CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	COMMON STOCK	ADDITIONAL PAID-IN CAPITAL	ACCUMULATED DEFICIT
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	$ 7	$ 605,495	$ (404,809)
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)
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	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)