CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Cumulative effect adjustment related to the adoption of ASC 842	COMMON STOCK	ADDITIONAL PAID-IN CAPITAL	ACCUMULATED DEFICIT	ACCUMULATED DEFICIT Cumulative effect adjustment related to the adoption of ASC 842	ACCUMULATED OTHER COMPREHENSIVE LOSS
Balance at beginning of period (in shares) at Dec. 31, 2018			68,210,742
Balance at beginning of period at Dec. 31, 2018	$ 200,693	$ (91)	$ 7	$ 605,495	$ (404,809)	$ (91)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercises of common stock options and sales of common stock under Employee Stock Purchase Plan (in shares)			78,164
Exercises of common stock options and sales of common stock under Employee Stock Purchase Plan	353			353
Stock-based compensation expense	4,827			4,784	43
Net loss	(26,154)				(26,154)
Balance at end of period (in shares) at Mar. 31, 2019			68,288,906
Balance at end of period at Mar. 31, 2019	179,628		$ 7	610,632	(431,011)		0
Balance at beginning of period (in shares) at Dec. 31, 2018			68,210,742
Balance at beginning of period at Dec. 31, 2018	200,693	$ (91)	$ 7	605,495	(404,809)	$ (91)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(80,789)
Balance at end of period (in shares) at Sep. 30, 2019			68,360,051
Balance at end of period at Sep. 30, 2019	134,367		$ 7	620,009	(485,646)		(3)
Balance at beginning of period (in shares) at Mar. 31, 2019			68,288,906
Balance at beginning of period at Mar. 31, 2019	179,628		$ 7	610,632	(431,011)		0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercises of common stock options and sales of common stock under Employee Stock Purchase Plan (in shares)			71,145
Exercises of common stock options and sales of common stock under Employee Stock Purchase Plan	489			489
Stock-based compensation expense	3,893			3,893
Net loss	(23,845)				(23,845)
Balance at end of period (in shares) at Jun. 30, 2019			68,360,051
Balance at end of period at Jun. 30, 2019	160,165		$ 7	615,014	(454,856)		0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	4,995			4,995
Other comprehensive loss	(3)						(3)
Net loss	(30,790)				(30,790)
Balance at end of period (in shares) at Sep. 30, 2019			68,360,051
Balance at end of period at Sep. 30, 2019	134,367		$ 7	620,009	(485,646)		$ (3)
Balance at beginning of period (in shares) at Dec. 31, 2019			71,573,196
Balance at beginning of period at Dec. 31, 2019	152,195		$ 7	677,504	(525,316)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercises of common stock options (in shares)			128,373
Exercises of common stock options	588			588
Stock-based compensation expense	8,527			8,527
Proceeds from issuance of common stock from public offering, net of underwriting fees and issuance costs (in shares)			2,077,500
Proceeds from issuance of common stock, net of commissions and offering costs of $904	39,088			39,088
Net loss	(22,492)				(22,492)
Balance at end of period (in shares) at Mar. 31, 2020			73,779,069
Balance at end of period at Mar. 31, 2020	177,906		$ 7	725,707	(547,808)
Balance at beginning of period (in shares) at Dec. 31, 2019			71,573,196
Balance at beginning of period at Dec. 31, 2019	152,195		$ 7	677,504	(525,316)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(76,161)
Balance at end of period (in shares) at Sep. 30, 2020			74,680,999
Balance at end of period at Sep. 30, 2020	153,700		$ 7	755,170	(601,477)
Balance at beginning of period (in shares) at Mar. 31, 2020			73,779,069
Balance at beginning of period at Mar. 31, 2020	177,906		$ 7	725,707	(547,808)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercises of common stock options and sales of common stock under Employee Stock Purchase Plan (in shares)			542,256
Exercises of common stock options and sales of common stock under Employee Stock Purchase Plan	5,876			5,876
Stock-based compensation expense	10,206			10,206
Net loss	(31,820)				(31,820)
Balance at end of period (in shares) at Jun. 30, 2020			74,321,325
Balance at end of period at Jun. 30, 2020	162,168		$ 7	741,789	(579,628)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercises of common stock options (in shares)			359,674
Exercises of common stock options	2,801			2,801
Stock-based compensation expense	10,580			10,580
Net loss	(21,849)				(21,849)
Balance at end of period (in shares) at Sep. 30, 2020			74,680,999
Balance at end of period at Sep. 30, 2020	$ 153,700		$ 7	$ 755,170	$ (601,477)