Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balance (in shares) at Dec. 31, 2016	45,333,790
Balance at Dec. 31, 2016	$ 45	$ 240,977	$ (246,362)	$ 3	$ (5,337)
Stock-based compensation		4,294			4,294
Net proceeds from issuance of common stock in connection with equity financings (in shares)	5,401,099
Net proceeds from issuance of common stock in connection with equity financings	$ 6	15,688			15,694
Issuance of common stock upon exercise of stock options (in shares)	69,372
Issuance of common stock upon exercise of stock options		105			105
Issuance of common stock upon ESPP purchase (in shares)	94,893
Issuance of common stock upon ESPP purchase		246			246
Change in unrealized gains and losses on investments				(3)	(3)
Net loss			(51,508)		(51,508)
Balance (in shares) at Dec. 31, 2017	50,899,154
Balance at Dec. 31, 2017	$ 51	261,310	(297,870)		(36,509)
Stock-based compensation		5,168			5,168
Net proceeds from issuance of common stock in connection with equity financings (in shares)	27,364,301
Net proceeds from issuance of common stock in connection with equity financings	$ 27	81,498			81,525
Issuance of common stock upon exercise of stock options (in shares)	135,385
Issuance of common stock upon exercise of stock options		401			401
Issuance of common stock upon ESPP purchase (in shares)	182,360
Issuance of common stock upon ESPP purchase		275			275
Change in unrealized gains and losses on investments
Net loss			(47,149)		(47,149)
Issuance of common stock upon exercise of warrants (in shares)	176,730
Issuance of common stock upon exercise of warrants		542			542
Balance (in shares) at Dec. 31, 2018	78,757,930
Balance at Dec. 31, 2018	$ 78	349,194	(345,019)		4,253
Cumulative effect adjustment for adoption of ASU No. 2016-02 | Accounting Standards Update 2016-02 [Member]			153		153
Stock-based compensation		5,057			5,057
Net proceeds from issuance of common stock in connection with equity financings (in shares)	500,000
Net proceeds from issuance of common stock in connection with equity financings		1,233			$ 1,233
Issuance of common stock upon exercise of stock options (in shares)	111,702				111,702
Issuance of common stock upon exercise of stock options		270			$ 270
Issuance of common stock upon ESPP purchase (in shares)	203,469
Issuance of common stock upon ESPP purchase	$ 1	472			473
Change in unrealized gains and losses on investments
Net loss			(53,240)		(53,240)
Issuance of warrants related to debt financing		383			383
Balance (in shares) at Dec. 31, 2019	79,573,101
Balance at Dec. 31, 2019	$ 79	$ 356,609	$ (398,106)		$ (41,418)