CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2018	$ 196,355	$ 3	$ 292,187	$ (77)	$ (95,758)
Beginning balance, Shares at Dec. 31, 2018		37,358,526
Vesting of common stock from option exercises	18		18
Vesting of common stock from option exercises, Shares		31,584
Issuance of common stock from option exercises	21		21
Issuance of common stock from option exercises, Shares		18,876
Issuance of common stock pursuant to employee stock purchase plan	396		396
Issuance of common stock pursuant to employee stock purchase plan, Shares		28,311
Stock-based compensation	1,243		1,243
Other comprehensive gain (loss)	83			83
Net loss	(23,852)				(23,852)
Ending balance at Mar. 31, 2019	174,264	$ 3	293,865	6	(119,610)
Ending balance, Shares at Mar. 31, 2019		37,437,297
Beginning balance at Dec. 31, 2018	196,355	$ 3	292,187	(77)	(95,758)
Beginning balance, Shares at Dec. 31, 2018		37,358,526
Other comprehensive gain (loss)	331
Net loss	(79,749)
Ending balance at Sep. 30, 2019	257,822	$ 4	433,071	254	(175,507)
Ending balance, Shares at Sep. 30, 2019		43,956,070
Beginning balance at Mar. 31, 2019	174,264	$ 3	293,865	6	(119,610)
Beginning balance, Shares at Mar. 31, 2019		37,437,297
Issuance of common stock in follow-on offering, net of discounts and issuance costs	134,525	$ 1	134,524
Issuance of common stock in follow-on offering, net of discounts and issuance costs, Shares		6,388,889
Vesting of common stock from option exercises	18		18
Vesting of common stock from option exercises, Shares		31,024
Issuance of common stock from option exercises	55		55
Issuance of common stock from option exercises, Shares		12,404
Stock-based compensation	1,970		1,970
Other comprehensive gain (loss)	180			180
Net loss	(26,270)				(26,270)
Ending balance at Jun. 30, 2019	284,742	$ 4	430,432	186	(145,880)
Ending balance, Shares at Jun. 30, 2019		43,869,614
Vesting of common stock from option exercises	11		11
Vesting of common stock from option exercises, Shares		16,104
Issuance of common stock from option exercises	209		209
Issuance of common stock from option exercises, Shares		43,429
Issuance of common stock pursuant to employee stock purchase plan	433		433
Issuance of common stock pursuant to employee stock purchase plan, Shares		26,923
Stock-based compensation	1,986		1,986
Other comprehensive gain (loss)	68			68
Net loss	(29,627)				(29,627)
Ending balance at Sep. 30, 2019	257,822	$ 4	433,071	254	(175,507)
Ending balance, Shares at Sep. 30, 2019		43,956,070
Beginning balance at Dec. 31, 2019	258,507	$ 4	457,994	183	(199,674)
Beginning balance, Shares at Dec. 31, 2019		45,116,742
Vesting of common stock from option exercises	12		12
Vesting of common stock from option exercises, Shares		15,542
Issuance of common stock from option exercises	135		135
Issuance of common stock from option exercises, Shares		34,311
Issuance of common stock pursuant to employee stock purchase plan	537		537
Issuance of common stock pursuant to employee stock purchase plan, Shares		39,471
Stock-based compensation	2,883		2,883
Other comprehensive gain (loss)	(202)			(202)
Net loss	(35,331)				(35,331)
Ending balance at Mar. 31, 2020	226,541	$ 4	461,561	(19)	(235,005)
Ending balance, Shares at Mar. 31, 2020		45,206,066
Beginning balance at Dec. 31, 2019	$ 258,507	$ 4	457,994	183	(199,674)
Beginning balance, Shares at Dec. 31, 2019		45,116,742
Issuance of common stock from option exercises, Shares	41,593
Issuance of common stock pursuant to employee stock purchase plan, Shares	55,234
Other comprehensive gain (loss)	$ (183)
Net loss	(98,903)
Ending balance at Sep. 30, 2020	170,222	$ 4	468,795		(298,577)
Ending balance, Shares at Sep. 30, 2020		45,259,747
Beginning balance at Mar. 31, 2020	226,541	$ 4	461,561	(19)	(235,005)
Beginning balance, Shares at Mar. 31, 2020		45,206,066
Vesting of common stock from option exercises	4		4
Vesting of common stock from option exercises, Shares		1,011
Issuance of common stock from option exercises	16		16
Issuance of common stock from option exercises, Shares		3,576
Stock-based compensation	3,453		3,453
Other comprehensive gain (loss)	30			30
Net loss	(35,340)				(35,340)
Ending balance at Jun. 30, 2020	194,704	$ 4	465,034	11	(270,345)
Ending balance, Shares at Jun. 30, 2020		45,210,653
Vesting of common stock from option exercises	5		5
Vesting of common stock from option exercises, Shares		1,011
Issuance of common stock from option exercises	9		9
Issuance of common stock from option exercises, Shares		3,706
Issuance of common stock pursuant to employee stock purchase plan	401		401
Issuance of common stock pursuant to employee stock purchase plan, Shares		44,377
Stock-based compensation	3,346		3,346
Other comprehensive gain (loss)	(11)			$ (11)
Net loss	(28,232)				(28,232)
Ending balance at Sep. 30, 2020	$ 170,222	$ 4	$ 468,795		$ (298,577)
Ending balance, Shares at Sep. 30, 2020		45,259,747