Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	At-the-Market Offering	Common Stock	Common Stock At-the-Market Offering	Additional Paid-In Capital	Additional Paid-In Capital At-the-Market Offering	Accumulated Deficit	Accumulated Other Comprehensive Loss
Balance at Dec. 31, 2017	$ 150,307		$ 4		$ 300,906		$ (150,333)	$ (270)
Balance, shares at Dec. 31, 2017			35,759,000
Exercise of stock options	150				150
Exercise of stock options, shares			69,000
Stock-based compensation expense	1,881				1,881
Cumulative-effect adjustment from adoption of accounting standard | ASC 606	8,791						8,791
Net loss	(13,206)						(13,206)
Unrealized gain (loss) on available-for-sale securities	(56)							(56)
Balance at Mar. 31, 2018	147,867		$ 4		302,937		(154,748)	(326)
Balance, shares at Mar. 31, 2018			35,828,000
Balance at Dec. 31, 2018	126,714		$ 4		322,993		(196,170)	(113)
Balance, shares at Dec. 31, 2018			38,834,000
Issuance of stock		$ 1,150				$ 1,150
Issuance of stock, shares				243,000
Exercise of stock options	$ 8				8
Exercise of stock options, shares	6,000		6,000
Stock-based compensation expense	$ 1,683				1,683
Cumulative-effect adjustment from adoption of accounting standard | ASC 2018-07					71		(71)
Net loss	(23,687)						(23,687)
Unrealized gain (loss) on available-for-sale securities	106							106
Balance at Mar. 31, 2019	$ 105,974		$ 4		$ 325,905		$ (219,928)	$ (7)
Balance, shares at Mar. 31, 2019			39,083,000