accretion of redeemable convertible preferred stock to redemption value		$ 1,993	$ 3,661	$ 4,595
Other comprehensive income, net	(43)							$ (43)
Net loss	(73,708)								(73,708)
Ending balance at Sep. 30, 2020	$ (269,070)					1,063			(270,133)
Temporary equity, Ending balance, shares at Sep. 30, 2020	73,987,407	33,200,000	20,583,328	20,204,079
Temporary equity, Ending balance at Sep. 30, 2020	$ 225,033	$ 46,162	$ 84,769	$ 94,102
Ending balance, shares at Sep. 30, 2020					3,195,355
Beginning balance at Jun. 30, 2020	(234,838)					2			(234,840)
Temporary equity, Beginning balance, shares at Jun. 30, 2020		33,200,000	20,583,328	20,204,079
Temporary equity, Beginning balance at Jun. 30, 2020		$ 45,493	$ 83,524	$ 92,559
Beginning balance, shares at Jun. 30, 2020					3,010,852
Issuance of common stock in connection with license agreement	2,660					2,660
Issuance of common stock in connection with license agreement, shares					177,318
Exercise of options to purchase common stock	38					38
Exercise of options to purchase common stock, shares					7,185
Stock-based compensation expense	1,820					1,820
Accretion of redeemable convertible preferred stock to redemption value	(3,457)					(3,457)
Temporary equity, Accretion of redeemable convertible preferred stock to redemption value		$ 669	$ 1,245	$ 1,543
Net loss	(35,293)								(35,293)
Ending balance at Sep. 30, 2020	$ (269,070)					$ 1,063			$ (270,133)
Temporary equity, Ending balance, shares at Sep. 30, 2020	73,987,407	33,200,000	20,583,328	20,204,079
Temporary equity, Ending balance at Sep. 30, 2020	$ 225,033	$ 46,162	$ 84,769	$ 94,102
Ending balance, shares at Sep. 30, 2020					3,195,355